How do I update a list after selection in gridview (ASP.NET)

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Hello,

I'm back again and have another question:

I have a simple GridView ona webpage with several names in it:

- Alex
- Thomas
- Kate
- George

I want to add the names in a list after I select a name.

I use the SelectedIndexChanged event.

I have this code:

Public Class StudentNames
Inherits System.Web.UI.Page
Public List As New Generic.List(Of String)

VB

Protected Sub GridView1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles GridView1.SelectedIndexChanged

Name = GridView1.SelectedRow.Cells(0).Text

List.Add(Name)

I click on the name Alex in the GridView and Alex is added to the list. But when I click on Thomas in the GridView, it looks like the name Alex is gone from the list.

I have a Textbox1 where I count the selected names.

Textbox1.text = List.count

List.count = 1. After the add of the name Alex, I expect two items in list and not one.

What I'm doing wrong?

Sorry for my bad English.

Thanks in advance.

What I have tried:

Search on Microsoft website and Google

Posted 5-Sep-20 23:36pm

Coder4EU

Updated 9-Sep-20 23:01pm

v15

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3 solutions

Solution 1

I think you should harness the GridView event for 'Selected' / 'Clicked' - Im on my Mac so I can't easily tell you what that is, but if you're in Forms Design mode and select the GridView there's a way of seeing the events - so select the event and define a handler for it .. actually, this DataGridView.SelectionChanged Event (System.Windows.Forms) | Microsoft Docs[^] may help - there's even an event for Rows Added for example ... have a look at SelectionChanged

Knowing that, if you're still stuck you can Google 'VB.Net Gridview SelectionChanged example' and that should help you along

Posted 6-Sep-20 0:19am

Garth J Lancaster

Comments

Coder4EU 6-Sep-20 6:31am

Thanks for the reply. I use the SelectedIndexChanged event for my code, but this won't work correctly. It looks like the list is overwritten when I add a new Name (the old name is gone).

Garth J Lancaster 6-Sep-20 6:36am

"but it won't work correctly.' not really helpful - why don't you use Improve question and add the SelectionIndexChanged event handler code you have, try to describe as clearly as possible why it doesn't work, and we'll see what we can do/suggest .. don't fret too much about the 'bad English' bit, we are quite tolerant unless the meaning is obscured, then we can always ask you

Coder4EU 6-Sep-20 6:43am

Thank you for the reply and sorry for my unclear question. I improved my question. I hope my question is more clear.

Garth J Lancaster 6-Sep-20 6:52am

sorry, without seeing the code all I can do is shrug - I can't set up a test from here either .. maybe someone else will understand from your description

Coder4EU 6-Sep-20 7:07am

Thanks for the replies. I appreciate your efforts

Solution 2

Because every time that event fires you create a new list which will contain just one name. Your list should be declared and initialised outside of the event handler, so each event will add the name to the existing list.

Posted 6-Sep-20 2:42am

Richard MacCutchan

Comments

Coder4EU 6-Sep-20 9:03am

Thank you for the reply. I declared my list outside of the event handler, but the List.Count is still 1 after selection of another name in the GridView.

Richard MacCutchan 6-Sep-20 9:12am

Yes, but you are still creating a new temporary one in the event handler.

Coder4EU 6-Sep-20 9:15am

Thank you for the reply. Maybe my reply was a bit unclear.

I removed the list from the event handler and placed it outside the event handler. This result also in list.count is 1. I updated my question with my recent code.

Richard MacCutchan 6-Sep-20 10:06am

Something else must be happening. You need to put a breakpoint on that code and use the debugger to trace through it to find out what.

Coder4EU 7-Sep-20 13:04pm

Thank you for the reply. I will insert a breakpoint and analyze what is going wrong.

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Richard Deeming · Accepted Answer · 2020-09-09T23:01:00

Quote:

VB.NET

Public Class StudentNames
Inherits System.Web.UI.Page
    Public List As New Generic.List(Of String)

Every request to your page, including a post-back, will create a new instance of your page class to process the request.

Anything stored in a field within that class will not be persisted between requests.

You'll need to store your list in ViewState for the changes to persist between requests.

VB.NET

Public Property List As Generic.List(Of String)
    Get
        Dim value As String() = DirectCast(ViewState("List"), String())
        If value Is Nothing Then Return New Generic.List(Of String)
        Return New Generic.List(Of String)(value)
    End Get
    Set(ByVal value As Generic.List(Of String))
        If value Is Nothing Then Throw New ArgumentNullException()
        ViewState("List") = value.ToArray()
    End Set
End Property

ASP.NET State Management Overview | Microsoft Docs[^]