Reputation: 373
I am new to Async and Await and have created a simple project in order to understand how it works. For this, I have a simple Windows Form application that has 2 elements:
When I click the button, it should display all completed Items in the TextBox. This is the code I have written:
private async void btnGetCompletedItems_Click(object sender, EventArgs e)
{
QueueSystem queueSystem = QueueSystem.NewInstance(75);
Stopwatch watch = new Stopwatch();
watch.Start();
await Task.Run(() => GetCompletedItems(queueSystem));
watch.Stop();
lblTime.Text = $"{watch.ElapsedMilliseconds.ToString()} ms";
}
private void GetCompletedItems(QueueSystem queueSystem)
{
foreach (var item in queueSystem.GetCompletedItems())
{
txtItems.Text += $"{txtItems.Text}{item.ItemKey}{Environment.NewLine}";
}
}
However, I am getting an error in
txtItems.Text += $"{txtItems.Text}{item.ItemKey}{Environment.NewLine}";
The error says
Additional information: Cross-thread operation not valid: Control 'txtItems' accessed from a thread other than the thread it was created on.
I checked in Debug and a new thread was created for GetCompletedItems(). When I read about Async and Await, I read that it doesn't necessarily create a new thread but it seems to have created a new one for some reason.
Is my implementation and understanding of Async and Await wrong? Is it possible to use Async and Await in a Windows Forms application?
Upvotes: 7
Views: 5351
Reputation: 37770
When I read about Async and Await, I read that it doesn't necessarily create a new thread
This is true for regular async methods. Consider this:
private async void button1_Click(object sender, EventArgs e)
{
Trace.WriteLine(Thread.CurrentThread.ManagedThreadId);
await DoesNothing();
}
private async Task DoesNothing()
{
// outputs the same thread id as similar line as from above;
// in particlar, for WinForms this means, that at this point
// we are still at UI thread
Trace.WriteLine(Thread.CurrentThread.ManagedThreadId);
await Task.Delay(1);
}
but it seems to have created a new one for some reason
This is what Task.Run
is intended for:
Queues the specified work to run on the ThreadPool
In other words, it pushes anything you pass it as a delegate to a thread pool thread. Since we are in WinForms, this means, that anonymous method () => GetCompletedItems(queueSystem)
will be executed at thread pool thread, not at UI one.
Here's code sample from above with little change:
private async void button1_Click(object sender, EventArgs e)
{
Trace.WriteLine(Thread.CurrentThread.ManagedThreadId);
await Task.Run(DoesNothing);
}
private async Task DoesNothing()
{
// outputs DIFFERENT thread id;
// in particlar, for WinForms this means, that at this point
// we are not at UI thread, and we CANNOT access controls directly
Trace.WriteLine(Thread.CurrentThread.ManagedThreadId);
await Task.Delay(1);
}
Upvotes: 3
Reputation: 162
You can access the thread from another thread in a following way. It does helps to avoid the cross thread exception in your application.
private void Thread()
{
this.Invoke((System.Action)(() => {
//your thread call or definition
});
}
Upvotes: 2
Reputation: 126
You cannot access UI thread on a different thread. This should help
private async void btnGetCompletedItems_Click(object sender, EventArgs e)
{
QueueSystem queueSystem = QueueSystem.NewInstance(75);
Stopwatch watch = new Stopwatch();
watch.Start();
var results = await Task.Run(() => queueSystem.GetCompletedItems());
foreach (var item in results)
{
txtItems.Text += $"{txtItems.Text}{item.ItemKey}{Environment.NewLine}";
}
watch.Stop();
lblTime.Text = $"{watch.ElapsedMilliseconds.ToString()} ms";
}
Upvotes: 8