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

Question

I get the following exception thrown:

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

This is my code:

if (InvokeRequired)
{
    BeginInvoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
}
else
    Invoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);

I found pages about this topic on this site but I don't know what is wrong.

Answer 1

The difference between Invoke and BeginInvoke is that the former is synchronous (waits for completion) while the later is asynchronous (sort of fire-and-forget). However, both work by posting a message to the UI message loop which will cause the delegate to be executed when it gets to that message.

The InvokeRequired property determines whether you need to Invoke at all or if it is already on the correct thread, not whether you want synchronous or asynchronous calling. If InvokeRequired is false you are (in theory) already running on the UI thread and can simply perform synchronous actions directly (or still BeginInvoke if you need to fire them off asynchronously). This also means you can't use Invoke if InvokeRequired is false, because there's no way for the message loop on the current thread to continue. So that's one big problem with your code above, but not necessarily the error you're reporting. You can actually use BeginInvoke in either case, if you watch out for recursive invocation, and so on.

However, you can't use either one without a window handle. If the Form/Control has been instantiated but not initialized (ie. before it is first shown) it may not have a handle yet. And the handle gets cleared by Dispose(), such as after the Form is closed. In either case InvokeRequired will return false because it is not possible to invoke without a handle. You can check IsDisposed, and there is also a property IsHandleCreated which more specifically tests if the handle exists. Usually, if IsDisposed is true (or if IsHandleCreated is false) you want to punt into a special case such as simply dropping the action as not applicable.

So, the code you want is probably more like:

if (IsHandleCreated)
{
    // Always asynchronous, even on the UI thread already.  (Don't let it loop back here!)
    BeginInvoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
    return; // Fired-off asynchronously; let the current thread continue.

    // WriteToForm will be called on the UI thread at some point in the near future.
}
else
{
    // Handle the error case, or do nothing.
}

Or maybe:

if (IsHandleCreated)
{
    // Always synchronous.  (But you must watch out for cross-threading deadlocks!)
    if (InvokeRequired)
        Invoke(new UpdateTextFieldDelegate(WriteToForm), finished, numCount);
    else
        WriteToForm(finished, numCount); // Call the method (or delegate) directly.

    // Execution continues from here only once WriteToForm has completed and returned.
}
else
{
    // Handle the error case, or do nothing.
}