NEWBIE - Displaying Selection of Listbox Item

  • Thread starter Thread starter RockNRoll
  • Start date Start date
R

RockNRoll

Greetings again,

I have a Listbox that is populated from a SQL Statement. I am attempting to
get the value of the selected item with the following code:

Sub ListBox1_SelectedIndexChanged(sender As Object, e As EventArgs)
response.write(ListBox1.SelectedItem.value)
End Sub

I have the datatext and datavalue fields both set to the same field in my
listbox (Just a list of Job numbers) and I have autopostback set to "True."

The issue is that when I select an item from the listbox, I get an error
message stating:
Object reference not set to an instance of an object.
Below is my code in entirety for further reference if needed. Thank you.
<%@ Page Language="VB" %>
<%@ Register TagPrefix="wmx" Namespace="Microsoft.Matrix.Framework.Web.UI"
Assembly="Microsoft.Matrix.Framework, Version=0.6.0.0, Culture=neutral,
PublicKeyToken=6f763c9966660626" %>
<%@ Register TagPrefix="n0" Namespace="MyListView" Assembly="MyListView" %>
<script runat="server">

Sub Page_Load (Src as Object, E As EventArgs)

Dim str_Connection as String = "server = 'XXXX'; user id = 'XX';
password = 'XX'; database = 'XXXX'"
Dim db_Connection as System.Data.IDbConnection = New
System.Data.SqlClient.SqlConnection(str_Connection)

Dim str_SQLQuery as String = "SELECT [PA01101].[PACONTNUMBER] FROM
[PA01101]"
dim db_Command as System.Data.IDbCommand = New
System.Data.SqlClient.SqlCommand
db_Command.CommandText = str_SQLQuery
db_Command.Connection = db_Connection

db_Connection.Open
Dim Rdr_DataReader as System.Data.IDataReader =
db_Command.ExecuteReader(System.Data.CommandBehavior.CloseConnection)

ListBox1.DataSource = Rdr_Datareader
ListBox1.DataBind()


End Sub

Sub ListBox1_SelectedIndexChanged(sender As Object, e As EventArgs)

response.write(ListBox1.SelectedItem.value)

End Sub

</script>
<html>
<head>
</head>
<body>
<form runat="server">
<p>
<asp:ListBox id="ListBox1" runat="server"
DATATEXTFIELD="PACONTNUMBER" DATAVALUEFIELD="PACONTNUMBER" Height="159px"
Width="305px" OnSelectedIndexChanged="ListBox1_SelectedIndexChanged"
AUTOPOSTBACK="True"></asp:ListBox>
</p>
<p>
<asp:Label id="Label1" runat="server">Label</asp:Label>
</p>
<p>
&nbsp;
</p>
</form>
</body>
</html>
 
Wrap your databinding in Page_Load in an

If IsPostBack Then

End If

and it should work. You only need to bind the data for the first time
the page is loaded. Postbacks cause Page_Load to happen, but by then
your items are in the ViewState, and don't have to be bound again.

See, you're posting back, and re-assigning data to the object. This
wipes out anything that was posted back to the server, causing there to
be no item selected.
 
Mike,

Thanks for your help. I'm starting to understand this stuff now.

Mike Newton said:
Wrap your databinding in Page_Load in an

If IsPostBack Then

End If

and it should work. You only need to bind the data for the first time
the page is loaded. Postbacks cause Page_Load to happen, but by then
your items are in the ViewState, and don't have to be bound again.

See, you're posting back, and re-assigning data to the object. This
wipes out anything that was posted back to the server, causing there to
be no item selected.
Greetings again,

I have a Listbox that is populated from a SQL Statement. I am attempting to
get the value of the selected item with the following code:

Sub ListBox1_SelectedIndexChanged(sender As Object, e As EventArgs)
response.write(ListBox1.SelectedItem.value)
End Sub

I have the datatext and datavalue fields both set to the same field in my
listbox (Just a list of Job numbers) and I have autopostback set to "True."

The issue is that when I select an item from the listbox, I get an error
message stating:
Object reference not set to an instance of an object.
Below is my code in entirety for further reference if needed. Thank you.
<%@ Page Language="VB" %>
<%@ Register TagPrefix="wmx" Namespace="Microsoft.Matrix.Framework.Web.UI"
Assembly="Microsoft.Matrix.Framework, Version=0.6.0.0, Culture=neutral,
PublicKeyToken=6f763c9966660626" %>
<%@ Register TagPrefix="n0" Namespace="MyListView" Assembly="MyListView" %>
<script runat="server">

Sub Page_Load (Src as Object, E As EventArgs)

Dim str_Connection as String = "server = 'XXXX'; user id = 'XX';
password = 'XX'; database = 'XXXX'"
Dim db_Connection as System.Data.IDbConnection = New
System.Data.SqlClient.SqlConnection(str_Connection)

Dim str_SQLQuery as String = "SELECT [PA01101].[PACONTNUMBER] FROM
[PA01101]"
dim db_Command as System.Data.IDbCommand = New
System.Data.SqlClient.SqlCommand
db_Command.CommandText = str_SQLQuery
db_Command.Connection = db_Connection

db_Connection.Open
Dim Rdr_DataReader as System.Data.IDataReader =
db_Command.ExecuteReader(System.Data.CommandBehavior.CloseConnection)

ListBox1.DataSource = Rdr_Datareader
ListBox1.DataBind()


End Sub

Sub ListBox1_SelectedIndexChanged(sender As Object, e As EventArgs)

response.write(ListBox1.SelectedItem.value)

End Sub

</script>
<html>
<head>
</head>
<body>
<form runat="server">
<p>
<asp:ListBox id="ListBox1" runat="server"
DATATEXTFIELD="PACONTNUMBER" DATAVALUEFIELD="PACONTNUMBER" Height="159px"
Width="305px" OnSelectedIndexChanged="ListBox1_SelectedIndexChanged"
AUTOPOSTBACK="True"></asp:ListBox>
</p>
<p>
<asp:Label id="Label1" runat="server">Label</asp:Label>
</p>
<p>
&nbsp;
</p>
</form>
</body>
</html>
Click to expand...
Click to expand...
 

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.

Ask a Question

Similar Threads

problem with multiple values of Listbox 9
[LISTBOX] AutoPostBack not fired with EnableViewState = False 2
Binding data to DropDownList... 5
ListBox and PostBackUrl event 3
Selecting Multiple Selection Listbox in a TemplateField? 1
ListBox selected item 6
Postback problem? 1
ListBox? 2

Back
Top