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Specified cast not valid

Hello All!

I am getting teh above error message on the following code:

Private Sub Button1_Click(B yVal sender As System.Object, ByVal e As
System.EventArg s) Handles Button1.Click

Dim dgt As DataGridTableSt yle = Main_DataGrid.T ableStyles(0)

Dim cm As CurrencyManager = CType(Me.Bindin gContext _
(Main_DataSet.T ables(dgt.Mappi ngName)), CurrencyManager )
' Get the Rectangle of the clicked cell.
Dim cellRect As Rectangle
Dim cellrow As Integer = 5
Dim cellcolumn As Integer = 1
cellRect = Main_DataGrid.G etCellBounds(5, 1)
' Get the clicked DataGridTextBox Column.

Dim gridCol As New MyGridColumn()
gridCol = CType(dgt.GridC olumnStyles(cel lcolumn),
MyGridColumn)
' Get the Graphics object for the form.
Dim g As Graphics = Main_DataGrid.C reateGraphics()
' Create two new Brush objects: a fore brush and back brush.
Dim fBrush As New SolidBrush(Colo r.Yellow)
Dim bBrush As New SolidBrush(Colo r.Yellow)
' Invoke the Paint method to paint the cell with the brushes.
gridCol.PaintCo l(g, cellRect, cm, cellrow, bBrush, fBrush,
False)
My MyGridColumn class looks like this:

Public Class MyGridColumn
Inherits DataGridTextBox Column
Public Sub PaintCol(ByVal g As Graphics, ByVal cellRect As
Rectangle, _
ByVal cm As CurrencyManager , ByVal rowNum As Integer, ByVal bBrush
As Brush, _
ByVal fBrush As Brush, ByVal isVisible As Boolean)
Me.Paint(g, cellRect, cm, rowNum, bBrush, fBrush, isVisible)
End Sub
End Class
Some of you may recognize this; it is only a slight revision of an
exam given by Microsoft at MSDN. I thought I would be safe using that
code. I get the error at "gridcol = CTYPE" statment. Everything looks
right to me! I have a valid GridTableStyles[0] and a
GridColumnStyle s[1].

My only thought: I can't seem to set a variable to a 'type' of
GridColumnStyle s. Can anyone tell me if it is correct to reference a
GridColumnstyle directly in this way. ALL SUGGESTIONS WOULD BE
APPRECIATED!!!
Nov 20 '05 #1
3 1695
Hi Mike,

I'm not entirely sure what you're doing but ..

dgt.GridColumnS tyles is a collection of DataGridColumnS tyle objects and
these cannot be referenced by a MyGridColumn object.

Are you sure that you want something from dgt.GridColumnS tyles and not an
actual column?

Regards,
Fergus
Nov 20 '05 #2
Thank you for responding,

In variable:

dgt.GridColumnS tyles(cellcolum n)

dgt is my datagridtablest yle and cellcolumn is an integer = 1. This
is actually
a reference then for dataGridColumnS tyle(0) which is the
DataGridTextBox Column column I am after. Just in
case this was the problem however, I change this reference to the
actual of DataGridTextBox Column2 the result is the same.

Have you, or anyone else out there attempted to use CTYPE to convert a
parent class object into a child object (which inherited the parent)?
I don't see why it shouldn't work!
"Fergus Cooney" <fi******@tesco .net> wrote in message news:<uu******* *******@TK2MSFT NGP12.phx.gbl>. ..
Hi Mike,

I'm not entirely sure what you're doing but ..

dgt.GridColumnS tyles is a collection of DataGridColumnS tyle objects and
these cannot be referenced by a MyGridColumn object.

Are you sure that you want something from dgt.GridColumnS tyles and not an
actual column?

Regards,
Fergus

Nov 20 '05 #3
Hi Mike,

My apologies, I took DataGridTextBox Column at its word and didn't look it
up. Given that it inherits from DataGridColumnS tyle I would have thought it
would be called DataGridTextBox ColumnStyle. It fooled me!.

Anyway, I can answer your question now that I see the problem.

If dgt.GridColumnS tyles(cellcolum n) contains a MyGridColumn (style) then
you can cast it. If it contains a descendant (child or further) of
MyGridColumn then you would be also able to cast it.

But not an <ancestor> (parent or further).

Let's get away from Data objects and just go to objects and references.

An Object variable can reference anything but it will only give you access
to the things that the Object class provides. Let's say it referenced a
String. To get the String methods you would have to cast it to a string first
(if Option Strict is on), or it would be obtained at runtime through late
binding.

Dim O As Object = "Some text"
Dim I As Integer = O.Length 'Fails to compile (OS on), or late binds.
Dim I As Integer = DirectCast (O, String).Length 'No problems.

But the converse is not possible. There's no way that a String reference
can reference an Object.

Dim O As Object = "Some text"
Dim S As String = O 'Fails to compile (OS on)
'or fine at run-time because O references a
String.

Dim O As New Object
Dim S As String = O 'Fails to compile (OS on)
'or gives Invalid Cast at run-time.

This is because a String reference is expecting to be able to access <any
and all> of the methods of Strings. Object, of course, doesn't provide any of
the extra facilities that Strings have. Hence - Bang!

Back to your problem, then.

I expect that dgt.GridColumnS tyles(cellcolum n) is an actual
DataGridTextBox Column. And you're trying to cast it to a derivative,
MyGridColumn, with its extra facilities. But, as, DataGridTextBox Column
doesn't have them - Bang!

That's the problem. There are two types of solution.

One is to keep the derived class and ensure that the dgt.GridColumnS tyles
collection is composed of MyGridColumns, either entirely, or just with those
that'll have a PaintCol requirement.

The other solution is to abandon the derived class. Then you might pass
gridCol to ColPaint as a parameter.

Regards,
Fergus
Nov 20 '05 #4

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