Inheritance - Can't assign object to extended class

[Mark G]

I want to extend the DataGridTextBox so I can access its IsInputKey protected method. I created the below class to do this

Public Class myDataGridTextBo
Inherits DataGridTextBo

Public Function CheckInputKey(ByVal keyData As Keys) As Boolea
CheckInputKey = IsInputKey(keyData
End Functio

Protected Overrides Function IsInputKey(ByVal keyData As Keys) As Boolea
'Some code her
End Functio

End Clas

My problem however occurs when I try to instantiate the object using DataGridTextBoxColumn.Textbox which is of type 'TextBox'. For example

Dim tbcName As DataGridTextBoxColum
Dim txtTB As TextBo
Dim txtDGTB As DataGridTextBo
Dim txtmyDGTB As myDataGridTextBo

tbcName = New DataGridTextBoxColum
tbcName.MappingName = "AnyValue

txtTB = tbcName.TextBox '* This works (Example A
txtDGTB = tbcName.TextBox '* This works (Example B
txtmyDGTB = tbcName.TextBox '* This fails with InvalidCastException (Example C

Prior to trying to extend the DataGridTextBox class, I was using Example B without any problem. Now, since the DataGridTextBox extends the TextBox class and one can assign a TextBox object to it with no problem, it shows I'm obviously missing something when I try to further extend the DataGridTextBox class.

I've also tried extending the TextBox class the same way and get the same error so I know I'm doing something wrong (since DataGridTextBox is an extension of TextBox) and it works.

I've looked at many examples on MSDN and can't find any that do this. Any help would be greatly appreciated