Compiler allows Nullable function to return Not Nullable value

[GuiAguirres]

When designing a function with a nullable return value, the compiler stays quiet about trying to return a not nullable value, as this:

public long? GetNullableValue()
{
    long value = 10;
    return value;
}

This function signature states a function that returns a Nullable.

In my vision, the compiler should throw the same casting error when doing the opposite:

public long GetNullableValue()
{
    long? value = 10;
    return value;
}

Cannot implicitly convert type 'long?' to 'long'. An explicit conversion exists (are you missing a cast?)

Motivation for this proposal:

The following function returns 0, instead of null:

public long? GetNullableValue()
{
    long[] values = [10, 5, 2];
    return values.Where(x => x == 7).LastOrDefault();
}

But this one returns null:

public long? GetNullableValue()
{
    long[] values = [10, 5, 2];
    return values.Where(x => x == 7).Select(x => (long?)x).LastOrDefault();
}

Environment: macOS Air M2
dotnet 8

[CyrusNajmabadi]

This is by design. All types are implicitly convertible to the nullable version of that type.

When designing a function with a nullable return value, the compiler stays quiet about trying to return a not nullable value, as this:

Correct. The long value 10 is perfectly representable as a long? so the compiler allows implicitly converting this. Just like it allows implicitly converting a string to an object, and many other implicit conversions.

Motivation for this proposal:

You've used LastOrDefault on an IEnumerable<long> and an IEnumerable<long?> respectively. The default(long) is just 0, while default(long?) is null.

[TahirAhmadov]

If your method has a nullable value type return type, and all returns have a known not-null value, it is indeed a potential source of bugs. You can write an analyzer which would warn in those situations.

For IEnumerable<T> where T : struct, create and use an extension method like LastOrNull which returns T?, instead of LastOrDefault. Then longs.LastOrNull() will be long?, as desired.