c# - What do ref, val and out mean on method parameters?

I'm looking for a clear, concise and accurate answer.

Ideally as the actual answer, although links to good explanations welcome.

This also applies to VB.Net, but the keywords are different - ByRef and ByVal.

Answer

By default (in C#), passing an object to a function actually passes a copy of the reference to that object. Changing the parameter itself only changes the value in the parameter, and not the variable that was specified.

void Test1(string param)
{
    param = "new value";
}

string s1 = "initial value";
Test1(s1);

// s1 == "initial value"

Using out or ref passes a reference to the variable specified in the call to the function. Any changes to the value of an out or ref parameter will be passed back to the caller.

Both out and ref behave identically except for one slight difference: ref parameters are required to be initialised before calling, while out parameters can be uninitialised. By extension, ref parameters are guaranteed to be initialised at the start of the method, while out parameters are treated as uninitialised.

void Test2(ref string param)
{
    param = "new value";

}

void Test3(out string param)
{
    // Use of param here will not compile
    param = "another value";
}

string s2 = "initial value";
string s3;

Test2(ref s2);
// s2 == "new value"
// Test2(ref s3); // Passing ref s3 will not compile
Test3(out s2);
// s2 == "another value"
Test3(out s3);
// s3 == "another value"

Edit: As dp points out, the difference between out and ref is only enforced by the C# compiler, not by the CLR. As far as I know, VB has no equivalent for out and implements ref (as ByRef) only, matching the support of the CLR.