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;
        }

C# is not recognizing the T: The type or namespace name "T" cannot be found
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.

this T? makes no sense at all, as a class is nullable per definition. — MakePeaceGreatAgain
– MakePeaceGreatAgain, Commented Sep 19, 2019 at 14:50
Why you want that extension at all? It's the default behavior, just use candidate.Value and be excited about the incoming error if there is no value. — Tim Schmelter
– Tim Schmelter, Commented Sep 19, 2019 at 14:53
Read Eric Lippert's Vexing exceptions to find out why this is a bad idea in the first place. — Zohar Peled
– Zohar Peled, Commented Sep 19, 2019 at 14:53
Possible duplicate of Creating a nullable<T> extension method ,how do you do it? — Sinatr
– Sinatr, Commented Sep 19, 2019 at 14:59

Viktor Arsanov · Accepted Answer · 2019-09-19 14:56:31Z

8

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).

edited Sep 19, 2019 at 14:56

answered Sep 19, 2019 at 14:51

Viktor Arsanov

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