Run a function when a Task finishes

Question

I have want to run a function asynchronously to prevent UI freezing. Here the the button click event.

private void btnEncrypt_Click(object sender, EventArgs e)
{
    // Create new Vigenere object instant
    cryptor = new Vigenere(txtPassword.Text, txtBox.Text);

    // Run encryption async
    Task<string> T = new Task<string>(cryptor.Encrypt);
    T.Start();
}

Now I want the following function to be called when the Task T finishes with it's return value as a parameter like this:

private void Callback(string return_value)
{
    txtBox.Text = return_value

    // Some other stuff here
}

How to achieve this?

http://blogs.msdn.com/b/csharpfaq/archive/2012/06/26/understanding-a-simple-async-program.aspx — Alexei Levenkov, Dec 01 '15 at 17:33

score 16 · Answer 1 · answered Dec 01 '15 at 17:29

What you could do is use the ContinueWith(Action<Task>) method.

So you would have something like

Task<string> t = new Task<string>(cryptor.Encrypt);
T.Start();

Task continuationTask = t.ContinueWith((encryptTask) => {
    txtBox.Text = encryptTask.Result;
    ...
})

Effectively this just says do the action after the current task completes, completion could be successful running, faulting or exiting early due to cancellation. You would likely want to do some error handling to make sure you didn't try to use the result of a cancelled or faulted task.

But the problem is ContinueWith action doesn't execute on the main thread, it executes in the worker thread in my case! — Mohit Atray, May 17 '20 at 12:48

score 4 · Answer 2 · answered Dec 01 '15 at 17:41

First, add async to your button click event. Try:

private async void btnEncrypt_Click(object sender, EventArgs e)
{
     cryptor = new Vigenere(txtPassword.Text, txtBox.Text);
     await Task.Run(() => cryptor.Encrypt()).ContinueWith(result => { Callback(result.Result); });
}

score 1 · Answer 3 · answered Dec 01 '15 at 17:29

1

You want to use Task.Run and await on it. This will make the method execute on a thread-pool thread, and assign the returned string to the TextBox once it completes:

private async void btnEncrypt_Click(object sender, EventArgs e)
{
     cryptor = new Vigenere(txtPassword.Text, txtBox.Text);
     textBox.Text = await Task.Run(() => cryptor.Encrypt());
}

answered Dec 01 '15 at 17:29

Yuval Itzchakov

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I find that peculiar. What does `Encrypt` do? – Yuval Itzchakov Dec 01 '15 at 17:46
Encrypts the given text – Xbyte Finance Dec 01 '15 at 17:48
1

If this freezes the UI, than this doesn't seem like your actual code. Post a [mcve] of your problem. – Yuval Itzchakov Dec 01 '15 at 17:49
Iterates over the user's input char by char and encrypt it...If I replace that code with a `Thread.Sleep(10000)` the UI doesn't freeze. – Xbyte Finance Dec 01 '15 at 17:50

Run a function when a Task finishes

3 Answers3