Question

i have a long running function¹:

public string FindPasswordFromHash(String hash) {     ... } 

which is called like:

private void Button1_Click(object sender, EventArgs e) {     PasswordTextBox.Text = FindPasswordFromHash(HashTextBox.Text); } 

Now i want to convert it into the asynchronous BeginInvoke/EndInvoke delegate pattern:

private void Button1_Click(object sender, EventArgs e) {    MyAsyncDelegate asyncDelegate = new MyAsyncDelegate(HashTextBox.Text);    asyncDelegte.BeginInvoke(hash, CompleteCallback, null); }  private void CompleteCallback(IAsyncResult ar) {    MyAsyncDelegate asyncDelegate = ((AsyncResult)ar).AsyncDelegate;    PasswordTextBox.Text = asyncDelegate.EndInvoke(asyncResult); }  delegate string MyAsyncDelegate(String hash); 

Of course this doesn’t work because of a leaky abstraction of the way asynchronous delegates are implemented:

‘Cross-thread operation not valid: Control ‘PasswordTextBox’ accessed from a thread other than the thread it was created on.’

Given that the asynchronous delegate pattern was invented to convert long running operations into asynchronous operations – what is the proper technique to use BeginInvoke/EndInvoke as replacement for the synchronous call?

And more specifically, what is the method to force the callback to marshalled back to the calling thread?

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¹ Function name invented for example

Answer 1

You are handling the BeginInvoke() and EndInvoke() calls correctly. You just need to handle the fact that manipulating the GUI needs to be done on the GUI-thread.

Luckily the framework provides the Control.Invoke() method, which allows you to have code executed on the GUI thread.

I usually do something like this:

private void SetPasswordText(string password){   if(InvokeRequired){     MethodInvoker mi = () => SetPasswordText(password);     Invoke(mi);     return;   }   PasswordTextBox.Text = password; } 

For this particular case, you could also just do

private void RecoveryCompleteCallback(IAsyncResult ar) {    MyAsyncDelegate asyncDelegate = ((AsyncResult)ar).AsyncDelegate;    string password = asyncDelegate.EndInvoke(asyncResult);    Invoke(()=>{PasswordTextBox.Text = password;}); } 

If you using C# 2.0, you would do:

MethodInvoker mi = delegate(){ SetPasswordText(password); }; Invoke(mi); 

or

Invoke(delegate(){PasswordTextBox.Text = password;});