using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(M1(typeof(Derived1), typeof(List<>)));

        Console.WriteLine(M2(typeof(Derived1), typeof(List<>)));

    }

    public class Derived1 : List<int> {}

    public class Derived2 : List<int> {}

    public static Type M1(Type type, Type baseType) {

        Type baseTypeToCheck = type;

​

        while (baseTypeToCheck != null && baseTypeToCheck != typeof(object))

        {

            if (baseTypeToCheck.IsGenericType)

            {

                Type genericTypeToCheck = baseTypeToCheck.GetGenericTypeDefinition();

                if (genericTypeToCheck == baseType)

                {

                    return baseTypeToCheck;

                }

            }

​

            baseTypeToCheck = baseTypeToCheck.BaseType!;

        }

​

        return null;

    }

    public static Type M2(Type type, Type baseType) {

        Type ienumerableType = type.GetInterface("System.Collections.Generic.IEnumerable`1");

        if (ienumerableType == null)

        {

            return null;

        }

        Type closedType = baseType.MakeGenericType(ienumerableType.GetGenericArguments()[0]);

        return closedType.IsAssignableFrom(type) ? closedType : null;

    }

}