I came across this interesting scenario when I was looking into some issues. In a normal method you can pass the derived class instance when a base class instance is expected. In the example below passing dc1 to Method1 shows that you can indeed pass a derived class when a BaseClass was expected. But when you generate a generic List with a derived class and try passing it to function which was expecting a generic list with a base class it does not work. Strange, but I got the error mentioned below.

 

To work around it I made the list as a list of base class and limited to access of the derived class by checking on the type. It helps but is not as elegant as it should have been.

 

 

using System;

using System.Collections.Generic;

using System.Collections;

class BaseClass

{

    public int field1;

    public BaseClass()

    {

        field1 = 30;

    }

}

 

class DerivedClass : BaseClass

{

    int field2;

    DerivedClass()

    {

        field1 = 10;

        field2 = 20;

    }

    void Method1(BaseClass a)

    {

        Console.WriteLine("In method1 .." + field2);

    }

 

    void Method2(List<BaseClass> a)

    {

        for (int count = 0; count < a.Count; count++)

        {

            Console.WriteLine("In Method2 ... " + a[count].field1);

            // To be safe you check the type and so on ...

            //

            if (a[count].GetType() == typeof(DerivedClass))

            {

                Console.WriteLine("In method2 ... " + ((DerivedClass)a[count]).field2);

            }

        }

    }

 

    static void Main()

    {

        BaseClass bc1 = new BaseClass();

        DerivedClass dc1 = new DerivedClass();

        DerivedClass dc2 = new DerivedClass();

        dc2.Method1(dc1);

        // This will give you compilation error:

        //

        // List<DerivedClass> bcl = new List<DerivedClass>();

 

        // This will work

        //

        List<BaseClass> bcl = new List<BaseClass>();

        bcl.Add(dc1);

        bcl.Add(bc1);

        dc2.Method2(bcl);

    }

}