String clipboardData = (String)Clipboard.GetData(DataFormats.StringFormat);
                if (clipboardData != null)
                    if (Clipboard.ContainsText(TextDataFormat.UnicodeText))
                    {
                        Clipboard.Clear();
                        Clipboard.SetText(signer.SignXml(clipboardData).ToString()); Тут вылетает ExternalException
                    }

   в System.Windows.Forms.Clipboard.ThrowIfFailed(Int32 hr)
   в System.Windows.Forms.Clipboard.SetDataObject(Object data, Boolean copy, Int32 retryTimes, Int32 retryDelay)
   в System.Windows.Forms.Clipboard.SetText(String text, TextDataFormat format)
   в System.Windows.Forms.Clipboard.SetText(String text)
   в ClientKeysManager.MainForm.signDataToolStripMenuItem_Click(Object sender, EventArgs e) в D:\SourceSafeProjects\RemoteServer\ClientKeysManager\MainForm.cs:строка 123
   в System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e)
   в System.Windows.Forms.ToolStripMenuItem.OnClick(EventArgs e)
   в System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e)
   в System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e)
   в System.Windows.Forms.ToolStripItem.FireEventInteractive(EventArgs e, ToolStripItemEventType met)
   в System.Windows.Forms.ToolStripItem.FireEvent(EventArgs e, ToolStripItemEventType met)
   в System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea)
   в System.Windows.Forms.ToolStripDropDown.OnMouseUp(MouseEventArgs mea)
   в System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   в System.Windows.Forms.Control.WndProc(Message& m)
   в System.Windows.Forms.ScrollableControl.WndProc(Message& m)
   в System.Windows.Forms.ToolStrip.WndProc(Message& m)
   в System.Windows.Forms.ToolStripDropDown.WndProc(Message& m)
   в System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   в System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   в System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   в System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   в System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
   в System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   в System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   в System.Windows.Forms.Application.Run(Form mainForm)
   в ClientKeysManager.Program.Main() в D:\SourceSafeProjects\RemoteServer\ClientKeysManager\Program.cs:строка 17
   в System.AppDomain.nExecuteAssembly(Assembly assembly, String[] args)
   в System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   в Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   в System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   в System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   в System.Threading.ThreadHelper.ThreadStart()

                if (Clipboard.ContainsText())
                {
                    String clipboardData = Clipboard.GetText();
                    Clipboard.Clear();
                    Clipboard.SetText(signer.SignXml(clipboardData).ToString());
                }

ExternalException
 The Clipboard could not be cleared. This typically occurs when the Clipboard is being used by another process.

PM>ExternalException
PM> The Clipboard could not be cleared. This typically occurs when the Clipboard is being used by another process.
 
PM>

                if (Clipboard.ContainsText())
                {                    
                    String clipboardData = Clipboard.GetText();
                    Clipboard.SetDataObject(signer.SignXml(clipboardData), false, 100, 10);
                }