Nullable types and the ternary operator: why is `? 10 : null` forbidden?

Try this:

int? x = GetBoolValue() ? 10 : (int?)null;

Basically what is happening is that conditional operator is unable to determine the "return type" of the expression. Since the compiler implictitly decides that 10 is an int it then decides that the return type of this expression shall be an int as well. Since an int cannot be null (the third operand of the conditional operator) it complains.

By casting the null to a Nullable<int> we are telling the compiler explicitly that the return type of this expression shall be a Nullable<int>. You could have just as easily casted the 10 to int? as well and had the same effect.

The compiler first tries to evaluate the right-hand expression:

GetBoolValue() ? 10 : null

The 10 is an int literal (not int?) and null is, well, null. There's no implicit conversion between those two hence the error message.

If you change the right-hand expression to one of the following then it compiles because there is an implicit conversion between int? and null (#1) and between int and int? (#2, #3).

GetBoolValue() ? (int?)10 : null    // #1
GetBoolValue() ? 10 : (int?)null    // #2
GetBoolValue() ? 10 : default(int?) // #3