Peter Ritchie's MVP Blog : WinForms

"Object is currently in use elsewhere" error.

PeterRitchie — Mon, 28 Jan 2008 17:04:00 GMT

I was debugging what I thought was a strange exception the other day. The exception was an InvalidOperationException and the message was "Object is currently in use elsewhere". Unless you're familiar with this exception, it really doesn't offer much as to why the exception is occurring. There seems to be several stale threads on the Web about this issue, so I'd thought I'd post about it.

As it turns out it had to do with some code that was PInvoking some native graphics functions and the interaction with the WinForm that was hosting the drawing surface was to blame.

What's really happening with "Object is currently in use elsewhere" is that GDI+ is complaining that the device context (DC) that it is trying to use is already "in use". With WinForms, this generally means there is a recursive Graphics.GetHdc occurring. GetHdc must match a ReleaseHdc before any other GetHdc. Recursive means you have something like GetHdc->GetHdc->ReleaseHdc->ReleaseHdc, instead of GetHdc->ReleaseHdc->GetHdc->ReleaseHdc. Another possibility is that there is a missing call to ReleaseHdc. (i.e. GetHdc->GetHdc->ReleaseHdc)

Now, in my case there was some seemingly innocuous code like this:

    SafeNativeMethods.DrawSomeStuff(e.Graphics.GetHdc(), parameters);
    e.Graphics.DrawString(text, this.Font, Brushes.Black, point);

...no matching ReleaseHdc(), before the DrawString call.

The fix turned out to be really simple:

    try
    {
        SafeNativeMethods.DrawSomeStuff(e.Graphics.GetHdc(), parameters);
    }
    finally 
    {
        e.Graphics.ReleaseHdc();
    }
    e.Graphics.DrawString(text, this.Font, Brushes.Black, point);

You can also encounter this exception if you're drawing to a form from multiple threads. You'll likely also be encountering a cross-threading exception as well. The solution in this case is to not use multiple threads when accessing a form, including drawing.

Changing TextBox Text as an Undo-able Action

PeterRitchie — Mon, 11 Sep 2006 01:23:00 GMT

The TextBox class supports undoing the last action--inherited from TextBoxBase. Normally the user does this by pressing the undo key (Ctrl-Z if your keyboard doesn't have a specific Undo key) or by selecting "Undo" from the context menu. The last action can also be undone programmatically by calling TextBoxBase.Undo() (after calling CanUndo() to see if Undo() will work).

Changing the text in a TextBox so that the change can be undone is not so obvious though. Changing the Text property or the SelectedText property is not undo-able. .NET 2.0 added TextBox.Paste(String) (not inherited from TextBoxBase, it's inherent to TextBox) that replaces the currently selected text and is undoable. If you want to replace all the text while allowing the user to undo it, simply call TextBoxBase.SelectAll() before Paste(Sting). For example:

textBox.SelectAll();
textBox.Paste(text);

This side-effect, unfortunately, is not documented--which is why it "is not so obvious".

Docking With Run-Time-Created Controls

PeterRitchie — Fri, 07 Jul 2006 13:33:00 GMT

How to Dock Fill a control created at run-time....(read more)