Nullable reference type and generic

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If I have a method like that:

C#

void DoSomething<T>() {
  // do some validation on `T`
}

How could I know and differentiate at runtime if T is, say, string, or string? ?

With value type its easy, typeof(T), is either int or Nullable<int>, which are easy to differentiate.

But for string and string? I cant find any difference at runtime... :/

What I have tried:

I tried using the debugger to look at typeof(T) and also at the MethodInfo of Fooz. Both no luck :/

Posted 27-Sep-21 6:13am

Super Lloyd

Updated 27-Sep-21 6:32am

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Super Lloyd 27-Sep-21 12:14pm

nullable reference of course, is a new C# 9 feature.

Super Lloyd 27-Sep-21 12:25pm

for the record I try to implement

public class MyList<T> : IList<t>, IList

And it's very hard to implement IList.Add(object? o), I dunno if the current implementation supports null or not..

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Super Lloyd · Accepted Answer · 2021-09-27T06:32:00

Solution 1

mm... after consideration, and seemingly no possibility to get the desired info, I decided to simply reject null value in the non generic (IList) version

Posted 27-Sep-21 6:32am

Super Lloyd

Richard Deeming · Accepted Answer · 2021-09-27T06:32:00

You can't. When you compile:

C#

DoSomething<string?>();
DoSomething<string>();

the compiler emits:

C#

DoSomething<string>();
DoSomething<string>();

So the nullability of the type parameter is lost.

You could add a notnull constraint to the type parameter if it should never accept null. But without it, you can't differentiate between nullable and non-nullable reference types.