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

I just came across a weird error:

private bool GetBoolValue()
{
    //Do some logic and return true or false
}

Then, in another method, something like this:

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

Simple, if the method returns true, assign 10 to the Nullableint x. Otherwise, assign null to the nullable int. However, the compiler complains:

Error 1 Type of conditional expression cannot be determined because there is no implicit conversion between int and <null>.

Am I going nuts?

Solution 1:

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

Solution 2:

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.

Solution 3:

Try this:

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

Solution 4:

Incidentally, the Microsoft implementation of the C# compiler actually gets the type analysis of the conditional operator wrong in a very subtle and interesting (to me) way. My article on it is Type inference woes, part one (2006-05-24).

Solution 5:

Try one of these:

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

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