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ComboBox initialization/reinitialization fails with NullReferenceException #2432

M-Lipin opened this issue Nov 26, 2019 · 0 comments · Fixed by #2449


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@M-Lipin M-Lipin commented Nov 26, 2019

  • .NET Core Version: 3.0
  • Have you experienced this same bug with .NET Framework?: Yes

Problem description:
The issue is that the multi-form desktop application or application with complex control appearance behavior may fail with the error caused by NullReference which has ComboBox's origin. ComboBox initializes successfully and the issue does not reproduce in case this is shown initially on the forms and is not recreated while app lifecycle.

Actual behavior:
NullReferenceException and application crash:

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Windows.Forms.ComboBox.WmParentNotify(Message& m)
   at System.Windows.Forms.ComboBox.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Expected behavior:
No error.

Minimal repro:
Create the app with hidden dialog form and ComboBox on this form.
Start the app, trigger dialog form initialization and reinitialization without dialog showing.
Observe the error.

DevDiv issue:

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