Can't populate listbox

Matt Funke

Here's the relevant C#:

List emailChoices = new List();

string emailPermissionsTable = "EmailPermissions";
sqlConn.ConnectionString = sqlConnStr;
sqlConn.Open();
sqlComm.Connection = sqlConn;
sqlComm.CommandText = "SELECT * FROM " + emailPermissionsTable + " WHERE Username = '" + user + "';";
sqlDr = sqlComm.ExecuteReader();

emailChoices.Clear();
while (sqlDr.Read())
{
string emailAddy = sqlDr["EmailRecipient"].ToString();
emailChoices.Add(emailAddy);
}

emailListBox.DataSource = emailChoices;
emailListBox.DataBind();

... and the relevant code-behind:

I can verify that the emailChoices list is being populated correctly. Why aren't the list elements showing up in the listbox? What have I missed? (Even if I look at the page source, it doesn't seem that the list is being loaded.)

7 Replies

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Suthish Nair15y ago
Accepted answer
just re-write the code as below..

Why you need an extra variable..

while (sqlDr.Read())
{
emailListBox.Items.Add(sqlDr["EmailRecipient"].ToString());
}
Matt Funke15y ago
Suthish: Why you need an extra variable..

I ended up removing the extra variable, thanks, exactly the way you suggested. There seems to be a deeper problem that I don't have time to trace going on here... in the meantime, though, I'll just play with ListBox.Items as if it were a list of strings. It seems to be doing the job quite nicely.
Matt Funke15y ago
Pradeep: I am able to see correct output with below code, I don't see any issue. It must be something else which is breaking. Are you getting some error?

No errors. Sorry. And I can put this code into a brand-new project and have it work, so something deeper must be going on.
Sam Hobbs15y ago
I can think of two possibilities. One is that there is some kind of post-back problem osr something like that, but I am not familiar with ASP enough to help with that.

Another relatively common beginner mistake is that you might have more than one ListBox and the one that you put data into is not the same one that you see.

Or maybe something is happening to emailChoices. During execution, after the page is built and you see nothing in the ListBox, if you can, look at emailListBox.DataSource to see what it has.
Pradeep Chandraker15y ago
I am able to see correct output with below code, I don't see any issue. It must be something else which is breaking. Are you getting some error?

emailListBox.Items.Clear();
emailListBox.Items.Add("AAAAA");
emailListBox.Items.Add("BBBBB");
emailListBox.Items.Add("CCCCC");

emailListBox.DataBind();
Matt Funke15y ago
Thanks, Pradeep. I can verify that your code shows a populated listbox when I run it.

[EDIT]:
All right... here's something weird.

If I remove the SQL block, ignore the list entirely, and replace it with this:

emailListBox.Items.Clear();
emailListBox.Items.Add("AAAAA");
emailListBox.Items.Add("BBBBB");
emailListBox.Items.Add("CCCCC");

emailListBox.DataBind();

... nothing shows up in the listbox. But if I put a breakpoint after the DataBind() invocation, then use the Visual Studio IDE to investigate the emailListBox object, look at "Non-Public members", then "base", then "items", then "Non-Public members", then "listItems", I can see that the object seems to be populated correctly (the list items are in there). Does this yield any insight?
Pradeep Chandraker15y ago
If the values are correctly populated in emailChoices variable I don't see any issue with binding. It should work perfectly.

Can you simply try this? just for testing.

List emailChoices = new List();

emailChoices.Add("AAAA");
emailChoices.Add("BBBB");
emailChoices.Add("CCCC");
emailListBox.DataSource = emailChoices;
emailListBox.DataBind();

Let me know if you still see issue.