I have a scenario where I have created a parent class, ParentTest,
which implements an interface, ITest.  The ParentTest class has a
method which returns an instance of the interface type.  Each child
class has a method which calls the parent's TestMethod method in order
to simplify method calls and make them more intuitive for each sub
type for developers.

In the ChildTest class, I want to return an instance of the ChildTest
in the GetData method.  However, I receive the following error:

An unhandled exception of type 'System.InvalidCastException' occurred
in test1.exe

Additional information: Unable to cast object of type
'TestNamespace.ParentTest' to type 'TestNamespace.ChildTest'.

Below is a simple example of what I am trying to accomplish.  Can
someone tell me if I am way off base with this approach?  If so, how
would I set this up correctly?

using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace TestNamespace
{
   public enum TestEnum { A, B, C, D }

   public interface ITest
   {
       string Code {get;set;}
       string Description {get; set;}

       // Define simple method
       ITest TestMethod(TestEnum e, string s);
   }

   public class ParentTest : ITest
   {
       string _code;
       string _description;

       public ParentTest() : base() { }
       ParentTest(string e, string s)
           : base()
       {
           _code = e;
           _description = s;
       }

       public string Code {
           get
           {
               return _code;
           }
           set
           {
               _code = value;
           }
       }

       public string Description {
           get
           {
               return _description;
           }
           set
           {
               _description = value;
           }
       }

       public virtual ITest TestMethod(TestEnum e, string s)
       {
           // TODO: Create a method to do a look up by type and code
as passed by enum value and string
           Console.WriteLine("\n\nIn TestMethod.  e: " + e.ToString()
+ " s:" + s + "\n");
           ParentTest pt = new ParentTest(e.ToString(),
s);
           ITest i = pt as ITest;
           return i;
       }
   }

   public class ChildTest : ParentTest
   {
       public ChildTest() : base() { }

       public ChildTest GetData(string s)
       {
           ITest i = this.TestMethod(TestEnum.A, s);
           return (ChildTest)i;
       }

   }

   static class Program
   {
       [STAThread]
       static void Main()
       {
           ChildTest f = new ChildTest();
           ChildTest c = f.GetData("TestLookupCode");
           Console.WriteLine("\n\nCode: " + c.Code + " Description: "
+ c.Description + "\n\n\n");
       }
   }
}