Covariance & Contravariance, C#

Covariance & Contravariance

//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            /// ---Assignment compatibility---   
            string str = "test";   
            /*
             An object of a more derived type is assigned to an object of a less derived type.
            */   
            object obj = str; 
            Console.WriteLine("obj={0}",obj);
            
            /*
             Cannot implicitly convert type 'object' to 'string'. An explicit conversion exist.
            */
            string str2 = (string)obj;
            Console.WriteLine("str2={0}",str2);

/// ---Covariance共變數--- 
            IEnumerable<string> strings = new List<string>(){"123","abc"};  
            /* 
             An object that is instantiated with a more derived type argument   
             is assigned to an object instantiated with a less derived type argument.   
             Assignment compatibility is preserved. 
            */  
            IEnumerable<object> objects = strings;  
            foreach(var o in objects)
            {
                Console.WriteLine("object={0}",o);
            }
            
            /// ---Contravariance反變數---             
            Action<object> actObject = delegate(object o){Console.WriteLine("object={0}",o);};  
            /*
             An object that is instantiated with a less derived type argument   
             is assigned to an object instantiated with a more derived type argument.   
             Assignment compatibility is reversed.   
            */
            Action<string> actString = actObject;  
            actString("hello~");
        }

}
}


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