using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

​

    static bool IsMatchingThing1(IThing thing) => thing is AThing { Children: [BThing] };

    static bool IsMatchingThing2(IThing thing) => thing is AThing { Children: [BThing] };

}

​

internal interface IThing;

​

internal class AThing : IThing

{

    public List<IThing> Children { get; set; } = new();

}

​

internal class BThing : IThing

{

    public List<IThing> Children { get; set; } = new();

}