Invoke or BeginInvoke cannot be called on a control until the window handle has been created

Question

I have a SafeInvoke Control extension method similar to the one Greg D discusses here (minus the IsHandleCreated check).

I am calling it from a System.Windows.Forms.Form as follows:

public void Show(string text) {
    label.SafeInvoke(()=>label.Text = text);
    this.Show();
    this.Refresh();
}

Sometimes (this call can come from a variety of threads) this results in the following error:

System.InvalidOperationException occurred

Message= "Invoke or BeginInvoke cannot be called on a control until the window handle has been created."

Source= "System.Windows.Forms"

StackTrace:
at System.Windows.Forms.Control.MarshaledInvoke(Control caller, Delegate method, Object[] args, Boolean synchronous)
at System.Windows.Forms.Control.Invoke(Delegate method, Object[] args)
at System.Windows.Forms.Control.Invoke(Delegate method)
at DriverInterface2.UI.WinForms.Dialogs.FormExtensions.SafeInvoke[T](T control, Action`1 action) 
in C:\code\DriverInterface2\DriverInterface2.UI.WinForms\Dialogs\FormExtensions.cs:line 16

What is going on and how do I fix it? I know as much as it is not a problem of form creation, since sometimes it will work once and fail the next time so what could the problem be?

PS. I really really am awful at WinForms, does anyone know a good series of articles that explains the whole model and how to work with it?

Answer 1

It's possible that you're creating your controls on the wrong thread. Consider the following documentation from MSDN:

This means that InvokeRequired can return false if Invoke is not required (the call occurs on the same thread), or if the control was created on a different thread but the control's handle has not yet been created.

In the case where the control's handle has not yet been created, you should not simply call properties, methods, or events on the control. This might cause the control's handle to be created on the background thread, isolating the control on a thread without a message pump and making the application unstable.

You can protect against this case by also checking the value of IsHandleCreated when InvokeRequired returns false on a background thread. If the control handle has not yet been created, you must wait until it has been created before calling Invoke or BeginInvoke. Typically, this happens only if a background thread is created in the constructor of the primary form for the application (as in Application.Run(new MainForm()), before the form has been shown or Application.Run has been called.

Let's see what this means for you. (This would be easier to reason about if we saw your implementation of SafeInvoke also)

Assuming your implementation is identical to the referenced one with the exception of the check against IsHandleCreated, let's follow the logic:

public static void SafeInvoke(this Control uiElement, Action updater, bool forceSynchronous)
{
    if (uiElement == null)
    {
        throw new ArgumentNullException("uiElement");
    }

    if (uiElement.InvokeRequired)
    {
        if (forceSynchronous)
        {
            uiElement.Invoke((Action)delegate { SafeInvoke(uiElement, updater, forceSynchronous); });
        }
        else
        {
            uiElement.BeginInvoke((Action)delegate { SafeInvoke(uiElement, updater, forceSynchronous); });
        }
    }
    else
    {    
        if (uiElement.IsDisposed)
        {
            throw new ObjectDisposedException("Control is already disposed.");
        }

        updater();
    }
}

Consider the case where we're calling SafeInvoke from the non-gui thread for a control whose handle has not been created.

uiElement is not null, so we check uiElement.InvokeRequired. Per the MSDN docs (bolded) InvokeRequired will return false because, even though it was created on a different thread, the handle hasn't been created! This sends us to the else condition where we check IsDisposed or immediately proceed to call the submitted action... from the background thread!

At this point, all bets are off re: that control because its handle has been created on a thread that doesn't have a message pump for it, as mentioned in the second paragraph. Perhaps this is the case you're encountering?

Answer 2

I found the InvokeRequired not reliable, so I simply use

if (!this.IsHandleCreated)
{
    this.CreateHandle();
}

Answer 3

Here is my answer to a similar question:

I think (not yet entirely sure) that this is because InvokeRequired will always return false if the control has not yet been loaded/shown. I have done a workaround which seems to work for the moment, which is to simple reference the handle of the associated control in its creator, like so:

var x = this.Handle; 

(See http://ikriv.com/en/prog/info/dotnet/MysteriousHang.html)

Answer 4

The method in the post you link to calls Invoke/BeginInvoke before checking if the control's handle has been created in the case where it's being called from a thread that didn't create the control.

So you'll get the exception when your method is called from a thread other than the one that created the control. This can happen from remoting events or queued work user items...

EDIT

If you check InvokeRequired and HandleCreated before calling invoke you shouldn't get that exception.

Answer 5

If you're going to use a Control from another thread before showing or doing other things with the Control, consider forcing the creation of its handle within the constructor. This is done using the CreateHandle function.

In a multi-threaded project, where the "controller" logic isn't in a WinForm, this function is instrumental in Control constructors for avoiding this error.

Answer 6

Add this before you call method invoke:

while (!this.IsHandleCreated) 
   System.Threading.Thread.Sleep(100)