Question

Way I could not merge List and List? OOP says MyType2 is MyType…

using System;
using System.Collections.Generic;

namespace two_list_merge
{
    public class MyType
    {
        private int _attr1 = 0;

        public MyType(int i)
        {
            Attr1 = i;
        }

        public int Attr1
        {
            get { return _attr1; }
            set { _attr1 = value; }
        }
    }

    public class MyType2 : MyType
    {
        private int _attr2 = 0;

        public MyType2(int i, int j)
            : base(i)
        {
            Attr2 = j;
        }

        public int Attr2
        {
            get { return _attr2; }
            set { _attr2 = value; }
        }
    }

    class MainClass
    {
        public static void Main(string[] args)
        {
            int count = 5;
            List<MyType> list1 = new List<MyType>();
            for(int i = 0; i < count; i++)
            {
                list1[i] = new MyType(i);
            }

            List<MyType2> list2 = new List<MyType2>();
            for(int i = 0; i < count; i++)
            {
                list1[i] = new MyType2(i, i*2);
            }           

            list1.AddRange((List<MyType>)list2);
        }
    }
}

Answer 1

I’m going to assume that you’re not using C# 4.0.

In earlier versions of C#, this won’t work because the language doesn’t support contravariance and covariance of generic types.

Don’t worry about the academic jargon – they’re just the terms for the kinds of variance (i.e. variation) permitted.

Here’s a good article on the details:
http://blogs.msdn.com/b/csharpfaq/archive/2010/02/16/covariance-and-contravariance-faq.aspx

To make your code work, write this:

list1.AddRange(list2.Cast<MyType>());