Why use the 'ref' keyword when passing an object?

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.

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; }
        }
    }

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;
}