return ulong in inline-if [duplicate]

Question

Is there a reason why I can't use the following code?

ulong test(int a, int b)
{
    return a == b ? 0 : 1;
}

It shows me:

Cannot implicitly convert type 'int' to 'ulong'. An explicit conversion exists (are you missing a cast?)

The following will work:

ulong test(int a, int b)
{
    return false ? 0 : 1;
}

I know how to solve the problem. I just want to know the reason.

Thanks.

Answer 1

Let's take a look at the resulting IL code of the second method:

IL_0000:  nop
IL_0001:  ldc.i4.1
IL_0002:  conv.i8
IL_0003:  stloc.0
IL_0004:  br.s       IL_0006
IL_0006:  ldloc.0
IL_0007:  ret

At IL_0001 the literal 1 is pushed onto the stack (so the expression return false ? 0 : 1; is evaluated at compile-time and embedded into IL as a constant) and at IL_0002 this literal is converted into an Int64.

From MSDN:

A constant-expression (Section 7.15) of type int can be converted to type sbyte, byte, short, ushort, uint, or ulong, provided the value of the constant-expression is within the range of the destination type.

Since 1 is in the range of the ulong data type, such conversion will always succeed. The compiler knows that and therefore performs an implicit conversion.

Answer 2

Variables typed as int cannot be implicitly converted to ulong, because an int can represent a negative value, while a ulong cannot. A constant can be implicitly converted, provided that the value of the constant expression is non-negative.

Your second example is a constant expression.