Nullable generic extension method

Question

I want to write a generic extension method that throws an error if there is no value. SO I want something like:

public static T GetValueOrThrow(this T? candidate) where T : class
        {
            if (candidate.HasValue == false)
            {
                throw new ArgumentNullException(nameof(candidate));
            }

            return candidate.Value;
        }

1. C# is not recognizing the T: The type or namespace name "T" cannot be found
2. C# is not recognizing where: Constraints are not allowed on non generic declarations

Any idea if this works? What am I missing?

I also come up with:

public static T GetValueOrThrow<T>(this T? candidate) where T : class
        {
            if (candidate.HasValue == false)
            {
                throw new ArgumentNullException(nameof(candidate));
            }

            return candidate.Value;
        }

Now C# complains about the candidate: The type T must be a non nullable value type in order to use it as parameter T in the generic type or method Nullable

This is not related to comparing.

Answer 1

public static T GetValueOrThrow<T>(this Nullable<T> candidate) where T : struct // can be this T? as well, but I think with explicit type is easier to understand
{
    if (candidate.HasValue == false)
    {
        throw new ArgumentNullException(nameof(candidate));
    }
    return candidate.Value;
}

where T: class constrains to reference types, which can be null, but HasValue is property of Nullable type (which is value type as well as T).