Question

When I make an assignment to an out or ref parameter, is the value immediately assigned to the reference provided by the caller, or are the out and ref parameter values assigned to the references when the method returns? If the method throws an exception, are the values returned?

For example:

int callerOutValue = 1;
int callerRefValue = 1;
MyMethod(123456, out callerOutValue, ref callerRefValue);

bool MyMethod(int inValue, out int outValue, ref int refValue)
{
    outValue = 2;
    refValue = 2;

    throw new ArgumentException();

    // Is callerOutValue 1 or 2?
    // Is callerRefValue 1 or 2?
}

Answer 1

Since ref and out parameters allow a method to work with the actual references that the caller passed in, all changes to those references are reflected immediately to the caller when control is returned.

This means in your example above (if you were to catch the ArgumentException of course), outValue and refValue would both be set to 2.

It is also important to note that out and ref are identical concepts at an IL level – it is only the C# compiler that enforces the extra rule for out that requires that a method set its value prior to returning. So from an CLR perspective outValue and refValue have identical semantics and are treated the same way.