Nullable Enable and Generics default

[HavenDV]

After adding <Nullable>enable</Nullable> or #nullable enable, I ran into the following problem with my Generic methods:

This does not work:

public T? GetDefault<T>()
{
    return default;
}

This works with warning:

public T GetDefault<T>()
{
   return default;
}

This works individually, but not together.

public T? GetDefault<T>() where T : class
{
    return default;
}

public T? GetDefault<T>() where T : struct
{
    return default;
}

Logically, the first method should work.
What is the correct way out of this situation without creating several methods and suppressing warning?

P.S.
At the moment, I have not found anything better than this solution:

public T GetDefault<T>()
{
    return default!;
}

public void OtherMethod()
{
    string value1 = GetDefault<string>();
    string value2 = GetDefault<string?>();
    string? value3 = GetDefault<string?>();
}

[bbarry]

agree it is frustrating; not much to do about it I think now though.

Here is what I do:

[return: MaybeNull]
public T GetDefault<T>() => default!;

public void OtherMethod()
{
#pragma warning disable CS8600 // Converting null literal or possible null value to non-nullable type.
    string value1 = GetDefault<string>();
#pragma warning restore CS8600 // Converting null literal or possible null value to non-nullable type.
    string? value2 = GetDefault<string?>();
    string? value3 = GetDefault<string>();
}

[333fred]

C# 9 will bring T??: #3297.

Until then, you can use @bbarry's solution. Note that the pragma warning suppressions would only be needed if you attempted to assign a string? to a string, as is done on the first line of OtherMethod there.

[333fred]

You can see the full set of attributes that can inform nullable analysis here: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/attributes/nullable-analysis.

[HavenDV]

The interesting thing - Visual Studio offers this edit:

[CyrusNajmabadi]

Tagging @jcouv for VS issue above.

[HavenDV]

Is there a way to use the [MaybeNull] attribute in a .Net Standard 2.0 project?

[bbarry]

I think you might be able to declare the attributes as internal in the correct namespace within your assembly? I haven't tried because targeting .Net standard leaves too many warts for me to deal with so I gave up and multitarget just net48 and netcoreapp3.1. Hopefully net5/6+ will improve things and let me gain mono/uwp/xamarin/etc. runtime support before I actually need them.

dotnet/runtime#29617 (comment)

[HavenDV]

What about the situation where I need to deliver a nuget package that contains a public function with this attribute?

[bbarry]

The compiler is looking for an attribute on the type with the full name (no assembly though) of System.Diagnostics.CodeAnalysis.MaybeNullAttribute.

It isn't looking for a specific type.

This is similar to how async works in earlier versions of .net framework. The internal attribute would have the same name.

[YairHalberstadt]

The original example is now legal in C# 9

public T? GetDefault<T>()
{
    return default;
}