An expression of type ... cannot be handled by a pattern when class is sealed

Question

I am getting a rather strange error:

CS8121 An expression of type 'IGrouping<string, IMyInterface>' cannot be handled by a pattern of type 'MyClass1'.

The error is occuring on the line:

if(tGroup is MyClass1 myclass1)

However, no error is provided for MyClass2 which is not sealed.

1. What is causing this?
2. Other than not sealing MyClass1 what solutions are there?

---

using System;
using System.Collections.Generic;
using System.Linq;

namespace Demo
{

    class Program
    {

        static void Main()
        {

            IEnumerable<IMyInterface> myInterfaces = new List<IMyInterface>();
            foreach (IGrouping<String, IMyInterface> tGroup in myInterfaces.GroupBy(x => x.XXX))
            {
                if(tGroup is MyClass1 myclass1)
                {
                }
                if(tGroup is MyClass2 myClass2)
                {
                }
            }

        }

    }

    public interface IMyInterface 
    {
        String XXX { get; } 
    }

    public sealed class MyClass1 : IMyInterface
    {
        public String XXX { get; }
    }

    public class MyClass2 : IMyInterface
    {
        public String XXX { get; }
    }

}

Answer 1

If MyClass is sealed, then there will never be a subclass that inherits MyClass and implements IGrouping<string, IMyInterface> The compiler is smart enough to deduce that, and hence "complains".

If it is not sealed there could be something like this:

class WhatEver : MyClass, IGrouping<string, IMyInterface> { ... }

So the compile cannot rule out if the is will ever succeed.

edit I bellieve, what you actually want to do is something like this:

foreach (IGrouping<String, IMyInterface> tGroup in myInterfaces.GroupBy(x => x.XXX))
{
    foreach(IMyInterface item in tGroup)
    {
        if(item is MyClass1 myclass1)
        {
        }
        if(item is MyClass2 myClass2)
        {
        }
    }
}