How does the compiler understand Nullables?

Question

If I have a method:

protected int CalculateActualDuration(DateTime? startDate, DateTime? endDate) {
        if (startDate.HasValue && endDate.HasValue) { 
            return Math.Abs((int)(endDate.Value.Subtract(startDate.Value).TotalMinutes));
        }
        else {
            return 0;
        }
    }

I am able to call the method by passing in both a DateTime? and a DateTime. So how does the compiler understand the difference?

Does that mean if I pass in a DateTime Value, the if statement will essentially be soemthing like

if (true && true)

and all the *.value has been changed to the proper objects? So all endDate.Value are all now EndDates?

Does the compiler cast all parameters that are not Nullables into Nullables during runtime?

Answer 1

Everything in your method remains the same, and the startDate and endDate parameters are still instances of the Nullable<T> struct.

When you pass a "normal" DateTime to the method, you're taking advantage of the implicit conversion specified in the Nullable<T> struct:

public static implicit operator Nullable<T>(T value) {
    return new Nullable<T>(value);
}

From the MSDN page linked above:

If the value parameter is not null, the Value property of the new Nullable value is initialized to the value parameter and the HasValue property is initialized to true.