I have a generic class to represent either a result or an error, but not both:

public class Result<TValue, TError>
    where TValue : notnull
    where TError : notnull
{
    private readonly bool succeeded;
    private readonly TValue? value;
    private readonly TError? error;

    public static Result<TValue, TError> Success(TValue value) =>
        new(true, value ?? throw new ArgumentNullException(nameof(value)), default);

    public static Result<TValue, TError> Failure(TError error) =>
        new(false, default, error ?? throw new ArgumentNullException(nameof(error)));

    protected Result(bool succeeded, TValue? value, TError? error)
    {
        this.succeeded = succeeded;
        this.value = value;
        this.error = error;
    }
    
    public bool Successful(
        [NotNullWhen(true)] out TValue? value,
        [NotNullWhen(false)] out TError? error)
    {
        if (succeeded)
        {
            value = this.value!;
            error = default;
            return true;
        }
        else
        {
            value = default;
            error = this.error!;
            return false;
        }
    }
}

This works as I want for classes. For example, if TValue is object then the value field and value parameters are object?. However, I've realised that when a generic type parameter is a value type, decorating it with the ? doesn't make it Nullable. This is presumably because doing so would change the underlying type rather than just commenting on its nullability. Is there any way to reference the Nullable version of a generic type?

CodePudding user response：

This is language limitation. See official docs

If the type argument for T is a value type, T? references the same value type, T. For example, if T is an int, the T? is also an int.