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Pass by Reference VS Static Variable

From my understanding passing a variable by reference means that the original variable that was passed into a function's parameters is changed. But also from my understanding a static variable means that the variable is changed when used as a parameter of a function. So don't both of these do the same thing?

From my understanding passing a variable by reference means that the original variable that was passed into a function's parameters is changed.
That location is passed, so yes; since the caller supplied the location, the caller can see the change.
But also from my understanding a static variable means that the variable is changed when used as a parameter of a function.
That's not what that means at all. A static field just exists as a field once per type (or per combination of generic type arguments). That's all it means. When you pass the value of a static field to a method, the value from the static field is read once and copied onto the stack, and that copy is passed to the method. The value of the static field will not be changed during the call.
Perhaps the confusion here is actually "reference types", not "pass by reference"; if a static field is actually a reference to an object, then changes to the object will be observed by all callers. But it isn't the field that changed: it is the object.

passing concurrentdictionary to another method

I'm facing some problem in passing ConcurrentDictionary to another method with an out parameter.
In main method,
Method1(1,2,dictionary);
public override int Method1(int x,int y, out ConcurrentDictionary<string,int> dictionary)
{
if(dictionary.IsEmpty)
{
do something
}
}
The error message i got is "use of unassigned out parameter dictionary". And I need to preserve the content of dictionary throughout the codes. Appreciate for the help.

What do you think "out" means?
"out" is a bit like "ref". "ref" and "out" work with .NET reference types. "ref" indicates that the method can change the object the variable references. i.e. change what piece of memory the variable is pointing to. "out" indicates that it is expected that the method will define what object the variable references.
i.e. with out parameters you must instantiate the instance of the parameter within the method
e.g
public override int Method1(int x,int y, out ConcurrentDictionary<string,int> dictionary)
{
dictionary = new ConcurrentDictionary<string,int>();
// It doesn't make sense to check if it is empty here as it will always be empty
// if(dictionary.IsEmpty)
// {

Because dictionary is an out parameter, you have to guarantee that dictionary is assigned to by the time Method1 finishes. If you don't want to change dictionary, you could just assign it to itself.

Are ILists passed by value?

Passing Value Type parameters to functions in c# is by value unless you use the ref or out keyword on the parameter. But does this also apply to Reference Types?
Specifically I have a function that takes an IList<Foo>. Will the list passed to my function be a copy of the list with copy of its contained objects? Or will modifications to the list also apply for the caller? If so - Is there a clever way I can go about passing a copy?
public void SomeFunction()
{
IList<Foo> list = new List<Foo>();
list.Add(new Foo());
DoSomethingWithCopyOfTheList(list);
..
}
public void DoSomethingWithCopyOfTheList(IList<Foo> list)
{
// Do something
}

All parameters are passed by value unless you explicitly use ref or out. However, when you pass an instance of a reference type, you pass the reference by value. I.e. the reference itself is copied, but since it is still pointing to the same instance, you can still modify the instance through this reference. I.e. the instance is not copied. The reference is.
If you want to make a copy of the list itself, List<T> has a handy constructor, that takes an IEnumerable<T>.

You're not alone; this confuses a lot of people.
Here's how I like to think of it.
A variable is a storage location.
A variable can store something of a particular type.
There are two kinds of types: value types and reference types.
The value of a variable of reference type is a reference to an object of that type.
The value of a variable of value type is an object of that type.
A formal parameter is a kind of variable.
There are three kinds of formal parameters: value parameters, ref parameters, and out parameters.
When you use a variable as an argument corresponding to a value parameter, the value of the variable is copied into the storage associated with the formal parameter. If the variable is of value type, then a copy of the value is made. If the variable is of reference type, then a copy of the reference is made, and the two variables now refer to the same object. Either way, a copy of the value of the variable is made.
When you use a variable as an argument corresponding to an out or ref parameter the parameter becomes an alias for the variable. When you say:
void M(ref int x) { ...}
...
int y = 123;
M(ref y);
what you are saying is "x and y now are the same variable". They both refer to the same storage location.
I find that much easier to comprehend than thinking about how the alias is actually implemented -- by passing the managed address of the variable to the formal parameter.
Is that clear?

The list is passed by reference, so if you modify the list in SomeFunction, you modify the list for the caller as well.
You can create a copy of a list by creating a new one:
var newList = new List<Foo>(oldList);

your list is passed by reference. If you want to pass a copy of the list you can do:
IList<Foo> clone = new List<Foo>(list);
if you add/remove elements in clone it won't modify list
but the modifications of the elements themselves will be taken into account in both lists.

When you pass reference type by value (without ref or out keywords) you may modify this reference type inside this method and all changes will reflect to callers code.
To solve your problem you may explicitly create a copy and pass this copy to your function, or you may use:
list.AsReadOnly();

When passing reference types, you pass the reference. This is an important concept.
If you pass a reference
byref, you pass the reference (pointer) directly.
byval, you pass a copy of the reference (pointer).
A reference is not the instance referenced. A reference is analagous to a pointer.
To pass a copy of the instance of a referencetype, you first must make a copy yourself and pass a reference to the copy. As such then you will not be modifying the original instance.

Why ref and out in C#?

While using keyword ref, calling code needs to initialize passed arguments, but with keyword out we need not do so.
Why don't we use out everywhere?
What is exact difference between the two?
Please give example of a situation in which we need to use ref and can't use out?

The answer is given in this MSDN article. From that post:
The two parameter passing modes
addressed by out and ref are subtly
different, however they are both very
common. The subtle difference between
these modes leads to some very common
programming errors. These include:
not assigning a value to an out
parameter in all control flow paths
not assigning a value to variable
which is used as a ref parameter
Because the C# language assigns
different definite assignment rules to
these different parameter passing
modes, these common coding errors are
caught by the compiler as being
incorrect C# code.
The crux of the decision to include
both ref and out parameter passing
modes was that allowing the compiler
to detect these common coding errors
was worth the additional complexity of
having both ref and out parameter
passing modes in the language.

out is a special form of ref where the referenced memory should not be initialized before the call.
In this case the C# compiler enforces that the out variable is assigned before the method returns and that the variable is not used before it has been assigned.
Two examples where out doesn't work but ref does:
void NoOp(out int value) // value must be assigned before method returns
{
}
void Increment(out int value) // value cannot be used before it has been assigned
{
value = value + 1;
}

None of these answers satisfied me, so here's my take on ref versus out.
My answer is a summary of the following two pages:
ref (C# Reference)
out (C# Reference)
Compare
Both the method definition and the calling method must explicitly use the ref / out keyword
Both keywords cause parameters to be passed by reference (even value types)
However, there is no boxing when a value type is passed by reference
Properties cannot be passed via out or ref, because properties are really methods
ref / out are not considered to be part of the method signature at compile time, thus methods cannot be overloaded if the only difference between them is that one of the methods takes a ref argument and the other takes an out argument
Contrast
ref
Must be initialized before it is passed
Can use to pass a value to the method
out
Does not have to be initialized before it is passed
Calling method is required to assign a value before the method returns
Can not use to pass a value to the method
Examples
Won't compile because only difference in method signatures is ref / out:
public void Add(out int i) { }
public void Add(ref int i) { }
Using ref keyword:
public void PrintNames(List<string> names)
{
int index = 0; // initialize first (#1)
foreach(string name in names)
{
IncrementIndex(ref index);
Console.WriteLine(index.ToString() + ". " + name);
}
}
public void IncrementIndex(ref int index)
{
index++; // initial value was passed in (#2)
}
Using out keyword:
public void PrintNames(List<string> names)
{
foreach(string name in names)
{
int index; // not initialized (#1)
GetIndex(out index);
Console.WriteLine(index.ToString() + ". " + name);
}
}
public void GetIndex(out int index)
{
index = IndexHelper.GetLatestIndex(); // needs to be assigned a value (#2 & #3)
}
Author's Random Remarks
In my opinion, the concept of using the out keyword is similar to using the Output enum value of ParameterDirection for declaring output parameters in ADO.NET
Arrays in C# are passed by reference, but in order for a reassignment of the array reference to affect the reference in the calling code, the ref keyword must be used
Example:
public void ReassignArray(ref int[] array)
{
array = new int[10]; // now the array in the calling code
// will point to this new object
}
For more info on reference types versus value types, see Passing Reference-Type Parameters (C# Programming Guide)

A contrived example of when you'd need to use ref and not out is as follows:
public void SquareThisNumber(ref int number)
{
number = number * number;
}
int number = 4;
SquareThisNumber(ref number);
Here we want number to be an in-out variable, so we use ref. If we had used out, the compiler would have given an error saying we initialized an out param before using it.

The ref keyword allows you to change the value of a parameter. The method being called can be an intermediate link in the calling chain. A method using the out keyword can only be used at the beginning of a calling chain.
Another advantage is that the existing value can be used in the logic of the method and still hold the return value.
In Oracle functions have explicit IN (default and what you get if you don't set a direction) IN/OUT and OUT parameters. The equivalent is normal (just the parameter), ref [parameter], and out [parameter].

The compiler knows that out variables shouldn't set before the call. This allows them be just declared before use. However it knows that it must be set before the function it's used in returns.

When we pass the value while calling the method prefixed by the out keyword, it treats it entirely different like we are not passing it to the method. Instead we are actually collecting (outing) the value of the out variable from the definition section of the method to the method out variable parameter where we are calling that method.
So out variable is the output of processing done with in the method definition, and this is the reason why we need to create it, initialize it, and modify it within the definition only.
An out variable is used when we need return multiple values from a particular method.
While in case of a ref variable we need to initialize it first as its memory location is transfered to method definition as parameter. Think what would happen if we are not initializing it before passing?

Difference between ref and out parameters in .NET [duplicate]

This question already has answers here:
What's the difference between the 'ref' and 'out' keywords?
(28 answers)
Closed 4 years ago.
What is the difference between ref and out parameters in .NET? What are the situations where one can be more useful than the other? What would be a code snippet where one can be used and another can't?

They're pretty much the same - the only difference is that a variable you pass as an out parameter doesn't need to be initialized but passing it as a ref parameter it has to be set to something.
int x;
Foo(out x); // OK
int y;
Foo(ref y); // Error: y should be initialized before calling the method
Ref parameters are for data that might be modified, out parameters are for data that's an additional output for the function (eg int.TryParse) that are already using the return value for something.

Why does C# have both 'ref' and 'out'?
The caller of a method which takes an out parameter is not required to assign to the variable passed as the out parameter prior to the call; however, the callee is required to assign to the out parameter before returning.
In contrast ref parameters are considered initially assigned by the caller. As such, the callee is not required to assign to the ref parameter before use. Ref parameters are passed both into and out of a method.
So, out means out, while ref is for in and out.
These correspond closely to the [out] and [in,out] parameters of COM interfaces, the advantages of out parameters being that callers need not pass a pre-allocated object in cases where it is not needed by the method being called - this avoids both the cost of allocation, and any cost that might be associated with marshaling (more likely with COM, but not uncommon in .NET).

ref and out both allow the called method to modify a parameter. The difference between them is what happens before you make the call.
ref means that the parameter has a value on it before going into the function. The called function can read and or change the value any time. The parameter goes in, then comes out
out means that the parameter has no official value before going into the function. The called function must initialize it. The parameter only goes out
Here's my favorite way to look at it: ref is to pass variables by reference. out is to declare a secondary return value for the function. It's like if you could write this:
// This is not C#
public (bool, string) GetWebThing(string name, ref Buffer paramBuffer);
// This is C#
public bool GetWebThing(string name, ref Buffer paramBuffer, out string actualUrl);
Here's a more detailed list of the effects of each alternative:
Before calling the method:
ref: The caller must set the value of the parameter before passing it to the called method.
out: The caller method is not required to set the value of the argument before calling the method. Most likely, you shouldn't. In fact, any current value is discarded.
During the call:
ref: The called method can read the argument at any time.
out: The called method must initialize the parameter before reading it.
Remoted calls:
ref: The current value is marshalled to the remote call. Extra performance cost.
out: Nothing is passed to the remote call. Faster.
Technically speaking, you could use always ref in place of out, but out allows you to be more precise about the meaning of the argument, and sometimes it can be a lot more efficient.

Example for OUT : Variable gets value initialized after going into the method. Later the same value is returned to the main method.
namespace outreftry
{
class outref
{
static void Main(string[] args)
{
yyy a = new yyy(); ;
// u can try giving int i=100 but is useless as that value is not passed into
// the method. Only variable goes into the method and gets changed its
// value and comes out.
int i;
a.abc(out i);
System.Console.WriteLine(i);
}
}
class yyy
{
public void abc(out int i)
{
i = 10;
}
}
}
Output:
10
===============================================
Example for Ref : Variable should be initialized before going into the method. Later same value or modified value will be returned to the main method.
namespace outreftry
{
class outref
{
static void Main(string[] args)
{
yyy a = new yyy(); ;
int i = 0;
a.abc(ref i);
System.Console.WriteLine(i);
}
}
class yyy
{
public void abc(ref int i)
{
System.Console.WriteLine(i);
i = 10;
}
}
}
Output:
0
10
=================================
Hope its clear now.

A ref variable needs to be initialized before passing it in.
An out variable needs to be set in your function implementation
out parameters can be thought of as additional return variables (not input)
ref parameters can be thought of as both input and output variables.

Ref and Out Parameters:
The out and the ref parameters are used to return values in the same variable, that you pass as an argument of a method. These both parameters are very useful when your method needs to return more than one value.
You must assigned value to out parameter in calee method body, otherwise the method won't get compiled.
Ref Parameter : It has to be initialized before passing to the Method.
The ref keyword on a method parameter causes a method to refer to the same variable that was passed as an input parameter for the same method. If you do any changes to the variable, they will be reflected in the variable.
int sampleData = 0;
sampleMethod(ref sampleData);
Ex of Ref Parameter
public static void Main()
{
int i = 3; // Variable need to be initialized
sampleMethod(ref i );
}
public static void sampleMethod(ref int sampleData)
{
sampleData++;
}
Out Parameter : It is not necessary to be initialized before passing to Method.
The out parameter can be used to return the values in the same variable passed as a parameter of the method. Any changes made to the parameter will be reflected in the variable.
int sampleData;
sampleMethod(out sampleData);
Ex of Out Parameter
public static void Main()
{
int i, j; // Variable need not be initialized
sampleMethod(out i, out j);
}
public static int sampleMethod(out int sampleData1, out int sampleData2)
{
sampleData1 = 10;
sampleData2 = 20;
return 0;
}

out:
In C#, a method can return only one value. If you would like to return more than one value, you can use the out keyword. The out modifier returns as return-by-reference. The simplest answer is that the keyword “out” is used to get the value from the method.
You don't need to initialize the value in the calling function.
You must assign the value in the called function, otherwise the compiler will report an error.
ref:
In C#, when you pass a value type such as int, float, double etc. as an argument to the method parameter, it is passed by value. Therefore, if you modify the parameter value, it does not affect argument in the method call. But if you mark the parameter with “ref” keyword, it will reflect in the actual variable.
You need to initialize the variable before you call the function.
It’s not mandatory to assign any value to the ref parameter in the method. If you don’t change the value, what is the need to mark it as “ref”?

Ref parameters aren't required to be set in the function, whereas out parameters must be bound to a value before exiting the function. Variables passed as out may also be passed to a function without being initialized.

out specifies that the parameter is an output parameters, i.e. it has no value until it is explicitly set by the method.
ref specifies that the value is a reference that has a value, and whose value you can change inside the method.

out parameters are initialized by the method called, ref parameters are initialized before calling the method. Therefore, out parameters are used when you just need to get a secondary return value, ref parameters are used to get a value and potentially return a change to that value (secondarily to the main return value).

The ref keyword is used to pass values by reference. (This does not preclude the passed values being value-types or reference types). Output parameters specified with the out keyword are for returning values from a method.
One key difference in the code is that you must set the value of an output parameter within the method. This is not the case for ref parameters.
For more details look at http://www.blackwasp.co.uk/CSharpMethodParameters.aspx

An out parameter is a ref parameter with a special Out() attribute added. If a parameter to a C# method is declared as out, the compiler will require that the parameter be written before it can be read and before the method can return. If C# calls a method whose parameter includes an Out() attribute, the compiler will, for purposes of deciding whether to report "undefined variable" errors, pretend that the variable is written immediately before calling the method. Note that because other .net languages do not attach the same meaning to the Out() attribute, it is possible that calling a routine with an out parameter will leave the variable in question unaffected. If a variable is used as an out parameter before it is definitely assigned, the C# compiler will generate code to ensure that it gets cleared at some point before it is used, but if such a variable leaves and re-enters scope, there's no guarantee that it will be cleared again.

ref will probably choke on null since it presumably expects to be modifying an existing object. out expects null, since it's returning a new object.

out and ref are exactly the same with the exception that out variables don't have to be initialized before sending it into the abyss. I'm not that smart, I cribbed that from the MSDN library :).
To be more explicit about their use, however, the meaning of the modifier is that if you change the reference of that variable in your code, out and ref will cause your calling variable to change reference as well. In the code below, the ceo variable will be a reference to the newGuy once it returns from the call to doStuff. If it weren't for ref (or out) the reference wouldn't be changed.
private void newEmployee()
{
Person ceo = Person.FindCEO();
doStuff(ref ceo);
}
private void doStuff(ref Person employee)
{
Person newGuy = new Person();
employee = newGuy;
}

This The out and ref Paramerter in C# has some good examples.
The basic difference outlined is that out parameters don't need to be initialized when passed in, while ref parameters do.

They are subtly different.
An out parameter does not need to be initialized by the callee before being passed to the method. Therefore, any method with an out parameter
Cannot read the parameter before assigning a value to it
Must assign a value to it before returning
This is used for a method which needs to overwrite its argument regardless of its previous value.
A ref parameter must be initialized by the callee before passing it to the method. Therefore, any method with a ref parameter
Can inspect the value before assigning it
Can return the original value, untouched
This is used for a method which must (e.g.) inspect its value and validate it or normalize it.

out has gotten a new more succint syntax in C#7
https://learn.microsoft.com/en-us/dotnet/articles/csharp/whats-new/csharp-7#more-expression-bodied-members
and even more exciting is the C#7 tuple enhancements that are a more elegant choice than using ref and out IMHO.