C# TaskFactory.StartNew changing DataSource causes System.InvalidOperationException "Access from another thread"

Question

I'm currently building an app that relies on DataGridView to receive updates via WAMP protocol, which involves working with new message events. My event handler looks like this:

 private async void NewMessage(object sender, MessageEventArgs e)
 {
      await Task.Factory.StartNew(() => DataHolder.TableSource.Add(new 
      CustomData(e.Name, e.Surname, 
      e.Whatever, e.WhoTheHellCares)));
 }

When such an event occurs, an exception is thrown: System.InvalidOperationException "An attempt to access control element which was created in a different thread".

DataHolder is a static class that exists within same namespace with the form class that has this event handler, DataHolder.TableSource is a BindingList<T> which is bound in Form.Load event to the DataGridView control created in Form1.Designer.cs .

I've read an answer to a related issue here, that mentioned await being able to automatically marshall something to UI thread if needed, but my wild and incompetent guess is that await does not recognize a databinding, so it must be told explicitly to do so, how though?

I need a .net 4.5 solution here or proof that tasks and awaits are unable to solve my problem. But I think they are more than able. Just that I have trouble to apply it to my own situation here.

UPDATE

Wow, what the hell.. Even when my handler looks like this, it still gives me the same exception.

 private void NewMessage(object sender, MessageEventArgs e)
 {
      DataHolder.TableSource.Add(new 
      CustomData(e.Name, e.Surname, 
      e.Whatever, e.WhoTheHellCares));
 }

I guess it has something to do with the class itself that is firing an event. Well.. I tried D:

UPDATE I've used debugging tools and here's the deal - the event itself is already nested in another thread. Will try to rewrite the source code for the library I'm using so that it will support progress reports. Wish me f`kin luck D:

Answer 1

You are misunderstanding async/await.

Your method will be divided in 2 parts: the before await and the after await. So when code runs it will execute like this:

1. before await
2. AWAIT IN ASYNC THREAD
3. execute your awaited method (what is inside Task.StartNew) in another thread^{(not necessarily)}
4. When step 3 is complete it will wakeup (marshal) the execution of step 2
5. run after await

So what is happening is that what is being marshaled to UI thread is the execution of the second part of your method not what is inside the task.

There is more to it than this, but I think this is the steps I used to understand it.

To solve this remove async completely as you are not doing a long process:

private void NewMessage(object sender, MessageEventArgs e)
{
     DataHolder.TableSource.Add(new CustomData(e.Name, e.Surname, 
     e.Whatever, e.WhoTheHellCares));
}

Or if you want to be async and you are in a WinForms control use BeginInvoke

private void NewMessage(object sender, MessageEventArgs e)
{
    this.BeginInvoke((Action)(() => DataHolder.TableSource.Add(new 
      CustomData(e.Name, e.Surname, 
      e.Whatever, e.WhoTheHellCares))));
}

Answer 2

An attempt to access control element which was created in a different thread

The issue is that your code is touching a UI element (or something that it relies on) from something other than the main UI thread. You must not do this.

I know that you don't want to use Invoke but it really is the way to solve this problem. These links (and hundreds like it) discuss it:

• How to update the GUI from another thread in C#?
• Can you access UI elements from another thread? (get not set)
• Cross-thread operation not valid: Control accessed from a thread other than the thread it was created on

Based on your comment, another approach you could consider is Looking for .NET 4.5 Progress<T> source code . Have a look at how it uses SynchronizationContext .