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C# passing class member as ref parameter

I do not understand why it is possible to pass a class instance member as ref parameter to a function.
The object could be moved by garbage collection while the function is executing, invalidating the reference.
Yet this seems to be allowed and works. Does it mean "ref" parameters are more than just a native pointer under the hood?
class A
{
public int Value;
}
class Test
{
static void F(ref int value)
{
value = 7;
}
static void Main()
{
A obj = new A();
// obj can be moved by GC, so "ref obj.Value" cannot be a native pointer under the hood
F(ref obj.Value);
System.Console.WriteLine(obj.Value); // Prints 7
}
}

Does it mean "ref" parameters are more than just a native pointer under the hood?
Yes, that's exactly what it means. If it were just a pointer, they'd call it a pointer instead of a reference. Instead, for references the GC knows about the original object and correctly tracks things, so the reference stays with the object and the object won't be collected until the method exits.

Thread doesn't work with void as ThreadStart [duplicate]

Is there any way to create a non-static thread method at .NET?
Show me code please.
The following code doesn't work:
ThreadStart ts = delegate { drawFloorAround(); };
public void drawFloorAround()
{
...
}
Gives this error -> "A field initializer cannot reference the non-static field, method, or property".
If I change the method do static, it works. But I don't want to.

... gives this error "A field initializer cannot reference the non-static field, method, or property".
Read the error message more carefully. It is telling you precisely what is wrong. A field initializer cannot reference a non-static method. That's because the compiler is trying to protect you from this bug:
class C
{
int foo;
int bar = GetBar();
public C(int newFoo)
{
this.foo = newFoo;
}
private int GetBar() { return this.foo + 1; }
}
You do "new C(123)". What is bar set to? If this were legal code then it would be set to 1, not 124. Why? Because first foo gets initialized to zero, then GetBar() gets called, then the constructor body sets this.foo to 123.
To prevent this bug it is simply illegal to reference an instance method or field in a field initializer.
Now, you might reasonably point out that in your code, you do not use the instance method, you only reference it. You never actually call it. This actually is safe. However, the rules of C# are designed to be simple and conservative; even though we could prove that this case is safe, we take the conservative, simple path and say that any reference to the instance in a field initializer is illegal.
If I change the method to static, it works.
Correct. In that case, the method does not depend upon instance state which has not yet been set up.
But I don't want to.
OK, then your only other choice is to stop using a field initializer. Put the initialization in the constructor; you then take responsibility for ensuring that the initialization does not accidentally use uninitialized state.

If you mean is it possible to start a thread with a non-static method - i.e. an instance method - then yes it is. But the same rules apply as to calling an instance method directly - you can only do it if you have an instance. For example, if you have an instance in a variable called foo then you can write this:
ThreadStart ts = delegate { foo.DrawFloorAround(); };
If you don't already have an instance that you can use, then you must first create one:
ThreadStart ts = delegate { new Foo().DrawFloorAround(); };
If you don't want to create an instance then your method probably should be static.

Yes
public class DoSomthing
{
public void Do()
{
Thread t = new Thread(DoInBackground);
t.Start();
}
public void DoInBackground()
{
// ....
}
}

Edit: the problem in the example code is that it is a field initialiser. Move this code to an explicit constructor:
ThreadStart ts;
public TypeName() {//constructor
ts = this.SomeMethod;
}
private void SomeMethod() {....}
Any method can act as a ThreadStart as log ad it takes no args and returns void. IMO the easiest option is a lambda or anon method as this allows closures:
ThreadStart ts = delegate {
someObj.DoSomething(x, y, "z");
};
But for an instance method that returns void and takes no args:
var obj = /* init obj */
ThreadStart ts = obj.SomeMethod;
Then
var thread = new Thread(ts);

Initializers run before constructors, so you have no instance to set it to. Set the value in your constructor and you should be okay.
class DoesNotWork {
public Action ts = Frob; // doesn't work, cannot access non-static method
void Frob() { }
}
class ThisIsFine {
public Action ts;
public ThisIsFine() { ts = Frob; }
void Frob();
}

It's important for anyone moving to C# from vb.net to note that the rules have changed between VB.net and C#. Under the vb.net (IMHO better) rules, initializers run between the call to mybase.new and the following statement of the constructor; it is permissible for field initializers to make reference to fields and properties of the current object. While this can cause problems if done carelessly, it allows variable initialization (and in some cases cleanup) to be handled at the same place in source code as the declaration. Anyone migrating to C# needs to recognize this difference in initializer handling. While it's possible in vb.net to properly dispose an iDisposable which is created in an initializer, that is not possible in C# without a severe kludge using a threadstatic variable.

Is it meaningless to pass Dictionary<TKey,TValue> by ref? [duplicate]

If I am passing an object to a method, why should I use the ref keyword? Isn't this the default behaviour anyway?
For example:
class Program
{
static void Main(string[] args)
{
TestRef t = new TestRef();
t.Something = "Foo";
DoSomething(t);
Console.WriteLine(t.Something);
}
static public void DoSomething(TestRef t)
{
t.Something = "Bar";
}
}
public class TestRef
{
public string Something { get; set; }
}
The output is "Bar" which means that the object was passed as a reference.

Pass a ref if you want to change what the object is:
TestRef t = new TestRef();
t.Something = "Foo";
DoSomething(ref t);
void DoSomething(ref TestRef t)
{
t = new TestRef();
t.Something = "Not just a changed t, but a completely different TestRef object";
}
After calling DoSomething, t does not refer to the original new TestRef, but refers to a completely different object.
This may be useful too if you want to change the value of an immutable object, e.g. a string. You cannot change the value of a string once it has been created. But by using a ref, you could create a function that changes the string for another one that has a different value.
It is not a good idea to use ref unless it is needed. Using ref gives the method freedom to change the argument for something else, callers of the method will need to be coded to ensure they handle this possibility.
Also, when the parameter type is an object, then object variables always act as references to the object. This means that when the ref keyword is used you've got a reference to a reference. This allows you to do things as described in the example given above. But, when the parameter type is a primitive value (e.g. int), then if this parameter is assigned to within the method, the value of the argument that was passed in will be changed after the method returns:
int x = 1;
Change(ref x);
Debug.Assert(x == 5);
WillNotChange(x);
Debug.Assert(x == 5); // Note: x doesn't become 10
void Change(ref int x)
{
x = 5;
}
void WillNotChange(int x)
{
x = 10;
}

You need to distinguish between "passing a reference by value", and "passing a parameter/argument by reference".
I've written a reasonably long article on the subject to avoid having to write carefully each time this comes up on newsgroups

In .NET when you pass any parameter to a method, a copy is created. In value types means that any modification you make to the value is at the method scope, and is lost when you exit the method.
When passing a Reference Type, a copy is also made, but it is a copy of a reference, i.e. now you have TWO references in memory to the same object. So, if you use the reference to modify the object, it gets modified. But if you modify the reference itself - we must remember it is a copy - then any changes are also lost upon exiting the method.
As people have said before, an assignment is a modification of the reference, thus is lost:
public void Method1(object obj) {
obj = new Object();
}
public void Method2(object obj) {
obj = _privateObject;
}
The methods above does not modifies the original object.
A little modification of your example
using System;
class Program
{
static void Main(string[] args)
{
TestRef t = new TestRef();
t.Something = "Foo";
DoSomething(t);
Console.WriteLine(t.Something);
}
static public void DoSomething(TestRef t)
{
t = new TestRef();
t.Something = "Bar";
}
}
public class TestRef
{
private string s;
public string Something
{
get {return s;}
set { s = value; }
}
}

Since TestRef is a class (which are reference objects), you can change the contents inside t without passing it as a ref. However, if you pass t as a ref, TestRef can change what the original t refers to. i.e. make it point to a different object.

With ref you can write:
static public void DoSomething(ref TestRef t)
{
t = new TestRef();
}
And t will be changed after the method has completed.

Think of variables (e.g. foo) of reference types (e.g. List<T>) as holding object identifiers of the form "Object #24601". Suppose the statement foo = new List<int> {1,5,7,9}; causes foo to hold "Object #24601" (a list with four items). Then calling foo.Length will ask Object #24601 for its length, and it will respond 4, so foo.Length will equal 4.
If foo is passed to a method without using ref, that method might make changes to Object #24601. As a consequence of such changes, foo.Length might no longer equal 4. The method itself, however, will be unable to change foo, which will continue to hold "Object #24601".
Passing foo as a ref parameter will allow the called method to make changes not just to Object #24601, but also to foo itself. The method might create a new Object #8675309 and store a reference to that in foo. If it does so, foo would no longer hold "Object #24601", but instead "Object #8675309".
In practice, reference-type variables don't hold strings of the form "Object #8675309"; they don't even hold anything that can be meaningfully converted into a number. Even though each reference-type variable will hold some bit pattern, there is no fixed relationship between the bit patterns stored in such variables and the objects they identify. There is no way code could extract information from an object or a reference to it, and later determine whether another reference identified the same object, unless the code either held or knew of a reference that identified the original object.

This is like passing a pointer to a pointer in C. In .NET this will allow you to change what the original T refers to, personally though I think if you are doing that in .NET you have probably got a design issue!

By using the ref keyword with reference types you are effectively passing a reference to the reference. In many ways it's the same as using the out keyword but with the minor difference that there's no guarantee that the method will actually assign anything to the ref'ed parameter.

ref mimics (or behaves) as a global area just for two scopes:
Caller
Callee.

If you're passing a value, however, things are different. You can force a value to be passed by reference. This allows you to pass an integer to a method, for example, and have the method modify the integer on your behalf.

Extension methods, which is a better choice

Following is a test class
public class Test
{
public int a;
}
Following are the Extension methods I have created:
public static class Extension
{
public static void Do1(this Test t,int value)
{
t.a = t.a + value;
}
public static Test Do2(this Test t,int value)
{
t.a = t.a + value;
return t
}
}
Code Usage:
Test t = new Test();
t.a = 5;
Both the following calls lead to same result for t.a, which is 10:
t.Do1(5)
t = t.Do2(5)
There are many instances in my code where I need to implement a similar logic, which one is better, one of them is passing reference by value and internally updating it, other is returning the updated reference. Is using one of them safer, if this kind of code ever gets into multi threaded wrapper, provided all the thread safety is taken care of. Normally to update the referenced variable we need a ref or out keyword, which is like pointer to a pointer, instead of a separate pointer to same memory location as in this case, but here in extension methods, I cannot use them. Please let me know if the question needs further clarity

In your example it does not make sense to return the tvariable. t is a reference, so setting t.a updates the object already. There's no need for ref, out or returning t. One reason for returning t would be to allow you to use method chaining.
You only need ref or out if you want to actually change the reference, not the content of the reference.

You are actually misunderstanding sense of ref and out keywords. Those are used, if you want to replace whole referenced object inside your method, for simple property level update they are not needed at all.
In your example, as Test is a class (reference type), there is no actual difference between two methods, but returning initial Test object as in Do2 method is just pointless, as object was already updated. So best of two will be the first implementation:
public static class Extension
{
public static void Do1(this Test t,int value)
{
t.a = t.a + value;
}
}
Going back to Do2 method - as I said before, referenced object value is already updated inside a method, so there is even no point in assigning return value to initial variable:
t.Do2(5)
is the same as
t.Do(5)

Does assigning an object to other creates a copy?

I tried with the below code, I got the output as 1000. I heard assigning object must share the reference instead of copying the entire object memory. Here the result is different.Can anyone help.
public aaaaa ad = new aaaaa();
static void Main(string[] args)
{
Program p = new Program();
p.fun1();
p.fun2();
}
public void fun1()
{
using(smallclass s = new smallclass())
{
s.j = 1000;
ad.fun1(s);
}
}
public void fun2()
{
ad.fun2();
}
public class aaaaa
{
public smallclass h = new smallclass();
public void fun1(smallclass d)
{
h = d;
}
public void fun2()
{
Console.WriteLine(h.j);
}
}
public class smallclass:IDisposable
{
public int j = 9;
public void Dispose()
{
GC.SuppressFinalize(this);
}
}
Update:
I expect an object reference exception as the referenced memory is disposed in p.fun1();

Here is an simple example how assinging works
using System;
namespace ConsoleApplication1
{
internal class Program
{
private static smallclass objA = new smallclass();
private static smallclass objB = new smallclass();
private static void Main(string[] args)
{
showValues();
objA.value = 1000;
showValues();
objB = objA;
showValues();
objA.value = 1055;
showValues();
}
private static void showValues()
{
Console.WriteLine("objA.value: " + objA.value);
Console.WriteLine("objB.value: " + objB.value);
Console.ReadLine();
}
}
internal class smallclass : IDisposable
{
public int value = 0;
public void Dispose()
{
//Here you can remove eventHandlers
//or do some other stuff before the GC will play with it
}
}
}
Like you can see
first we create 2 objects objA and objB
than we show the values like expected they are both 0
after that we increase the value of objA to 1000
the value of objA a is 1000 and the value of objB remains at 0
NOW we assingning objA and objB
so the value of objB got also 1000
if we now change the value of objA to 1055
the value of objB get also changed
because objB is no more an separate object it now holds the same
reference like objA does
EDIT
And now i will show you how you get your Error based on your example
change your aaaaa class to:
public class aaaaa
{
public WeakReference<smallclass> h;
public void fun1(smallclass d)
{
h = new WeakReference<smallclass>(d);
}
public void fun2()
{
smallclass k;
if(h.TryGetTarget(out k))
Console.WriteLine(k.j);
else
Console.WriteLine("ERROR ERRROR ERROR");
}
}
and modify your static void Main(string[] args) to:
static void Main(string[] args)
{
Program p = new Program();
p.fun1();
GC.Collect();
p.fun2();
Console.Read();
}
Ok lets get through the changes
we are using the WeakReference<T> (you could also use WeakReference)
if the GC now comes across our object he can't find a StrongReference so can Collect it
now to the GC.Collect() YOU need to call it because it forced the GC to do his work (now at this moment)
and remember like i told you before IDisposable will get called from the GC before he destroys the object (AFAIK) so there is the place to put all the stuff that need to be done before the object will get destroyed

No, assingning is not a "new" statement, it copies.... a reference, it does not create a new object. For a class.
For a struct, it does so.
I suggest learning C# by reading the documentation or a book - those basics are normally handled to great detail in those.

You will not go far wrong if you think of every reference type variable, field, parameter, array slot, or other such storage location, has holding either "null", or "object #24601" [or some other number]. There are really only a handful things that can be done with references:
You may create a null reference
You may ask the system to create a new object and return a reference to it
You may copy one reference to another
You may check whether two references are equal to each other, or whether one is equal to null.
You may ask the system to perform some action upon the object identified by a reference
If myCar is a variable of some reference type, a statement like myCar.Color = CarColors.Blue won't affect the variable myCar at all. Instead, it will observe that myCar holds [e.g.] "Object #8675309", and then ask the system to access the Color property or field of object #8675309. Conversely, if otherCar happens to hold "object #90210", a statement of the form otherCar=myCar won't do anything with object #8675309, nor object #90210, but will instead replace the "90210" stored in otherCar with "8675309".
Objects are guaranteed to exist as long as any form of reference to them exists, but if there are two objects which, although referenced by each other, are not referenced by anything else in the universe, both objects may simultaneously cease to exist. This rule is absolute, but there are a couple of twists: code may request a WeakReference to an object; an object is guaranteed to exist as long as a weak reference to it exists, but if the system discovers that no strong references to an object exist, it will invalidate every WeakReference to it. Further, the system keeps a list of all objects that have would like to be notified if they are abandoned. If the system finds that this list holds the only reference to an object, it will move the object to a strongly-referenced list of objects whose Finalize method should run at the first convenient opportunity. When the object's Finalize method is run, the reference will be removed from that latter list. If no reference to the object has been stored anywhere in the mean time, the object will cease to exist.

I have replaced GC.SuppressFinalize with GC.Collect() in dispose function, however this is not freeing the memory.. and am receiving 1000 as a result.
I guess, as it holds other reference(the variable h), GC will not free the memory, even if we invoked it explicit.
So we can very well pass and assign the objects irrespective of the allocated(new) object going out of scope.
Please correct me If i am wrong.