CODEWITHSUNDEEP
c#asp.netmultithreadingwebforms
UpQuark
UpQuark

Reputation: 801

How do you update ASP.NET web forms from different threads?

In the last few days I've been trying to learn how to use ASP.NET Web Forms together with multithreading the hard way by building a simple applet using both and I've been struggling with aspects of interactions between different threads and the UI.

I've resolved some multithreading issues in some other questions (and also learned after waaaaaay too long that web forms and WPF are not the same thing) but now I'm running into trouble finding the best way to update UI elements based on data acquired in multiple threads.

Here's my code:

Default.aspx

public partial class _Default : System.Web.UI.Page
{
    private NlSearch _search;
    private static int _counter = 0;
    private static SortedList<long, SearchResult> resultsList = new SortedList<long, SearchResult>();

    protected void Page_Load(object sender, EventArgs e)
    {
        _search = new NlSearch();
    }

    protected void AddSearchMethod(object sender, EventArgs e)
    {
        var text = SearchForm.Text;
        new Task(() => MakeRequest(text));
    }

    protected void UpdateMethod(object sender, EventArgs e)
    {
        resultsLabel.Text = "";
        foreach (var v in resultsList.Values)
        {
            resultsLabel.Text += v.SearchTerm + ": " + v.Count + " occurances<br/>";
        }
    }

    protected void ClearSearchMethod(object sender, EventArgs e)
    {
        resultsLabel.Text = "";
        resultsList.Clear();
    }

    protected void MakeRequest(string text)
    {
        _counter++;
        SearchResult s = new SearchResult
        {
            SearchTerm = text,
            Count = _search.MakeRequests(text)
        };
        resultsList.Add(_counter, s);
    }
}

I've tried quite a few versions of the same basic thing. NlSearch.MakeRequest (called by MakeRequests) sends an HTTP POST request to an outside web site imitating a search bar input, and then extracts an integer from the markup indicating how many results came back.

The current simple UI revolves around a SearchForm textfield, an "Add Search" button, an "Update Label" button a "Clear Search" method, and a ResultsLabel that displays results. The AddSearch button creates a new task that calls MakeRequest, which calls the method to send the HTTP request and then stores the results in the order they were sent in a static sorted list.

So now ideally in a good UI I would like to just update the label every time a thread returns, however I've tried using ContinueWhenAll and a few other task functions and the problem seems to be that other threads do not have the ability to change the UI.

I have also tried running a new thread on page load that updates the label every few seconds, but this likewise failed.

Because I haven't been able to implement this correctly, I've had to use the "Update Label" button which literally just tells the label to display what's currently in the static list. I would really like to get rid of this button but I can't figuer out how to get my threads to make UI changes.

Upvotes: 1

Views: 3879

Answers (4)

jwheron
jwheron

Reputation: 2562

You mentioned WPF in your question -- are you perhaps instead looking for a Windows application, like WinForms? I think that perhaps the term "web forms" has confused the situation. Web forms are just webpages with some (okay, a lot) of added in Microsoft functionality.

It sounds like you're trying to send updates to a webpage from a thread in code. The web doesn't work that way. I'd suggest reading through the ASP.NET Page Life Cycle Overview if you're actually trying to design webpages. Other answers have suggested AJAX functionality (which is where the web page executes some JavaScript that goes out and talks to a web server).

Upvotes: 2

chris
chris

Reputation: 37440

In general, trying to do threading in a web app is a bad idea. Web servers are designed for this, but spinning off new threads or processes should be avoided if at all possible. While there used to be a mechanism (and maybe there still is) to "push" results to a client, there are better solutions available today.

What you're describing is exactly the problem that AJAX is intended to solve.

Upvotes: 2

Kees
Kees

Reputation: 1418

If you want to run your code asynchonous you may want to use the Async Await keywords instead of managing threads yourself. See information about Asynchronous Programming with Async and Await

Do not let your threads get tangled up ;)

Upvotes: 0

mtzaldo
mtzaldo

Reputation: 105

Have you ever hear about AJAX before? I think you're a thinking as application dev instead of web dev.

Upvotes: 0

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