CODEWITHSUNDEEP
winformssilverlightasynchronousthread-safety
montelof
montelof

Reputation: 501

Why this Asynchronous example works without Dispatcher nor Control.BeginInvoke?

I was testing an Asynchronous example I wrote in other post, I modified it to show some info in a textbox. what happened next I was not expecting. I don't know why it does not throw an exception when modifying a control from another thread. am I blind or why I don't see it?

here is the example, it works the same for silverlight and WinForms:

    int rand=0;
    public MainPage()
    {
        InitializeComponent();
    }

    public Func<Action<int, int>, Action<int>> DownloadDataInBackground = (callback) =>
    {
        return (c) =>
        {
            WebClient client = new WebClient();
            Uri uri = new Uri(string.Format("https://www.google.com/search?q={0}", c));
            client.DownloadStringCompleted += (s, e2) =>
            {
                callback(c, e2.Result.Length);
            };
            client.DownloadStringAsync(uri);
        };
    };

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        int callid = rand++;
        Debug.WriteLine("Executing CallID #{0}", callid);
        DownloadDataInBackground((c3, r3) =>this.textBox1.Text+=string.Format("The result for the callid {0} is {1} \n", c3, r3))(callid);
    }

Tap the button pretty fast, it wont fail.

your help will be very appreciated.

Edit: added picture showing that windows forms always execute controls modification from the main thread, but, why if it is supposed to be another one? winfors mainthread

Upvotes: 1

Views: 158

Answers (1)

AnthonyWJones
AnthonyWJones

Reputation: 189439

The actual answer to why your code doesn't fail in the way you expect it to is that the WebClient invokes its events on the UI thread. Hence you aren't modifying your control on a different thread as you seem to imagine you are.

Upvotes: 1

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