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

Answer 1

The problem is that the ternary operator is inferring type based on your first parameter assignment...10 in this case, which is an int, not a nullable int.

You might have better luck with:

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

Answer 2

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

Answer 3

Just add an explict cast.

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

It is the ternary operator that gets confused - the second argument is an integer and so is the third argument exspected to be an integer, too, and null does not fit.

Answer 4

Try this:

int? result = condition ? 10 : default(int?);

Answer 5

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

The reason you see this is because behind the scenes you're using Nullable and you need to tell C# that your "null" is a null instance of Nullable.

Answer 6

It's because the compiler determines the type of the conditional operator by its second and third operand, not by what you assign the result to. There is no direct cast between an integer and an null reference that the compiler can use to determine the type.