C# ref keyword usage

Question

I understand (or at least I believe I do) what it means to pass an instance of a class to a method by ref versus not passing by ref. When or under

Answer 1

Put succinctly, you would pass a value as a ref parameter if you want the function you're calling to be able to alter the value of that variable.

This is not the same as passing a reference type as a parameter to a function. In those cases, you're still passing by value, but the value is a reference. In the case of passing by ref, then an actual reference to the variable is sent; essentially, you and the function you're calling "share" the same variable.

Consider the following:

public void Foo(ref int bar)
{
    bar = 5;
}

...

int baz = 2;

Foo(ref baz);

In this case, the baz variable has a value of 5, since it was passed by reference. The semantics are entirely clear for value types, but not as clear for reference types.

public class MyClass
{
    public int PropName { get; set; }
}

public void Foo(MyClass bar)
{
    bar.PropName = 5;
}

...

MyClass baz = new MyClass();

baz.PropName = 2;

Foo(baz);

As expected, baz.PropName will be 5, since MyClass is a reference type. But let's do this:

public void Foo(MyClass bar)
{
    bar = new MyClass();

    bar.PropName = 5;
}

With the same calling code, baz.PropName will remain 2. This is because even though MyClass is a reference type, Foo has its own variable for bar; bar and baz just start out with the same value, but once Foo assigns a new value, they are just two different variables. If, however, we do this:

public void Foo(ref MyClass bar)
{
    bar = new MyClass();

    bar.PropName = 5;
}

...

MyClass baz = new MyClass();

baz.PropName = 2;

Foo(ref baz);

We'll end up with PropName being 5, since we passed baz by reference, making the two functions "share" the same variable.