NRE opening combobox dropdown menu

[Andrus]

SWF DataGridView contains combobox control.
Activating combobox and pressing F4 to open dropdown menu causes strange NRE
(see below).

When I enter some valid value to combobox, press tab, shift+tab , 44
dropdown menu opens OK.

How to fix ?

How to use VCSE 2008 with .net framework source code to find the line in

System.Windows. Forms.DataGridV iewComboBoxCell .ComboBox_DropD own()

which causes this error ?


Andrus.

System.NullRefe renceException was unhandled
Message="Object reference not set to an instance of an object."
Source="System. Windows.Forms"
StackTrace:
at
System.Windows. Forms.DataGridV iewComboBoxCell .ComboBox_DropD own(Object
sender, EventArgs e)
at System.EventHan dler.Invoke(Obj ect sender, EventArgs e)
at System.Windows. Forms.ComboBox. OnDropDown(Even tArgs e)
at myApp.myComboBo x.OnDropDown(Ev entArgs e) in myComboBox.cs:l ine 71
at System.Windows. Forms.ComboBox. WmReflectComman d(Message& m)
at System.Windows. Forms.ComboBox. WndProc(Message & m)
at
System.Windows. Forms.Control.C ontrolNativeWin dow.OnMessage(M essage& m)
at System.Windows. Forms.Control.C ontrolNativeWin dow.WndProc(Mes sage&
m)
at System.Windows. Forms.NativeWin dow.DebuggableC allback(IntPtr hWnd,
Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows. Forms.UnsafeNat iveMethods.Send Message(HandleR ef
hWnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at System.Windows. Forms.Control.S endMessage(Int3 2 msg, IntPtr wparam,
IntPtr lparam)
at System.Windows. Forms.Control.R eflectMessageIn ternal(IntPtr hWnd,
Message& m)
at System.Windows. Forms.Control.W mCommand(Messag e& m)
at System.Windows. Forms.Control.W ndProc(Message& m)
at System.Windows. Forms.Scrollabl eControl.WndPro c(Message& m)
at
System.Windows. Forms.Control.C ontrolNativeWin dow.OnMessage(M essage& m)
at System.Windows. Forms.Control.C ontrolNativeWin dow.WndProc(Mes sage&
m)
at System.Windows. Forms.NativeWin dow.DebuggableC allback(IntPtr hWnd,
Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows. Forms.UnsafeNat iveMethods.Call WindowProc(IntP tr
wndProc, IntPtr hWnd, Int32 msg, IntPtr wParam, IntPtr lParam)
at System.Windows. Forms.NativeWin dow.DefWndProc( Message& m)
at System.Windows. Forms.ComboBox. DefChildWndProc (Message& m)
at System.Windows. Forms.ComboBox. ChildWndProc(Me ssage& m)
at
System.Windows. Forms.ComboBox. ComboBoxChildNa tiveWindow.WndP roc(Message& m)
at System.Windows. Forms.NativeWin dow.DebuggableC allback(IntPtr hWnd,
Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows. Forms.UnsafeNat iveMethods.Disp atchMessageW(MS G&
msg)
at
System.Windows. Forms.Applicati on.ComponentMan ager.System.Win dows.Forms.Unsa feNativeMethods .IMsoComponentM anager.FPushMes sageLoop(Int32
dwComponentID, Int32 reason, Int32 pvLoopData)
at
System.Windows. Forms.Applicati on.ThreadContex t.RunMessageLoo pInner(Int32
reason, ApplicationCont ext context)
at
System.Windows. Forms.Applicati on.ThreadContex t.RunMessageLoo p(Int32 reason,
ApplicationCont ext context)
at System.Windows. Forms.Applicati on.Run(Form mainForm)
.....

[Andrus]

Re: NRE opening combobox dropdown menu

I was unable to reproduce your issue using a simple DataGridView and a

ComboBoxColumn. F4 worked just as expected. It does, however, appear
that
you have a custom ComboBox control in the grid and I would take a closer
look
at line 71 in myComboBox.cs.

line 71 is only line, base.OnDropDown (e) in OnDropDown()

protected override void OnDropDown(Syst em.EventArgs e) {
base.OnDropDown (e);

}

after I removed this line by creating empty procedure:

protected override void OnDropDown(Syst em.EventArgs e) {

}

NullReferenceEx ception disappears.

Any idea why ?

Andrus.

[=?Utf-8?B?TW9ydGVuIFdlbm5ldmlrIFtDIyBNVlBd?=]

Re: NRE opening combobox dropdown menu

"Andrus" wrote:

I was unable to reproduce your issue using a simple DataGridView and a
ComboBoxColumn. F4 worked just as expected. It does, however, appear
that
you have a custom ComboBox control in the grid and I would take a closer
look
at line 71 in myComboBox.cs.

>
line 71 is only line, base.OnDropDown (e) in OnDropDown()
>
protected override void OnDropDown(Syst em.EventArgs e) {
base.OnDropDown (e);
>
}
>
after I removed this line by creating empty procedure:
>
protected override void OnDropDown(Syst em.EventArgs e) {
>
}
>
NullReferenceEx ception disappears.
>
Any idea why ?
>
Andrus.
>
>

I couldn't say. If you have VS 2008 you could try stepping into the
base.OnDropDown (e) method and see what it does.

--
Happy Coding!
Morten Wennevik [C# MVP]