Type of conditional expression cannot be determined because there is no implicit conversion between 'int' and

Question

Why does this not compile? ``` int? number = true ? 5 : null; ``` Type of conditional expression cannot be determined because there is no implicit conversion between 'int' and <null>

Accepted Answer

The spec (§7.14) says that for conditional expression `b ? x : y`, there are three possibilities, either `x` and `y` both have a type and certain good conditions are met, only one of `x` and `y` has a type and certain good conditions are met, or a compile-time error occurs. Here, "certain good conditions" means certain conversions are possible, which we will get into the details of below.

Now, let's turn to the germane part of the spec:

If only one of `x` and `y` has a type, and both `x` and `y` are implicitly convertible to that type, then that is the type of the conditional expression.

The issue here is that in

```
int? number = true ? 5 : null;
```

only one of the conditional results has a type. Here `x` is an `int` literal, and `y` is `null` which does not have a type and `null` is not implicitly convertible to an `int`1. Therefore, "certain good conditions" aren't met, and a compile-time error occurs.

There are two ways around this:

```
int? number = true ? (int?)5 : null;
```

Here we are still in the case where only one of `x` and `y` has a type. Note that `null` still does not have a type yet the compiler won't have any problem with this because `(int?)5` and `null` are both implicitly convertible to `int?` (§6.1.4 and §6.1.5).

The other way is obviously:

```
int? number = true ? 5 : (int?)null;
```

but now we have to read a different clause in the spec to understand why this is okay:

If `x` has type `X` and `y` has type `Y` then

If an implicit conversion (§6.1) exists from `X` to `Y`, but not from `Y` to `X`, then `Y` is the type of the conditional expression.

If an implicit conversion (§6.1) exists from `Y` to `X`, but not from `X` to `Y`, then `X` is the type of the conditional expression.

Otherwise, no expression type can be determined, and a compile-time error occurs.

Here `x` is of type `int` and `y` is of type `int?`. There is no implicit conversion from `int?` to `int`, but there is an implicit conversion from `int` to `int?` so the type of the expression is `int?`.

1: Note further that the type of the left-hand side is ignored in determining the type of the conditional expression, a common source of confusion here.

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

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