Question

Possible Duplicate:
C# okay with comparing value types to null

I came a cross something I find strange in the C# (4.0) compiler just now.

int x = 0;
if (x == null) // Only gives a warning - 'expression is always false'
    x = 1;

int y = (int)null; // Compile error
int z = (int)(int?)null; // Compiles, but runtime error 'Nullable object must have a value.'

If you cannot assign null to an int, why does the compiler allow you to compare them (It gives a warning only)?

Interestingly, the compiler does not allow the following:

struct myStruct
{
};

myStruct s = new myStruct();
if (s == null) // does NOT compile
    ;

Why does the struct example not compile, but the int example does?

Answer 1

When the comparison is made, the compiler tries to make it so both operands of the comparison have compatible types if possible.

It had an int value and a constant null value (with no particular type). The only compatible type between the two values is int? so they are coerced to int? and compared as int? == int?. Some int value as an int? is definitely non-null and null is definitely null. The compiler realizes that and since a non-null value is not equal to a definite null value, the warning is given.