mbomb007 mbomb007 - 4 months ago 32
Vb.net Question

ASP.NET CheckBox not checked on postback without weird hack

I have a

GridView
with a checkbox column. On clicking a button, all rows with the checkbox checked should be removed. I somehow stumbled upon a strange and hacky solution, and I have no idea why it works. I already searched through related SO questions already.

Related code:

Protected Sub Page_Init(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Init
' I have no idea why this is needed for the checkboxes to work...
Dim x = imageGridView.Rows
End Sub


Protected Sub RemoveButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles removeButton.Click

For Each row As GridViewRow In imageGridView.Rows
Dim selectCheckBox As CheckBox = DirectCast(row.Cells(0).FindControl("selectCheckBox"), CheckBox)
If selectCheckBox.Checked Then
Dim fileName As String = row.Cells(1).Text
ImageList.Remove(ImageList.FindLast(Function(r) r.FileName = fileName))
End If
Next
imageGridView.DataSource = ImageList
imageGridView.DataBind()
End Sub


Aspx:

<asp:GridView ID="imageGridView" runat="server">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:CheckBox ID="selectCheckBox" runat="server" />
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>


The line
Dim x = imageGridView.Rows
is needed for the rows to be removed. I found this after trying my
RemoveButton_Click
code in the
Page_Init
sub, then removing code until it didn't work anymore.
Dim x = imageGridView
is not enough, and it doesn't work to do the same thing in
Page_Load
.

My checkboxes are never disabled.

So, simply put, why is it necessary for me to reference
imageGridView.Rows
in the
Page_Init
for my code to work?

Answer

That is an interesting behavior. I reproduce the problem if I bind the data to the GridView in Page_Load on every postback. In that situation, the check boxes lose their selection state on postback, but not if we refer to imageGridView.Rows in Page_Init, as you observed.

The solution is to bind the data inside an If Not IsPostBack conditional block:

Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If Not IsPostBack Then
        imageGridView.DataSource = ImageList
        imageGridView.DataBind()
    End If
End Sub

In that case, however, we must NOT refer to imageGridView.Rows in Page_Init. Doing so causes the check boxes to lose their selection state (!?!).

From the source code of the GridView (assuming that this source is reliable), I notice that accessing the Rows collection triggers a call to EnsureChildControls which then calls CreateChildControls. I haven't been able to step into the .NET code to see what happens at that point. Calling these methods in the Page_Init event handler may come earlier than expected in the life cycle of the GridView.

By the way, accessing the HeaderRow and the FooterRow properties also triggers a call to EnsureChildControls, and has the same effect on the selection status of the check boxes.

Comments