[C#] Casting from derived class to base class

Melon00 · 06-06-2006, 08:34 AM

I have a class I am calling SimTreeNode which is a System.Windows.Forms.TreeNode type. I am planning on using this class within my TreeView class.

Class Definition:

Code:

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

namespace SimTree
{
    public class SimTreeNode : System.Windows.Forms.TreeNode
    {
        //public static explicit operator SimTreeNode(TreeNode tn)
        //{

        //}
    }
}

As you can see, I don't do anything more or less than the TreeNode class.

The problem is, when I try to use my SimTreeNode in place of any TreeNode in my TreeView class, it throws runtime errors.

Here is the snippet of code I am doing the conversion.

Code:

using System.Collections;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Text;
using System.Windows.Forms;
using System.Xml;
using System.Xml.Serialization;
using SimTree.Properties;
using System.Runtime.InteropServices;

namespace SimTree
{
    public partial class SimTree : System.Windows.Forms.TreeView
    {

          private void SimTree_ItemDrag(object sender, ItemDragEventArgs e)
          {
              // Do not allow anything other than devices to be dragged
              Point pt = this.PointToClient(Control.MousePosition);
              SimTreeNode tn = (SimTreeNode)this.GetNodeAt(pt); // <--- Runtime error here

              ...
          }
   }
}

Runtime Error:

Code:

Unable to cast object of type 'System.Windows.Forms.TreeNode' to type 'SimTree.SimTreeNode'.

I would like to know why I cant caste my derived type to my base type.

According to a million online forums, I don't need to make an explicit operator to caste to/from base types. I actually tried to make an explicit caste operator if you look at my comments in the SimTree.SimTreeNode class, but gave me a compiler error:

Code:

Error	1	'SimTree.SimTreeNode.explicit operator SimTree.SimTreeNode(System.Windows.Forms.TreeNode)': user-defined conversion to/from base class	C:\...\SimTree\SimTreeNode.cs	10	23	SimTree

This really doesn't make any sense to me as the compiler SHOULDN'T have anyway of telling how to convert a base type to a derived type without me telling it explicitly.

I am hitting brick walls left and right with this simple thing I want to do.