How can I fix "Cannot open clipboard: Access Denied" errors?

user_63116 0
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I am using the following code to copy text to the clipboard:

  Clipboard.Open;
  try
    Clipboard.AsText := GenerateClipboardText;
  finally
    Clipboard.Close;
  end;

Seemingly at random I get "Cannot open clipboard: Access Denied" errors. I'm guessing that these errors are caused by other application locking the clipboard, but I never seem to be doing anything with other applications that should cause the locks.

Strangely my users seem to be reporting more of the errors with Vista and Windows 7 than with XP.

Is there a way to check if the clipboard is locked before trying to access it?

delphi-2010 copy-paste clipboard delphi
6 Answers

Strangely my users seem to be reporting more of the errors with Vista and Windows 7 than with XP

This may have to do with how Vista/Win7 deal with clipboard viewer notification. While they still support the XP "clipboard viewer chain", which sends one notification message that must be re-sent to each listener in turn (and if one app fails to do this, the other apps aren't notified). Starting with Vista, apps are notified directly. And there's nothing to keep them from trying to access the clipboard all at once.

Analogy: I have 3 children. I have a cake. With XP rules, I tell the oldest child to have some cake, then tell the next oldest child to have a slice. She gets her slice, tells her brother, he gets his, and tells his brother, who gets his, and everything proceeds in an orderly fashion.
Problem: The middle child takes the cake to his room, doesn't tell the youngest, and the youngest misses out.

With Vista/Windows7, that system still exists. But newer apps can request to be notified immediately, by me, as soon as the cake arrives in the kitchen. I shout "cake is ready!" and they all show up at the same time and try to grab some. But there's only one serving knife, so they have to keep reaching for the knife, failing to get it, and waiting for the next opportunity.

user_2891350

First of all please notice that this probably isn't a problem in your application. Other applications locked the clipboard or messed up the notification chain and now your application fails to access it. When I do have problems like this I restart the computer and they magically go away... well... at least until I run again the application that creates the problem.

This code (not checked in Delphi) may help you. It won't fix the problem is the notification chain is broken (nothing except a PC restart will ever fix it) but it will fix the problem if an application is locking the clipboard for a while. Increase the MaxRetries if that pesky application keeps the clipboard locked for A REALLY LONG time (seconds):

procedure Str2Clipboard(CONST Str: string; iDelayMs: integer);
CONST
   MaxRetries= 5;
VAR RetryCount: Integer;
begin
 RetryCount:= 0;
 for RetryCount:= 1 to MaxRetries DO
  TRY
    inc(RetryCount);
    Clipboard.AsText:= Str;
    Break;
  EXCEPT
    on Exception DO
      if RetryCount = MaxRetries
      then RAISE Exception.Create('Cannot set clipboard')
      else Sleep(iDelayMs)
  END;
end;

Also, it may be a good idea to drop the 'raise' and convert it to a function and use it like this:

if not Str2Clipboard 
then Log.AddMsg('Dear user, other applications are blocking the clipboard. We have tried. We really did. But it didn''t work. Try again in a few seconds.');
user_462070
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