How to tell if an out parameter was set already?

Question

Is there a way to know if an out parameter was set already or not. This is the pseudocode for what I am looking for:

public virtual string blabla(long num, out bool bval)
    {
        if (!bval.HasValue)
            {
            //Do some default logic
            bval = defaultValue;
            }

        return blabla2(num, bval);
    }

Answer 1

You can't - you can't read the variable until it's been definitely assigned within your method. You should think of it as being like a local variable, declared but not assigned any value at the start of the method - but which you must assign a value to before you return. (It's okay not to have assigned a value to it if an exception is thrown.)

If you want a parameter which carries information as input to the method as well as propagating information out, you should use ref instead of out.

See my article on parameter passing for more information.

Answer 2

In addition to Jon's excellent answer, if you want the parameter to still be out, but need to see if it has been assigned a value at some place inside the method, you could use a local nullable type like follows:

public virtual string blabla(long num, out bool bval)
{
    bool? bvalLocal;

    ... //I'm assuming there is some code here that may or 
        //may not assign bvalLocal?

    // This whole if block may not be needed if the default
    // value is the default for the type (i.e. false) as
    // GetValueOrDefualt() will take care of that (see 
    // second to last line).
    if (!bvalLocal.HasValue)
    {
        //Do some default logic
        bvalLocal = defaultValue;
    }

    bval = bvalLocal.GetValueOrDefault();
    return blabla2(num, bval);
}