How to handle a blocked clipboard and other oddities

user_74302 0
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Over the course of the last couple of hours I have been tracking down a fairly specific bug with that occurs because another application has the clipboard open. Essentially as the clipboard is a shared resource (as per "Why does my shared clipboard not work?") and you attempt to execute

Clipboard.SetText(string)

or

Clipboard.Clear().

The following exception is thrown:

System.Runtime.InteropServices.ExternalException: Requested Clipboard operation did not succeed. 
    at System.Windows.Forms.Clipboard.ThrowIfFailed(Int32 hr)
    at System.Windows.Forms.Clipboard.SetDataObject(Object data, Boolean copy, Int32 retryTimes, Int32 retryDelay)
    at System.Windows.Forms.Clipboard.SetText(String text, TextDataFormat format)
    at System.Windows.Forms.Clipboard.SetText(String text)

My initial solution was to retry after a short pause, until I realised that Clipboard.SetDataObject has fields for the number of times and the length of the delay. .NET's default behaviour is to try 10 times with a 100 msec delay.

There is one final thing that has been noted by the end user. That is, despite the exception being thrown, the copy to clipboard operation still works. I haven't been able to find any further information about why this may be.

My current solution to the issue is just to silently ignore the exception... is this really the best way?

clipboard .net c#
8 Answers

As the clipboard is shared by all UI applications, you will run into this from time to time. Obviously, you don't want your application to crash if it failed to write to the clipboard, so gracefully handling ExternalException is reasonable. I would suggest presenting an error to the user if the SetObjectData call to write to the clipboard fails.

A suggestion would be to use (via P/Invoke) user32!GetOpenClipboardWindow to see if another application has the clipboard open. It will return the HWND of the window which has the clipboard open, or IntPtr.Zero if no application had it open. You could spin on the value until its IntPtr.Zero for a specified amount of time.

Phil_Price_565010

I've actually come up with my own solution and it seems to be working for me.

// This allows the clipboard to have something copied to it.
    public static void ClipboardPaste(String pasteString)
    {
        // This clears the clipboard
        Clipboard.Clear();

        // This is a "Try" of the statement inside {}, if it fails it goes to "Catch"
        // If it "Catches" an exception. Then the function will retry itself.
        try
        {
            // This, per some suggestions I found is a half second second wait before another
            // clipboard clear and before setting the clipboard
            System.Threading.Thread.Sleep(500);
            Clipboard.Clear();
            // This is, instead of using SetText another method to put something into
            // the clipboard, it includes a retry/fail set up with a delay
            // It retries 5 times with 250 milliseconds (0.25 second) between each retry.
            Clipboard.SetDataObject(pasteString, false, 5, 250);
        }
        catch (Exception)
        {
            ClipboardPaste(pasteString);
        }
    }

This is obviously C#, however these methods are exposed to all Visual Studios. I have obviously created a looping function, as well as attempted to force it into the clipboard with retries.

Essentially here's the flow. Let's say you want to place the word clipboard into the clipboard, anywhere in your code (assuming this function is defined).

  1. Call function ClipboardPaste("Clipboard");
  2. It will then clear the clipboard
  3. Then it will "try" to put your string into the clipboard.
  4. First it waits half a second (500 milliseconds)
  5. Clears the clipboard again
  6. Then it tries to put the string into the clipboard using SetDataObject
  7. SetDataObject if it fails will retry up to five times, with a 250 millisecond delay in between each retry.
  8. If the initial attempt fails, it catches the exception, the crash, then it tries it all over again.

Yes, this does have a flaw if you know your clipboard will always have an exception no matter what (infinite loop). However I have not run into an infinite loop with this method yet. The other flaw is that it can take a couple of seconds (essentially slowing down your applications) before it will work, while it's attempting it may freeze your application, once it succeeds the application will continue anyway.

user_12486610
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