How do I make my method to populate listbox?

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I have a listbox that I want to be populated when page loads. The listbox will be populated with names of people. These names are associated each person's email. My method is not working. When page loads the listbox is empty.

When I hover mouse over object staffEmails I can see count = 16. there are 16 names and the emails. When I expand the staffEmails I see [0] {FTACaseReseting.Models.GetRequestorInfoModel}. When I expand this I see DisplayName "Jim, Doe" Email "Jim.Doe@gmail.com". However my listbox is not populated with this name or other names.

I am not sure how to change my method so the listbox is populated with the names. Please help

Here is my method.

What I have tried:

C#

public async void PopulateAdditionalStaffEmailListBox()
        {
            try
            {
                List<GetRequestorInfoModel> staffEmails = new List<GetRequestorInfoModel>();
                staffEmails = await FTACaseReseting.Controllers.RequestorInfoController.GetAllRequestorInfoes();
                staffEmails = staffEmails.OrderBy(x => x.DisplayName).ToList();
                AdditionalStaffEmailListBox.DataSource = staffEmails;

                AdditionalStaffEmailListBox.DataSource = "DisplayName";
                AdditionalStaffEmailListBox.DataValueField = "Email";
                //Clear all selections in the listbox after populating
                AdditionalStaffEmailListBox.ClearSelection();
            }
            catch (Exception ex)
            {
                string errorMsg = string.Format("An error has occured in {0}. \nException:\n{1}", "AdditionalStaffEmailListBox()", ex.Message);
                Response.Write("<script>alert('" + errorMsg + "')</script>");
            }
        }

Posted 19-Dec-19 4:00am

Member 11403304

Updated 19-Dec-19 5:10am

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3 solutions

Solution 1

C#

AdditionalStaffEmailListBox.DataSource = staffEmails;

AdditionalStaffEmailListBox.DataSource = "DisplayName";

That second statement is destroying the data source list.

Posted 19-Dec-19 4:13am

Richard MacCutchan

Solution 2

You could try to replace

C#

AdditionalStaffEmailListBox.DataSource = "DisplayName";

with

C#

AdditionalStaffEmailListBox.DataTextField = "DisplayName";

Posted 19-Dec-19 4:32am

phil.o

Updated 19-Dec-19 4:54am

v2

Comments

Member 11403304 19-Dec-19 10:45am

The only options available for AdditionalStaffEmailListBox. are Dispose, Disposed SupportDisabledAttribute.

When I hover mouse over AdditionalStaffEmailListBox I see AdditionalStaffEmailListBox{System.Web.UI.WebControls.ListBox}

phil.o 19-Dec-19 10:53am

Please see my updated answer.

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Richard Deeming · Accepted Answer · 2019-12-19T05:11:00

Solution 3

Quote:

C#

public async void PopulateAdditionalStaffEmailListBox()

You've found one of the many reasons to avoid async void methods[^]. :)

Your Page_Load event handler calls PopulateAdditionalStaffEmailListBox. That method gets as far as the first await statement and returns.

The calling code has no way of knowing that the method has not completed yet, so it continues to run. The page is rendered and the content is sent back to the user - without any values in the list.

At some later point, the await completes, and the rest of the PopulateAdditionalStaffEmailListBox method runs. But it's too late - the response has already been sent back to the client. Anything you add to the list now will simply be thrown away.

There are a couple of ways to solve this. Both will require that you add Async="True" to your <%@ Page %> directive. And both will require that you change your PopulateAdditionalStaffEmailListBox method to return a Task.

Option 1:
Use the RegisterAsyncTask method[^]:

C#

public async Task PopulateAdditionalStaffEmailListBox() { ... }

public void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        Page.RegisterAsyncTask(new PageAsyncTask(PopulateAdditionalStaffEmailListBox));
    }
}

Option 2:
Use async model binding[^] (requires .NET 4.6 or later):

C#

public async Task<IEnumerable<GetRequestorInfoModel>> GetAdditionalStaffEmails()
{
    var staffEmails = await FTACaseReseting.Controllers.RequestorInfoController.GetAllRequestorInfoes();
    return staffEmails.OrderBy(x => x.DisplayName);
}

Markup:

ASP.NET

<asp:ListBox ID="AdditionalStaffEmailListBox" runat="server"
    SelectMethod="GetAdditionalStaffEmails"
    DataTextField="DisplayName"
    DataValueField="Email"
/>

Using Asynchronous Methods in ASP.NET 4.5 | Microsoft Docs[^]

Posted 19-Dec-19 5:11am

Richard Deeming

Updated 19-Dec-19 5:45am

v2

Comments

Member 11403304 19-Dec-19 11:24am

That is a great solution. But before I try it. I want you to see how my page is currently loaded. Here is that code. Maybe that will help you see what I am doing.
private void _Default_Load(object sender, EventArgs e)
{
this.PopulateAdditionalStaffEmailListBox();
}

Richard Deeming 19-Dec-19 11:29am

Yes, that's exactly as I thought it was. :)

Member 11403304 19-Dec-19 11:43am

After making the changes you suggested, the AdditionalStaffEmailListBox is populated. However the object is displayed instead of names. This is what is populated FTACaseReseting.Model.GetRequestorInfoModel

Richard Deeming 19-Dec-19 11:45am

You'll need to set the DataValueField and DataTextField in the markup. I've updated my answer.

Member 11403304 19-Dec-19 12:03pm

Yeeeah. That worked. Since I am in the process of developing a web application. You will probably see a lot of questions from me. I am curious if there was a way to send you an email with a question/s or post here and you will see it?

Richard Deeming 19-Dec-19 12:04pm

Post them on here - Ask a Question[^].

That way, other people can help as well. :)

Member 11403304 19-Dec-19 12:32pm

Now I would like to add the code I had before in try catch because I would like to catch any exception. When I added the try catch inside public async Task<ienumerable<getrequestorinfomodel>> GetAdditionalStaffEmails() method, I am getting a message '_Default.GetAdditionalStaffEmails()':not all code paths return a value. How to I solve this? I need a try catch

Richard Deeming 19-Dec-19 12:37pm

You'll need to return a default value from your catch block:

public async Task<IEnumerable<GetRequestorInfoModel>> GetAdditionalStaffEmails()
{
    try
    {
        var staffEmails = await FTACaseReseting.Controllers.RequestorInfoController.GetAllRequestorInfoes();
        return staffEmails.OrderBy(x => x.DisplayName);
    }
    catch (Exception ex)
    {
        string errorMsg = string.Format("An error has occured in {0}. \nException:\n{1}", "GetAdditionalStaffEmails()", ex.Message);
        Response.Write("<script>alert(" + HttpUtility.JavaScriptStringEncode(errorMsg, true) + ")</script>");
        return Enumerable.Empty<GetRequestorInfoModel>();
    }
}

NB: You'll need to encode the error message before you render it, in case it contains any characters which aren't valid as part of a JavaScript string.

Member 11403304 19-Dec-19 12:45pm

Thank you again Richard. That fixed my problem. I also understand why I need to do it this way i.e. encode the error message before rendering it. I am learning a lot as I continue to develop this web app

Member 11403304 19-Dec-19 13:13pm

I will post a new question because I am stuck on populating a RequestorDropDownList. A user will only be allowed to select one name from the list.