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POObjectDisposedException on call to Form's Invoke when it hasn't been Disposed
 

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  1. POObjectDisposedException on call to Form's Invoke when it hasn't been Disposed
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    <p>We get an <code>ObjectDisposedException</code> from a call to <code>Invoke</code> on a Form that hasn't yet been disposed. Here's some sample code demonstrating the problem:</p> <pre><code>public partial class Form2 : Form { void Form2_Load(object sender, EventArgs e) { // Start a task that does an Invoke on this control Task.Factory.StartNew(TaskWork); // Sleep here long enough to allow the task that does the Invoke // to execute to the point where it has: // a. Posted the message and // b. is waiting Thread.Sleep(500); // Cause ShowDialog to return by setting the DialogResult DialogResult = DialogResult.OK; } void TaskWork() { // This call doesn't return, but instead throws an ObjectDisposedException this.Invoke((MethodInvoker)(() =&gt; MessageBox.Show("Invoke succeeded"))); } } </code></pre> <p>Here's the calling code from Form1 (the main form) which I never close:</p> <pre><code>public partial class Form1 : Form { Form2 m_form2 = new Form2(); void Form1_Load(object sender, EventArgs e) { // Call ShowDialog, but don't dispose it. m_form2.ShowDialog(); // Cause the finalizers to run. This causes an AggregateException to be thrown // due to the unhandled ObjectDisposedException from the Task. GC.Collect(); } } </code></pre> <p>We dug in to the Microsoft source, and found the exception is created during the call to DestroyHandle (below). DestroyHandle is being called by ShowDialog as it is finishing.</p> <p>From source.NET\4\DEVDIV_TFS\Dev10\Releases\RTMRel\ndp\fx\src\WinForms\Managed\System\WinForms\Control.cs\1305376\Control.cs:</p> <pre><code>protected virtual void DestroyHandle() { // ... // If we're not recreating the handle, then any items in the thread callback list will // be orphaned. An orphaned item is bad, because it will cause the thread to never // wake up. So, we put exceptions into all these items and wake up all threads. // If we are recreating the handle, then we're fine because recreation will re-post // the thread callback message to the new handle for us. // if (!RecreatingHandle) { if (threadCallbackList != null) { lock (threadCallbackList) { Exception ex = new System.ObjectDisposedException(GetType().Name); while (threadCallbackList.Count &gt; 0) { ThreadMethodEntry entry = (ThreadMethodEntry)threadCallbackList.Dequeue(); entry.exception = ex; entry.Complete(); } } } } // ... } </code></pre> <h1>Questions:</h1> <ol> <li><p>Why is ShowDialog destroying the handle (when I might re-use this form)?</p></li> <li><p>Why am I getting an ObjectDisposedException--its very misleading (since its not disposed). Its as if the code expected the handle would be destroyed only when the object was disposed (which is what I was expecting).</p></li> <li><p>Should this be valid? That is, should I be allowed to Invoke on to a control after ShowDialog?</p></li> </ol> <h1>Notes:</h1> <ol> <li><p>Doing a second <code>.ShowDialog()</code> causes a new handle to be created.</p></li> <li><p>If after doing <code>.ShowDialog()</code> I try to do an <code>Invoke</code>, I get an InvalidOperationException stating that "Invoke or BeginInvoke cannot be called on a control until the window handle has been created."</p></li> <li><p>If after doing <code>.ShowDialog()</code> I access the <code>Handle</code> property, and then do an <code>Invoke</code>, it will succeed.</p></li> </ol>
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