This program generates compiler warning CS8620

Argument of type ‘IEnumerable<(string S1, string S2)>’ cannot be used
for parameter ‘e’ of type ‘IEnumerable<(string? s1, string? s2)>’ in
‘void Program.F(IEnumerable<(string? s1, string? s2)> e)’ due to
differences in the nullability of reference types.

But I’m not trying to pass off something that can be null as something that can’t, I’m doing the opposite! That should be perfectly fine!

Is there some way I can tell the compiler, "look I know it can’t be null, just pretend that it can be"? An opposite to the null-forgiving operator? A null-condemning operator?!

namespace MyNamespace;

public static class Program
{
    public static void Main(string[] args)
    {
        var L = new [] {new C("S1", "S2")};
        F(L);
    }

    public class C
    {
        public string S1 { get; }
        public string S2 { get; }

        public C(string s1, string s2)
        {
            S1 = s1;
            S2 = s2;
        }
    }

    public static void F(IEnumerable<C> e)
        => F(e.Select(c => (c.S1, c.S2)));

    public static void F(IEnumerable<(string? s1, string? s2)> e)
    {}
}

>Solution :

But I’m not trying to pass off something that can be null as something that can’t, I’m doing the opposite! That should be perfectly fine!

It would be fine if the original collection cannot be modified through the IEnumerable .. but it can be. Consider this code, which yes, is a bit of a hack, that’s the point:

public static void Main(string[] args)
{
    var L = new[] { ("S1", "S2") };
    F(L);
    Console.WriteLine(String.Join(", ", L));
}

public static void F(IEnumerable<(string? s1, string? s2)> e)
{
    if (e is (string? s1, string? s2)[] array)
        array[0].s1 = null;
}

L is an array of tuples of regular strings (not string?). The body of the if in F is executed, and a null ends up in that tuple anyway. So the compiler warning about that call to F is not wrong.