Question

This is very strange, maybe someone can explain what’s happening, or this is a bug (though I tend to think that this is probably just something intricate about C#).

The following code throws the error “Cannot implicitly convert type ‘uint?’ to ‘uint’.”:

public void Test(UInt32? p)
{ 
    UInt32 x = p;
}

However, this code works without error:

public void Test(UInt32? p)
{
    UInt32 x = p ?? 1;
}

Huh? Why does this work? Why would the coalesce operator cause implicit conversion of UInt32? (nullable) to UInt32 (non-nullable), while the first error message says that there is no implicit conversion between those types?

Answer 1

Think of it this way.

Nullable<T> is a value type. Obviously, it can’t really be null. Thus, the expression:

p == null

Is really syntactic sugar for this:

!p.HasValue

Similarly, the expression:

p ?? 1

Is syntactic sugar for:

p.HasValue ? p.Value : 1

From the preceding, it’s clear that the expression p ?? 1 is actually evaluating to a uint, which is why the code works without a cast.