Constraint required default(T) is a valid T

[TonyValenti]

I have a method and I want to specific a constraint on T such that:

T can be a struct
or T can be a struct?
or T can be an object?

Essentially I want to make sure that default(T) will always result in a valid ```T```` value.

How can I do that?

[jnm2]

Can you explain what you mean by "valid T value"? default(T) is always going to return something that may be null unless you constrain T : struct. default(object) is (object?)null.

using System;
using System.Diagnostics.CodeAnalysis;

#nullable enable

public class C
{
    public T? M<T>() => default(T);
}

[TonyValenti]

I want to enable my class to work with structs (nullable or not) and nullable classes.

[bernd5]

it works with unconstrained generics. You can write e.g.:

Something<int> t1;
Something<int?> t2;
Something<object> t3;
Something<object?> t4;

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

[jnm2]

I want to enable my class to work with structs (nullable or not) and nullable classes.

@TonyValenti That is exactly what my suggestion of returning T? achieves.

[TonyValenti]

Maybe I need to clarify:

using System;
#nullable enable

namespace ConsoleApp9 {

    public class Foo<T> {
        public T Default() {
            return default(T);
        }
    }

}

How do I constrain T in the above such that default(T) will not produce a warning and I can use my class for int, int? and string? but not string?

[AartBluestoke]

if you're returning T? does a Foo return Nullable?
otherwise if T is an object type default(T) is null, which will of course warn when returned through a T(non-nullable)

[Joe4evr]

if you're returning T? does a Foo return Nullable?

No.

[jnm2]

No.

And the reason for this is that the runtime doesn't provide this facility.

[TonyValenti]

@jnm2 - Oh wow. Interesting! I didn't realize that T? could return a value that cannot contain null.

[jnm2]

T? means "nullable" unless T is not constrained to class or struct, in which case it means "defaultable."
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/unconstrained-type-parameter-annotations