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How does C# LINQ change the data source and how do I prevent it? [duplicate]

Some guy asked me this question couple of months ago and I couldn't explain it in detail. What is the difference between a reference type and a value type in C#?
I know that value types are int, bool, float, etc and reference types are delegate, interface, etc. Or is this wrong, too?
Can you explain it to me in a professional way?

Your examples are a little odd because while int, bool and float are specific types, interfaces and delegates are kinds of type - just like struct and enum are kinds of value types.
I've written an explanation of reference types and value types in this article. I'd be happy to expand on any bits which you find confusing.
The "TL;DR" version is to think of what the value of a variable/expression of a particular type is. For a value type, the value is the information itself. For a reference type, the value is a reference which may be null or may be a way of navigating to an object containing the information.
For example, think of a variable as like a piece of paper. It could have the value "5" or "false" written on it, but it couldn't have my house... it would have to have directions to my house. Those directions are the equivalent of a reference. In particular, two people could have different pieces of paper containing the same directions to my house - and if one person followed those directions and painted my house red, then the second person would see that change too. If they both just had separate pictures of my house on the paper, then one person colouring their paper wouldn't change the other person's paper at all.

Value type:
Holds some value not memory addresses
Example:
Struct
Storage:
TL;DR: A variable's value is stored wherever it is decleared. Local variables live on the stack for example, but when declared inside a class as a member it lives on the heap tightly coupled with the class it is declared in.
Longer: Thus value types are stored wherever they are declared.
E.g.: an int's value inside a function as a local variable would be stored on the stack, whilst an in int's value declared as member in a class would be stored on the heap with the class it is declared in. A value type on a class has a lifetype that is exactly the same as the class it is declared in, requiring almost no work by the garbage collector. It's more complicated though, i'd refer to #JonSkeet's book "C# In Depth" or his article "Memory in .NET" for a more concise explenation.
Advantages:
A value type does not need extra garbage collection. It gets garbage collected together with the instance it lives in. Local variables in methods get cleaned up upon method leave.
Drawbacks:
When large set of values are passed to a method the receiving variable actually copies so there are two redundant values in memory.
As classes are missed out.it losses all the oop benifits
Reference type:
Holds a memory address of a value not value
Example:
Class
Storage:
Stored on heap
Advantages:
When you pass a reference variable to a method and it changes it indeed changes the original value whereas in value types a copy of the given variable is taken and that's value is changed.
When the size of variable is bigger reference type is good
As classes come as a reference type variables, they give reusability, thus benefitting Object-oriented programming
Drawbacks:
More work referencing when allocating and dereferences when reading the value.extra overload for garbage collector

I found it easier to understand the difference of the two if you know how computer allocate stuffs in memory and know what a pointer is.
Reference is usually associated with a pointer. Meaning the memory address where your variable reside is actually holding another memory address of the actual object in a different memory location.
The example I am about to give is grossly over simplified, so take it with a grain of salt.
Imagine computer memory is a bunch of PO boxes in a row (starting w/ PO Box 0001 to PO Box n) that can hold something inside it. If PO boxes doesn't do it for you, try a hashtable or dictionary or an array or something similar.
Thus, when you do something like:
var a = "Hello";
the computer will do the following:
allocate memory (say starting at memory location 1000 for 5 bytes) and put H (at 1000), e (at 1001), l (at 1002), l (at 1003) and o (at 1004).
allocate somewhere in memory (say at location 0500) and assigned it as the variable a.
So it's kind of like an alias (0500 is a).
assign the value at that memory location (0500) to 1000 (which is where the string Hello start in memory). Thus the variable a is holding a reference to the actual starting memory location of the "Hello" string.
Value type will hold the actual thing in its memory location.
Thus, when you do something like:
var a = 1;
the computer will do the following:
allocate a memory location say at 0500 and assign it to variable a (the same alias thing)
put the value 1 in it (at memory location 0500).
Notice that we are not allocating extra memory to hold the actual value (1).
Thus a is actually holding the actual value and that's why it's called value type.

This is from a post of mine from a different forum, about two years ago. While the language is vb.net (as opposed to C#), the Value Type vs. Reference type concepts are uniform throughout .net, and the examples still hold.
It is also important to remember that within .net, ALL types technically derive from the base type Object. The value types are designed to behave as such, but in the end they also inherit the functionality of base type Object.
A. Value Types are just that- they represent a distinct area in memory where a discrete VALUE is stored. Value types are of fixed memory size and are stored in the stack, which is a collection of addresses of fixed size.
When you make a statement like such:
Dim A as Integer
DIm B as Integer
A = 3
B = A
You have done the following:
Created 2 spaces in memory sufficient to hold 32 bit integer values.
Placed a value of 3 in the memory allocation assigned to A
Placed a value of 3 in the memory allocation assigned to B by assigning it the same value as the held in A.
The Value of each variable exists discretely in each memory location.
B. Reference Types can be of various sizes. Therefore, they can't be stored in the "Stack" (remember, the stack is a collection of memory allocations of fixed size?). They are stored in the "Managed Heap". Pointers (or "references") to each item on the managed heap are maintained in the stack (Like an Address). Your code uses these pointers in the stack to access objects stored in the managed heap. So when your code uses a reference variable, it is actually using a pointer (or "address" to an memory location in the managed heap).
Say you have created a Class named clsPerson, with a string Property Person.Name
In this case, when you make a statement such as this:
Dim p1 As clsPerson
p1 = New clsPerson
p1.Name = "Jim Morrison"
Dim p2 As Person
p2 = p1
In the case above, the p1.Name Property will Return "Jim Morrison", as you would expect. The p2.Name property will ALSO return "Jim Morrison", as you would Iintuitively expect. I believe that both p1 and p2 represent distinct addresses on the Stack. However, now that you have assigned p2 the value of p1, both p1 and p2 point to the SAME LOCATION on the managed heap.
Now COnsider THIS situation:
Dim p1 As clsPerson
Dim p2 As clsPerson
p1 = New clsPerson
p1.Name = "Jim Morrison"
p2 = p1
p2.Name = "Janis Joplin"
In this situation, You have created one new instance of the person Class on the Managed Heap with a pointer p1 on the Stack which references the object, and assigned the Name Property of the object instance a value of "Jim Morrison" again. Next, you created another pointer p2 in the Stack, and pointed it at the same address on the managed heap as that referenced by p1 (when you made the assignement p2 = p1).
Here comes the twist. When you the Assign the Name property of p2 the value "Janis Joplin" you are changing the Name property for the object REFERENCED by Both p1 and p2, such that, if you ran the following code:
MsgBox(P1.Name)
'Will return "Janis Joplin"
MsgBox(p2.Name)
'will ALSO return "Janis Joplin"Because both variables (Pointers on the Stack) reference the SAME OBJECT in memory (an Address on the Managed Heap).
Did that make sense?
Last. If you do THIS:
DIm p1 As New clsPerson
Dim p2 As New clsPerson
p1.Name = "Jim Morrison"
p2.Name = "Janis Joplin"
You now have two distinct Person Objects. However, the minute you do THIS again:
p2 = p1
You have now pointed both back to "Jim Morrison". (I am not exactly sure what happened to the Object on the Heap referenced by p2 . . . I THINK it has now gone out of scope. This is one of those areas where hopefullly someone can set me straight . . .). -EDIT: I BELIEVE this is why you would Set p2 = Nothing OR p2 = New clsPerson before making the new assignment.
Once again, if you now do THIS:
p2.Name = "Jimi Hendrix"
MsgBox(p1.Name)
MsgBox(p2.Name)
Both msgBoxes will now return "Jimi Hendrix"
This can be pretty confusing for a bit, and I will say one last time, I may have some of the details wrong.
Good Luck, and hopefully others who know better than me will come along to help clarify some of this . . .

value data type and reference data type
1) value( contain the data directly )
but
reference ( refers to the data )
2) in value( every variable has its own copy)
but
in reference (more than variable can refer to some objects)
3) in value (operation variable can`t effect on other variable )
but
in reference (variable can affect other )
4) value types are(int, bool, float)
but
reference type are (array , class objects , string )

Value Type:
Fixed memory size.
Stored in Stack memory.
Holds actual value.
Ex. int, char, bool, etc...
Reference Type:
Not fixed memory.
Stored in Heap memory.
Holds memory address of actual value.
Ex. string, array, class, etc...

"Variables that are based on value types directly contain values. Assigning one value type variable to another copies the contained value. This differs from the assignment of reference type variables, which copies a reference to the object but not the object itself." from Microsoft's library.
You can find a more complete answer here and here.

Sometimes explanations won't help especially for the beginners. You can imagine value type as data file and reference type as a shortcut to a file.
So if you copy a reference variable you only copy the link/pointer to a real data somewhere in memory. If you copy a value type, you really clone the data in memory.

This is probably wrong in esoterical ways, but, to make it simple:
Value types are values that are passed normally "by value" (so copying them). Reference types are passed "by reference" (so giving a pointer to the original value). There isn't any guarantee by the .NET ECMA standard of where these "things" are saved. You could build an implementation of .NET that is stackless, or one that is heapless (the second would be very complex, but you probably could, using fibers and many stacks)
Structs are value type (int, bool... are structs, or at least are simulated as...), classes are reference type.
Value types descend from System.ValueType. Reference type descend from System.Object.
Now.. In the end you have Value Type, "referenced objects" and references (in C++ they would be called pointers to objects. In .NET they are opaque. We don't know what they are. From our point of view they are "handles" to the object). These lasts are similar to Value Types (they are passed by copy). So an object is composed by the object (a reference type) and zero or more references to it (that are similar to value types). When there are zero references the GC will probably collect it.
In general (in the "default" implementation of .NET), Value type can go on the stack (if they are local fields) or on the heap (if they are fields of a class, if they are variables in an iterator function, if they are variables referenced by a closure, if they are variable in an async function (using the newer Async CTP)...). Referenced value can only go to the heap. References use the same rules as Value types.
In the cases of Value Type that go on the heap because they are in an iterator function, an async function, or are referenced by a closure, if you watch the compiled file you'll see that the compiler created a class to put these variables, and the class is built when you call the function.
Now, I don't know how to write long things, and I have better things to do in my life. If you want a "precise" "academic" "correct" version, read THIS:
http://blogs.msdn.com/b/ericlippert/archive/2010/09/30/the-truth-about-value-types.aspx
It's 15 minutes I'm looking for it! It's better than the msdn versions, because it's a condensed "ready to use" article.

The simplest way to think of reference types is to consider them as being "object-IDs"; the only things one can do with an object ID are create one, copy one, inquire or manipulate the type of one, or compare two for equality. An attempt to do anything else with an object-ID will be regarded as shorthand for doing the indicated action with the object referred to by that id.
Suppose I have two variables X and Y of type Car--a reference type. Y happens to hold "object ID #19531". If I say "X=Y", that will cause X to hold "object ID #19531". Note that neither X nor Y holds a car. The car, otherwise known as "object ID #19531", is stored elsewhere. When I copied Y into X, all I did was copy the ID number. Now suppose I say X.Color=Colors.Blue. Such a statement will be regarded as an instruction to go find "object ID#19531" and paint it blue. Note that even though X and Y now refer to a blue car rather than a yellow one, the statement doesn't actually affect X or Y, because both still refer to "object ID #19531", which is still the same car as it always has been.

Variable types and Reference Value are easy to apply and well applied to the domain model, facilitate the development process.
To remove any myth around the amount of "value type", I will comment on how this is handled on the platform. NET, specifically in C # (CSharp) when called APIS and send parameters by value, by reference, in our methods, and functions and how to make the correct treatment of the passages of these values​​.
Read this article Variable Type Value and Reference in C #

Suppose v is a value-type expression/variable, and r is a reference-type expression/variable
x = v
update(v) //x will not change value. x stores the old value of v
x = r
update(r) //x now refers to the updated r. x only stored a link to r,
//and r can change but the link to it doesn't .
So, a value-type variable stores the actual value (5, or "h"). A reference-type varaible only stores a link to a metaphorical box where the value is.

Before explaining the different data types available in C#, it's important to mention that C# is a strongly-typed language. This means that each variable, constant, input parameter, return type and in general every expression that evaluates to a value, has a type.
Each type contains information that will be embedded by the compiler into the executable file as metadata which will be used by the common language runtime (CLR) to guarantee type safety when it allocates and reclaims memory.
If you wanna know how much memory a specific type allocates, you can use the sizeof operator as follows:
static void Main()
{
var size = sizeof(int);
Console.WriteLine($"int size:{size}");
size = sizeof(bool);
Console.WriteLine($"bool size:{size}");
size = sizeof(double);
Console.WriteLine($"double size:{size}");
size = sizeof(char);
Console.WriteLine($"char size:{size}");
}
The output will show the number of bytes allocated by each variable.
int size:4
bool size:1
double size:8
char size:2
The information related to each type are:
The required storage space.
The maximum and minimum values. For example, the type Int32 accepts values between 2147483648 and 2147483647.
The base type it inherits from.
The location where the memory for variables will be allocated at run time.
The kinds of operations that are permitted.
The members (methods, fields, events, etc.) contained by the type. For example, if we check the definition of type int, we will find the following struct and members:
namespace System
{
[ComVisible(true)]
public struct Int32 : IComparable, IFormattable, IConvertible, IComparable<Int32>, IEquatable<Int32>
{
public const Int32 MaxValue = 2147483647;
public const Int32 MinValue = -2147483648;
public static Int32 Parse(string s, NumberStyles style, IFormatProvider provider);
...
}
}
Memory management
When multiple processes are running on an operating system and the amount of RAM isn't enough to hold it all, the operating system maps parts of the hard disk with the RAM and starts storing data in the hard disk. The operating system will use than specific tables where virtual addresses are mapped to their correspondent physical addresses to perform the request. This capability to manage the memory is called virtual memory.
In each process, the virtual memory available is organized in the following 6 sections but for the relevance of this topic, we will focus only on the stack and the heap.
Stack
The stack is a LIFO (last in, first out) data structure, with a size-dependent on the operating system (by default, for ARM, x86 and x64 machines Windows's reserve 1MB, while Linux reserve from 2MB to 8MB depending on the version).
This section of memory is automatically managed by the CPU. Every time a function declares a new variable, the compiler allocates a new memory block as big as its size on the stack, and when the function is over, the memory block for the variable is deallocated.
Heap
This region of memory isn't managed automatically by the CPU and its size is bigger than the stack. When the new keyword is invoked, the compiler starts looking for the first free memory block that fits the size of the request. and when it finds it, it is marked as reserved by using the built-in C function malloc() and a return the pointer to that location. It's also possible to deallocate a block of memory by using the built-in C function free(). This mechanism causes memory fragmentation and has to use pointers to access the right block of memory, it's slower than the stack to perform the read/write operations.
Custom and Built-in types
While C# provides a standard set of built-in types representing integers, boolean, text characters, and so on, You can use constructs like struct, class, interface, and enum to create your own types.
An example of custom type using the struct construct is:
struct Point
{
public int X;
public int Y;
};
Value and reference types
We can categorize the C# type into the following categories:
Value types
Reference types
Value types
Value types derive from the System.ValueType class and variables of this type contain their values within their memory allocation in the stack. The two categories of value types are struct and enum.
The following example shows the member of the type boolean. As you can see there is no explicit reference to System.ValueType class, this happens because this class is inherited by the struct.
namespace System
{
[ComVisible(true)]
public struct Boolean : IComparable, IConvertible, IComparable<Boolean>, IEquatable<Boolean>
{
public static readonly string TrueString;
public static readonly string FalseString;
public static Boolean Parse(string value);
...
}
}
Reference types
On the other hand, the reference types do not contain the actual data stored in a variable, but the memory address of the heap where the value is stored. The categories of reference types are classes, delegates, arrays, and interfaces.
At run time, when a reference type variable is declared, it contains the value null until an object that has been created using the keywords new is assigned to it.
The following example shows the members of the generic type List.
namespace System.Collections.Generic
{
[DebuggerDisplay("Count = {Count}")]
[DebuggerTypeProxy(typeof(Generic.Mscorlib_CollectionDebugView<>))]
[DefaultMember("Item")]
public class List<T> : IList<T>, ICollection<T>, IEnumerable<T>, IEnumerable, IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T>
{
...
public T this[int index] { get; set; }
public int Count { get; }
public int Capacity { get; set; }
public void Add(T item);
public void AddRange(IEnumerable<T> collection);
...
}
}
In case you wanna find out the memory address of a specific object, the class System.Runtime.InteropServices provides a way to access to managed objects from unmanaged memory. In the following example, we are gonna use the static method GCHandle.Alloc() to allocate a handle to a string and then the method AddrOfPinnedObject to retrieve its address.
string s1 = "Hello World";
GCHandle gch = GCHandle.Alloc(s1, GCHandleType.Pinned);
IntPtr pObj = gch.AddrOfPinnedObject();
Console.WriteLine($"Memory address:{pObj.ToString()}");
The output will be
Memory address:39723832
References
Official documentation: https://learn.microsoft.com/en-us/cpp/build/reference/stack-stack-allocations?view=vs-2019

I think these two pictures describe it the best. This is the case in languages like C#, Java, JavaScript, and Python. For C++ references mean different, and the equivalent of reference types are pointer types (That's why you see in various documents of different languages that they are used interchangeably). One of the important things is the meaning of "Pass by Value" and "Pass by Reference". I think there are other questions about them on StackOverflow you can seek for.

There are many little details of the differences between value types and reference types that are stated explicitly by the standard and some of them are not easy to understand, especially for beginners.
See ECMA standard 33, Common Language Infrastructure (CLI). The CLI is also standardized by the ISO. I would provide a reference but for ECMA we must download a PDF and that link depends on the version number. ISO standards cost money.
One difference is that value types can be boxed but reference types generally cannot be. There are exceptions but they are quite technical.
Value types cannot have parameter-less instance constructors or finalizers and they cannot refer to themselves. Referring to themselves means for example that if there is a value type Node then a member of Node cannot be a Node. I think there are other requirements/limitations in the specifications but if so then they are not gathered together in one place.

Does just declaring a variable of a simple type allocates memory?

I read this question in C, and I have the same question in C#.
Is it correct to say that by the code
int a;
four unassigned bytes is reserved for a before it is initialized? Or initialization is where the bytes are allocated for the first time?

An int declaration without assignment will (as mentioned in the comments) will be optimized away by the compiler- it will not show up at all in the program.
Assuming you are in a function (as opposed to a type definition) and assign the int, the allocation will take place from stack space when the function begins (that is, the int is allocated from memory already dedicated to that thread, and will not result in a OS allocation).
On the other hand, if the int is part of a type definition (and is used) then it will increase the allocated space of each instance of that type, wherever that type ends up being allocated (heap for classes, heap or stack for structs depending on usage).
In no situation will it result in a pointer or reference for the int itself.
Some further explanation:
Th original question refers to an int by example. An int is a fixed-length (4-byte) data structure, so the compiler notes that the method containing the declaration in question will be needing 4 bytes to store it. Rather than allocating those 4 bytes on the heap (where it puts reference types) and incurring garbage collection overhead, the compiler will reserve 4 bytes in the area of memory reserved for the current thread's method local variables (the call stack). That memory had been allocated from the OS when the thread started and it is reserved for that thread alone. If the thread runs out of that space, it's called a stack overflow.
It's worth noting here that .NET really has 2 compilers- the C# compiler that converts C# code into IL, and the JIT compiler that converts IL into machine instructions at runtime. In my answer when I say "the compiler", I am doing some hand-waving about which compiler I mean exactly, but the result is the same.
As per the comments, if I do this...
void Foo() {
{
int a = 5;
Console.WriteLine(a);
}
{
int a = 7;
Console.WriteLine(a);
}
}
... then it is possible that the compiler could re-use the stack space allocated for the first variable a for the second, as they are semantically different. But that's an optimization.
It's also worth noting that the call stack includes other information besides method-local variables- it includes the parameters to the method, space for the return value (a pointer if the function returns a reference type), and the return address as well.
Finally, I'd add that in C# your methods may be inlined by the JIT compiler- this means that the code of a called method may be copied wholesale into the body of the caller to avoid the overhead of a method call. In that case, the stack frame will include space for the called method's local variables as well.

An int in C# is always 4 bytes (no matter what the value).
Remember that
int a;
is the same as
int a = 0;
When declaring an int, it is given the default value of 0.

Well, It depends on the variable type.
Since int is a ValueType it allocates all 4 bytes needed.
There are two main categories of variables in c# : Reference Types and Value Types.In case of value Types such as int all the memory needed for the variable allocates but when you declare a variable of a reference type before using new operator it just allocates a pointer and after creating the main object it allocates the space needed for storing the variable.
Class MyType
{
...
}
MyType s; // Just a pointer
s=new MyType(); //Now allocating happens.
For more information on Reference Types see MSDN Reference Types.
For more information on Value Types see MSDN Value Types.
MSDN :
There are two kinds of types in C#: reference types and value types.
Variables of reference types store references to their data (objects),
while variables of value types directly contain their data. With
reference types, two variables can reference the same object;
therefore, operations on one variable can affect the object referenced
by the other variable. With value types, each variable has its own
copy of the data, and it is not possible for operations on one
variable to affect the other (except in the case of ref and out
parameter variables, see ref (C# Reference) and out parameter modifier
(C# Reference)).

A simple declaration of a variable like in your example will contain a reference, until you create an object with new keyword.

I really don't understand the reference of C#

English is not my mother tongue, exactly I am a Chinese. I'll be sorry if I cannot express my idea clearly.
I used to programme with c++. I really dont's understand the reference of C#.Many people tell me
the reference is similar to the pointer in c++.Such as below
Person Foo = new Person(); // C#
Person *Foo = new Person(); // C++.
Many times it works well.While many C# coders also tell me that the instance of class is just
the Foo.Foo has name.whiched is called Foo.name.Foo has age,which is called Foo.age. It seems the
thought is more easy to be used to code.But this thought does not conform to Memory model .
Please tell me What the reference is in c#.a object,or more likely a pointer?Thx.

You will want to get familiar with the different between Reference Type and Value Type.
Example is below:
void Main()
{
Foo A = new Foo{Age = 16}; //A creates a space in memory for the data of Foo and points to that memory location
Foo B = A; //B just creates a variable and points to the memory location created by A
A.Bar();
B.Bar();
//Result
//Your Age is 16
//Your Age is 16
A.Age = 24;
A.Bar();
B.Bar();
//Result
//Your Age is 24
//Your Age is 24 <----- B is pointing to the same location as A and reflects the changes on call.
FooValueType E = new FooValueType{Age = 23};
FooValueType F = E;
E.Bar();
F.Bar();
//Result
//Your Age is 23
//Your Age is 23
E.Age = 56;
E.Bar();
F.Bar();
//Result
//Your Age is 56
//Your Age is 23
}
public class Foo //These object are by reference
{
public int Age { get; set; }
public void Bar()
{
Console.WriteLine("Your Age is {0}", Age);
}
}
public struct FooValueType //These object are by value
{
public int Age { get; set; }
public void Bar()
{
Console.WriteLine("Your Age is {0}", Age);
}
}

C# has garbage collection and by default, it does not support pointers. You can however, use pointers as outlined here http://msdn.microsoft.com/en-us/library/t2yzs44b(v=VS.80).aspx

With respect to the variable behaviors, there are two kinds of types in C#:
Value types.
Reference types.
Value type variables carry the actual object as their value. Assigning a value type variable assigned from another value type variable copies the whole object. In C++ terms, this is like using a variable of the class type (not a pointer to the class) like for example when you're making a stack allocated instance. C# however doesn't give you that choice, but restricts this behavior only to such types as structs.
Reference type variables carry a reference to the actual object as their value. A reference to the actual object will most probably at least in involve the memory address of the object (but the details are hidden from you). Assigning a reference type variable from another reference type variable copies the reference, not the object, so now the two variables point to the same object. In C++ terms, this is like having a pointer variable of the class. C# however defaults to this behavior for variables of such types as classes and interfaces (and there's no way to jump back to the other behavior, as far as I know). The major difference between C# references and C++ pointers is that C++ pointers support arithmetic, while C# references don't. This is one of the ways C# ensures what we call memory safety. (C# does offer pointers, but they are used rarely and a lot of the time they are useful in interoperability scenarios. It'll likely be a long time before you ever need to use a pointer in C#.)
In the end, you can think that the C# reference is close to the C++ pointer (but, as outlined above, there are differences because the languages behave differently and give you different degrees of control and protection).

In C# when you say Person Foo = new Person();
new Person() creates an object of the class Person by allocating memory in heap section
returns a reference to this object (i.e nothing but reference to memory allocated) of type Person.
While Foo catches this reference and with this reference you can access the objects properties i.e its methods and variables.
In C++,
It follows the same steps but in c++ you catch a reference to object in a pointer of type Person.
More specifically new internally calls operator new function to allocate a memory dynamically to create objects.
In the image shown below Person p (on stack) creates an reference to an object returned by new Person().
age and name are the member variables of the Object.So we can easily access them using p.name and p.age.
More to add about stack and heap memory allocation in both c# and c++
The reference Foo always gets a memory on stack
Where object i.e here new Person() gets memory on heap and in case of C# more specifically it gets memory on managed heap
Updates
When you say new Person() it internally allocates memory for the Object Of the type -person.This object is composed of Persons members like shown in image.
You then can do two thing
Either you catch this reference in variable of type Person
e.g Person p = new Person();
Here p is a reference to memory location of object.So I can easily access Objects member using p.
Just don't store the reference directly use it e.g new Person().age will give me Persons age.This is called an anonymous Object.

Have you googled that before asking? There are many top resources on the internet and also some nice e-books.
However, here´s the memory model used in C# (in Java and PHP nearly as well):
Every object is inherited from the class object. That means, that nearly everything is an object in C#.
You don´t need to use pointer anymore. There´s nearly no need in C# for it. When you create an object, you will get back an reference to the instance in the stack. You could also have many references to one single object in stack. The references are located in the heap. This is the default behaviour and you aren´t able to change it, unless you do native code.
There´s no need of deleting something. It will be automatically happened by the Garbage Collector. It will be delete every local variable which is created inside a method at the end of it. The Garbage Collector will also delete objects, which got no reference anymore.
Internal datatypes are always located at the stack, strings also. For strings, they made an exception here, strings are also treated as simple datatypes, not as an object.
Every internal datatype will be copied per default, if you e. g. have give an int value to a method. This pattern is called "Call-By-Value". Every object will be passed by a new reference. That means, if you give an object as a method paremter, you will just get inside the method a new reference to the object. This pattern is called "Call-By-Reference". It´s very important, that you might divide in them.
You can change Call-By-Value to Call-By-Reference using the "ref" keyword. You could also define only an out parameter. That means, that doesn´t matter what value you give to a method. It will only return something at the end of the method. This can be done by using the "out" keyword in C#.
In logical conclusion to this, you are not able to overflow the heap in C#. Because the stack would get an overflow before...
There´s no difference anymore between static and object-calls. It´s always the dot. So, you could call public members of a class in the same way you would also call public static members of the class. For static members, you just need to use the class name, or inside the class without any keyword before the variable name. You call object/class member by using a reference to it.
You could only inherit from one class. Multiple inheritance is not supported it C# (Java and PHP as well).
Stdout commands (>> and >) changes to "Console" or to any other stream reference, e. g. file output streams.
Edit:
A child class inherits everything from the base class, also private and protected members/methods. But you can only access public and protected members/methods, unless you dont´ make use of reflections.
Hope, this helps you in order to unterstand C# a bit more...

The reference is a pointer to an object.
You can use "real" pointers in C# in unsafe code: here is an example

How do you store an int or other "C# value types" on the heap (with C#)?

I'm engaged in educating myself about C# via Troelsen's Pro C# book.
I'm familiar with the stack and heap and how C# stores these sorts of things. In C++, whenever we use new we receive a pointer to something on the heap. However, in C# the behavior of new seems different to me:
when used with value types like an int, using new seems to merely call the int default constructor yet the value of such an int would still be stored on the stack
I understand that all objects/structs and such are stored on the heap, regardless of whether or not new is used.
So my question is: how can I instantiate an int on the heap? (And does this have something to do with 'boxing'?)

You can box any value type to System.Object type so it will be stored on the managed heap:
int number = 1;
object locatedOnTheHeap = number;
An other question is why you need this.
This is a classic example from the must-know MSDN paper: Boxing and Unboxing (C# Programming Guide)
When the CLR boxes a value type, it wraps the value inside a
System.Object and stores it on the managed heap.
Boxing is used to store value types in the garbage-collected heap.
Boxing is an implicit conversion of a value type to the type object or
to any interface type implemented by this value type. Boxing a value
type allocates an object instance on the heap and copies the value
into the new object.
.
I understand that all objects/structs and such are stored on the heap
BTW, IIRC sometimes JIT optimizes code so value type objects like of type like int are stored in the CPU registers rather than stack.

I do not know why you would want to do this, however, in theory you could indeed box your value. You would do this by boxing the int into an object (which is a reference type and will be placed on the stack:
object IAmARefSoIWillBeOnHeap = (object)1;
*As sll stated, you do not need the (object) as it will be an implicit cast. This is here merely for academic reasons, to show what is happening.
Here is a good article about reference versus value types, which is the difference of the heap versus the stack

A value type is "allocated" wherever it is declared:
As a local variable, typically on the stack (but to paraphrase Eric Lippert, the stack is an implementation detail, I suggest you read his excellent piece on his blog: The Truth About Value Types.)
As a field in a class, it expands the size of the instance with the size of the value type, and takes up space inside the instance
As such, this code:
var x = new SomeValueType();
does not allocate something on the heap by itself for that value type. If you close over it with an anonymous method or similar, the local variable will be transformed into the field of a class, and an instance of that class will be allocated on the heap, but in this case, the value type will be embedded into that class as a field.
The heap is for instances of reference types.
However, you've touched up something regarding boxing. You can box a value type value to make a copy of it and place that copy on the heap, wrapped in an object.
So this:
object x = new SomeValueType();
would first allocate the value type, then box it into an object, and store the reference to that object in x.

yet the value of such an int would still be stored on the stack
This is not necessarily true. Even when it is true, it's purely an implementation detail, and not part of the specification of the language. The main issue is that the type system does not necessarily correlate to the storage mechanism used by the runtime.
There are many cases where calling new on a struct will still result in an object that isn't on the stack. Boxing is a good example - when you box an object, you're basically pushing it into an object (effectively "copying" it to the heap), and referencing the object. Also, any time you're closing over a value type with a lambda, you'll end up "allocating on the heap."
That being said, I wouldn't focus on this at all - the issue really shouldn't about stack vs. heap in allocations, but rather about value type vs. reference type semantics and usage. As such, I'd strongly recommend reading Eric Lippert's The Truth About Value Types and Jon Skeet's References and Values. Both of these articles focus on the important aspects of struct vs. class semantics instead of necessarily looking at the storage.
As for ways to force the storage of an int on the heap, here are a couple of simple ones:
object one = 1; // Boxing
int two = 2; // Gets closed over, so ends up "on the heap"
Action closeOverTwo = () => { Console.WriteLine(two); }
// Do stuff with two here...
var three = new { Three = 3 }; // Wrap in a value type...

If you want an int on the heap, you can do this:
object o = 4;
But basically, you shouldn't want that. C# is designed for you not to think about such things. Here's a good place to start on that: http://blogs.msdn.com/b/ericlippert/archive/2009/04/27/the-stack-is-an-implementation-detail.aspx

So my question is: how can I instantiate an int on the heap? (And does
this have something to do with 'boxing'?)
Your understanding about objects and structs are correct. When you intialized either an object or a structure it goes on the heap.

What is the difference between a reference type and value type in c#?

Some guy asked me this question couple of months ago and I couldn't explain it in detail. What is the difference between a reference type and a value type in C#?
I know that value types are int, bool, float, etc and reference types are delegate, interface, etc. Or is this wrong, too?
Can you explain it to me in a professional way?

Your examples are a little odd because while int, bool and float are specific types, interfaces and delegates are kinds of type - just like struct and enum are kinds of value types.
I've written an explanation of reference types and value types in this article. I'd be happy to expand on any bits which you find confusing.
The "TL;DR" version is to think of what the value of a variable/expression of a particular type is. For a value type, the value is the information itself. For a reference type, the value is a reference which may be null or may be a way of navigating to an object containing the information.
For example, think of a variable as like a piece of paper. It could have the value "5" or "false" written on it, but it couldn't have my house... it would have to have directions to my house. Those directions are the equivalent of a reference. In particular, two people could have different pieces of paper containing the same directions to my house - and if one person followed those directions and painted my house red, then the second person would see that change too. If they both just had separate pictures of my house on the paper, then one person colouring their paper wouldn't change the other person's paper at all.

Value type:
Holds some value not memory addresses
Example:
Struct
Storage:
TL;DR: A variable's value is stored wherever it is decleared. Local variables live on the stack for example, but when declared inside a class as a member it lives on the heap tightly coupled with the class it is declared in.
Longer: Thus value types are stored wherever they are declared.
E.g.: an int's value inside a function as a local variable would be stored on the stack, whilst an in int's value declared as member in a class would be stored on the heap with the class it is declared in. A value type on a class has a lifetype that is exactly the same as the class it is declared in, requiring almost no work by the garbage collector. It's more complicated though, i'd refer to #JonSkeet's book "C# In Depth" or his article "Memory in .NET" for a more concise explenation.
Advantages:
A value type does not need extra garbage collection. It gets garbage collected together with the instance it lives in. Local variables in methods get cleaned up upon method leave.
Drawbacks:
When large set of values are passed to a method the receiving variable actually copies so there are two redundant values in memory.
As classes are missed out.it losses all the oop benifits
Reference type:
Holds a memory address of a value not value
Example:
Class
Storage:
Stored on heap
Advantages:
When you pass a reference variable to a method and it changes it indeed changes the original value whereas in value types a copy of the given variable is taken and that's value is changed.
When the size of variable is bigger reference type is good
As classes come as a reference type variables, they give reusability, thus benefitting Object-oriented programming
Drawbacks:
More work referencing when allocating and dereferences when reading the value.extra overload for garbage collector

I found it easier to understand the difference of the two if you know how computer allocate stuffs in memory and know what a pointer is.
Reference is usually associated with a pointer. Meaning the memory address where your variable reside is actually holding another memory address of the actual object in a different memory location.
The example I am about to give is grossly over simplified, so take it with a grain of salt.
Imagine computer memory is a bunch of PO boxes in a row (starting w/ PO Box 0001 to PO Box n) that can hold something inside it. If PO boxes doesn't do it for you, try a hashtable or dictionary or an array or something similar.
Thus, when you do something like:
var a = "Hello";
the computer will do the following:
allocate memory (say starting at memory location 1000 for 5 bytes) and put H (at 1000), e (at 1001), l (at 1002), l (at 1003) and o (at 1004).
allocate somewhere in memory (say at location 0500) and assigned it as the variable a.
So it's kind of like an alias (0500 is a).
assign the value at that memory location (0500) to 1000 (which is where the string Hello start in memory). Thus the variable a is holding a reference to the actual starting memory location of the "Hello" string.
Value type will hold the actual thing in its memory location.
Thus, when you do something like:
var a = 1;
the computer will do the following:
allocate a memory location say at 0500 and assign it to variable a (the same alias thing)
put the value 1 in it (at memory location 0500).
Notice that we are not allocating extra memory to hold the actual value (1).
Thus a is actually holding the actual value and that's why it's called value type.

This is from a post of mine from a different forum, about two years ago. While the language is vb.net (as opposed to C#), the Value Type vs. Reference type concepts are uniform throughout .net, and the examples still hold.
It is also important to remember that within .net, ALL types technically derive from the base type Object. The value types are designed to behave as such, but in the end they also inherit the functionality of base type Object.
A. Value Types are just that- they represent a distinct area in memory where a discrete VALUE is stored. Value types are of fixed memory size and are stored in the stack, which is a collection of addresses of fixed size.
When you make a statement like such:
Dim A as Integer
DIm B as Integer
A = 3
B = A
You have done the following:
Created 2 spaces in memory sufficient to hold 32 bit integer values.
Placed a value of 3 in the memory allocation assigned to A
Placed a value of 3 in the memory allocation assigned to B by assigning it the same value as the held in A.
The Value of each variable exists discretely in each memory location.
B. Reference Types can be of various sizes. Therefore, they can't be stored in the "Stack" (remember, the stack is a collection of memory allocations of fixed size?). They are stored in the "Managed Heap". Pointers (or "references") to each item on the managed heap are maintained in the stack (Like an Address). Your code uses these pointers in the stack to access objects stored in the managed heap. So when your code uses a reference variable, it is actually using a pointer (or "address" to an memory location in the managed heap).
Say you have created a Class named clsPerson, with a string Property Person.Name
In this case, when you make a statement such as this:
Dim p1 As clsPerson
p1 = New clsPerson
p1.Name = "Jim Morrison"
Dim p2 As Person
p2 = p1
In the case above, the p1.Name Property will Return "Jim Morrison", as you would expect. The p2.Name property will ALSO return "Jim Morrison", as you would Iintuitively expect. I believe that both p1 and p2 represent distinct addresses on the Stack. However, now that you have assigned p2 the value of p1, both p1 and p2 point to the SAME LOCATION on the managed heap.
Now COnsider THIS situation:
Dim p1 As clsPerson
Dim p2 As clsPerson
p1 = New clsPerson
p1.Name = "Jim Morrison"
p2 = p1
p2.Name = "Janis Joplin"
In this situation, You have created one new instance of the person Class on the Managed Heap with a pointer p1 on the Stack which references the object, and assigned the Name Property of the object instance a value of "Jim Morrison" again. Next, you created another pointer p2 in the Stack, and pointed it at the same address on the managed heap as that referenced by p1 (when you made the assignement p2 = p1).
Here comes the twist. When you the Assign the Name property of p2 the value "Janis Joplin" you are changing the Name property for the object REFERENCED by Both p1 and p2, such that, if you ran the following code:
MsgBox(P1.Name)
'Will return "Janis Joplin"
MsgBox(p2.Name)
'will ALSO return "Janis Joplin"Because both variables (Pointers on the Stack) reference the SAME OBJECT in memory (an Address on the Managed Heap).
Did that make sense?
Last. If you do THIS:
DIm p1 As New clsPerson
Dim p2 As New clsPerson
p1.Name = "Jim Morrison"
p2.Name = "Janis Joplin"
You now have two distinct Person Objects. However, the minute you do THIS again:
p2 = p1
You have now pointed both back to "Jim Morrison". (I am not exactly sure what happened to the Object on the Heap referenced by p2 . . . I THINK it has now gone out of scope. This is one of those areas where hopefullly someone can set me straight . . .). -EDIT: I BELIEVE this is why you would Set p2 = Nothing OR p2 = New clsPerson before making the new assignment.
Once again, if you now do THIS:
p2.Name = "Jimi Hendrix"
MsgBox(p1.Name)
MsgBox(p2.Name)
Both msgBoxes will now return "Jimi Hendrix"
This can be pretty confusing for a bit, and I will say one last time, I may have some of the details wrong.
Good Luck, and hopefully others who know better than me will come along to help clarify some of this . . .

value data type and reference data type
1) value( contain the data directly )
but
reference ( refers to the data )
2) in value( every variable has its own copy)
but
in reference (more than variable can refer to some objects)
3) in value (operation variable can`t effect on other variable )
but
in reference (variable can affect other )
4) value types are(int, bool, float)
but
reference type are (array , class objects , string )

Value Type:
Fixed memory size.
Stored in Stack memory.
Holds actual value.
Ex. int, char, bool, etc...
Reference Type:
Not fixed memory.
Stored in Heap memory.
Holds memory address of actual value.
Ex. string, array, class, etc...

"Variables that are based on value types directly contain values. Assigning one value type variable to another copies the contained value. This differs from the assignment of reference type variables, which copies a reference to the object but not the object itself." from Microsoft's library.
You can find a more complete answer here and here.

Sometimes explanations won't help especially for the beginners. You can imagine value type as data file and reference type as a shortcut to a file.
So if you copy a reference variable you only copy the link/pointer to a real data somewhere in memory. If you copy a value type, you really clone the data in memory.

This is probably wrong in esoterical ways, but, to make it simple:
Value types are values that are passed normally "by value" (so copying them). Reference types are passed "by reference" (so giving a pointer to the original value). There isn't any guarantee by the .NET ECMA standard of where these "things" are saved. You could build an implementation of .NET that is stackless, or one that is heapless (the second would be very complex, but you probably could, using fibers and many stacks)
Structs are value type (int, bool... are structs, or at least are simulated as...), classes are reference type.
Value types descend from System.ValueType. Reference type descend from System.Object.
Now.. In the end you have Value Type, "referenced objects" and references (in C++ they would be called pointers to objects. In .NET they are opaque. We don't know what they are. From our point of view they are "handles" to the object). These lasts are similar to Value Types (they are passed by copy). So an object is composed by the object (a reference type) and zero or more references to it (that are similar to value types). When there are zero references the GC will probably collect it.
In general (in the "default" implementation of .NET), Value type can go on the stack (if they are local fields) or on the heap (if they are fields of a class, if they are variables in an iterator function, if they are variables referenced by a closure, if they are variable in an async function (using the newer Async CTP)...). Referenced value can only go to the heap. References use the same rules as Value types.
In the cases of Value Type that go on the heap because they are in an iterator function, an async function, or are referenced by a closure, if you watch the compiled file you'll see that the compiler created a class to put these variables, and the class is built when you call the function.
Now, I don't know how to write long things, and I have better things to do in my life. If you want a "precise" "academic" "correct" version, read THIS:
http://blogs.msdn.com/b/ericlippert/archive/2010/09/30/the-truth-about-value-types.aspx
It's 15 minutes I'm looking for it! It's better than the msdn versions, because it's a condensed "ready to use" article.

The simplest way to think of reference types is to consider them as being "object-IDs"; the only things one can do with an object ID are create one, copy one, inquire or manipulate the type of one, or compare two for equality. An attempt to do anything else with an object-ID will be regarded as shorthand for doing the indicated action with the object referred to by that id.
Suppose I have two variables X and Y of type Car--a reference type. Y happens to hold "object ID #19531". If I say "X=Y", that will cause X to hold "object ID #19531". Note that neither X nor Y holds a car. The car, otherwise known as "object ID #19531", is stored elsewhere. When I copied Y into X, all I did was copy the ID number. Now suppose I say X.Color=Colors.Blue. Such a statement will be regarded as an instruction to go find "object ID#19531" and paint it blue. Note that even though X and Y now refer to a blue car rather than a yellow one, the statement doesn't actually affect X or Y, because both still refer to "object ID #19531", which is still the same car as it always has been.

Variable types and Reference Value are easy to apply and well applied to the domain model, facilitate the development process.
To remove any myth around the amount of "value type", I will comment on how this is handled on the platform. NET, specifically in C # (CSharp) when called APIS and send parameters by value, by reference, in our methods, and functions and how to make the correct treatment of the passages of these values​​.
Read this article Variable Type Value and Reference in C #

Suppose v is a value-type expression/variable, and r is a reference-type expression/variable
x = v
update(v) //x will not change value. x stores the old value of v
x = r
update(r) //x now refers to the updated r. x only stored a link to r,
//and r can change but the link to it doesn't .
So, a value-type variable stores the actual value (5, or "h"). A reference-type varaible only stores a link to a metaphorical box where the value is.

Before explaining the different data types available in C#, it's important to mention that C# is a strongly-typed language. This means that each variable, constant, input parameter, return type and in general every expression that evaluates to a value, has a type.
Each type contains information that will be embedded by the compiler into the executable file as metadata which will be used by the common language runtime (CLR) to guarantee type safety when it allocates and reclaims memory.
If you wanna know how much memory a specific type allocates, you can use the sizeof operator as follows:
static void Main()
{
var size = sizeof(int);
Console.WriteLine($"int size:{size}");
size = sizeof(bool);
Console.WriteLine($"bool size:{size}");
size = sizeof(double);
Console.WriteLine($"double size:{size}");
size = sizeof(char);
Console.WriteLine($"char size:{size}");
}
The output will show the number of bytes allocated by each variable.
int size:4
bool size:1
double size:8
char size:2
The information related to each type are:
The required storage space.
The maximum and minimum values. For example, the type Int32 accepts values between 2147483648 and 2147483647.
The base type it inherits from.
The location where the memory for variables will be allocated at run time.
The kinds of operations that are permitted.
The members (methods, fields, events, etc.) contained by the type. For example, if we check the definition of type int, we will find the following struct and members:
namespace System
{
[ComVisible(true)]
public struct Int32 : IComparable, IFormattable, IConvertible, IComparable<Int32>, IEquatable<Int32>
{
public const Int32 MaxValue = 2147483647;
public const Int32 MinValue = -2147483648;
public static Int32 Parse(string s, NumberStyles style, IFormatProvider provider);
...
}
}
Memory management
When multiple processes are running on an operating system and the amount of RAM isn't enough to hold it all, the operating system maps parts of the hard disk with the RAM and starts storing data in the hard disk. The operating system will use than specific tables where virtual addresses are mapped to their correspondent physical addresses to perform the request. This capability to manage the memory is called virtual memory.
In each process, the virtual memory available is organized in the following 6 sections but for the relevance of this topic, we will focus only on the stack and the heap.
Stack
The stack is a LIFO (last in, first out) data structure, with a size-dependent on the operating system (by default, for ARM, x86 and x64 machines Windows's reserve 1MB, while Linux reserve from 2MB to 8MB depending on the version).
This section of memory is automatically managed by the CPU. Every time a function declares a new variable, the compiler allocates a new memory block as big as its size on the stack, and when the function is over, the memory block for the variable is deallocated.
Heap
This region of memory isn't managed automatically by the CPU and its size is bigger than the stack. When the new keyword is invoked, the compiler starts looking for the first free memory block that fits the size of the request. and when it finds it, it is marked as reserved by using the built-in C function malloc() and a return the pointer to that location. It's also possible to deallocate a block of memory by using the built-in C function free(). This mechanism causes memory fragmentation and has to use pointers to access the right block of memory, it's slower than the stack to perform the read/write operations.
Custom and Built-in types
While C# provides a standard set of built-in types representing integers, boolean, text characters, and so on, You can use constructs like struct, class, interface, and enum to create your own types.
An example of custom type using the struct construct is:
struct Point
{
public int X;
public int Y;
};
Value and reference types
We can categorize the C# type into the following categories:
Value types
Reference types
Value types
Value types derive from the System.ValueType class and variables of this type contain their values within their memory allocation in the stack. The two categories of value types are struct and enum.
The following example shows the member of the type boolean. As you can see there is no explicit reference to System.ValueType class, this happens because this class is inherited by the struct.
namespace System
{
[ComVisible(true)]
public struct Boolean : IComparable, IConvertible, IComparable<Boolean>, IEquatable<Boolean>
{
public static readonly string TrueString;
public static readonly string FalseString;
public static Boolean Parse(string value);
...
}
}
Reference types
On the other hand, the reference types do not contain the actual data stored in a variable, but the memory address of the heap where the value is stored. The categories of reference types are classes, delegates, arrays, and interfaces.
At run time, when a reference type variable is declared, it contains the value null until an object that has been created using the keywords new is assigned to it.
The following example shows the members of the generic type List.
namespace System.Collections.Generic
{
[DebuggerDisplay("Count = {Count}")]
[DebuggerTypeProxy(typeof(Generic.Mscorlib_CollectionDebugView<>))]
[DefaultMember("Item")]
public class List<T> : IList<T>, ICollection<T>, IEnumerable<T>, IEnumerable, IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T>
{
...
public T this[int index] { get; set; }
public int Count { get; }
public int Capacity { get; set; }
public void Add(T item);
public void AddRange(IEnumerable<T> collection);
...
}
}
In case you wanna find out the memory address of a specific object, the class System.Runtime.InteropServices provides a way to access to managed objects from unmanaged memory. In the following example, we are gonna use the static method GCHandle.Alloc() to allocate a handle to a string and then the method AddrOfPinnedObject to retrieve its address.
string s1 = "Hello World";
GCHandle gch = GCHandle.Alloc(s1, GCHandleType.Pinned);
IntPtr pObj = gch.AddrOfPinnedObject();
Console.WriteLine($"Memory address:{pObj.ToString()}");
The output will be
Memory address:39723832
References
Official documentation: https://learn.microsoft.com/en-us/cpp/build/reference/stack-stack-allocations?view=vs-2019

I think these two pictures describe it the best. This is the case in languages like C#, Java, JavaScript, and Python. For C++ references mean different, and the equivalent of reference types are pointer types (That's why you see in various documents of different languages that they are used interchangeably). One of the important things is the meaning of "Pass by Value" and "Pass by Reference". I think there are other questions about them on StackOverflow you can seek for.

There are many little details of the differences between value types and reference types that are stated explicitly by the standard and some of them are not easy to understand, especially for beginners.
See ECMA standard 33, Common Language Infrastructure (CLI). The CLI is also standardized by the ISO. I would provide a reference but for ECMA we must download a PDF and that link depends on the version number. ISO standards cost money.
One difference is that value types can be boxed but reference types generally cannot be. There are exceptions but they are quite technical.
Value types cannot have parameter-less instance constructors or finalizers and they cannot refer to themselves. Referring to themselves means for example that if there is a value type Node then a member of Node cannot be a Node. I think there are other requirements/limitations in the specifications but if so then they are not gathered together in one place.