Create a Windows Clipboard Monitor in C# using SetClipboardViewer
Introduction
Applications such as GetRight
and ClipMate seem to
magically know when something is copied to the Clipboard. How is it done? This
example shows you how to use the Win32 API function SetClipboardViewer
to create a Clipboard Viewer application.
Using SetClipboardViewer
The Win32 API contains a function SetClipboardViewer which
allows an application to register itself as a viewer of Clipboard events. When
an item is copied to the Clipboard, windows notifies the Clipboard Viewer by
sending the WM_DRAWCLIPBOARD message. The application can then use the
appropriate function to get the contents of the Clipboard and perform
processing.
Example Application
I had a great idea one day (after copying and pasting internet links a few
hundred times) to create a small application that would read the Clipboard when
something was copied to it and extract any Internet URLs, UNC paths or
filenames that it found and put them on a menu in the system tray.
I spent a little time on it and it is in a working state but it is not
finished. One day I will get around to finishing it off but for the moment it
is a good sample to learn how SetClipboardViewer works.
The SetClipboardViewer Function
The SetClipboardViewer function adds the specified window to the
chain of Clipboard Viewers. Clipboard Viewer windows receive a WM_DRAWCLIPBOARD
message whenever the content of the Clipboard changes.
The SetClipboardViewer API declaration has the following syntax in
C#
[DllImport("User32.dll", CharSet=CharSet.Auto)]
public static extern IntPtr SetClipboardViewer(IntPtr hWndNewViewer);
The hWndNewViewer parameter is the handle to the window to be
added to the Clipboard chain.
When SetClipboardViewer is successful, the return value identifies
the next window in the Clipboard Viewer chain. This is important as a Clipboard
Viewer is required to pass Clipboard messages to the next window in the
Clipboard chain.
Setting up the Clipboard Viewer
I need to override the WndProc method of the
underlying Form so that the main form can respond to window messages from the
operating system. Normally .NET applications do not need to use WndProc
as the .NET Framework takes care of these events.
protected override void WndProc(ref Message m)
{
Next, call SetClipboardViewer to register the application
as a Clipboard Viewer. The window will now receive the WM_DRAWCLIPBOARD
message. Applications do not normally receive the WM_DRAWCLIPBOARD
message, only those that call SetClipboardViewer.
_ClipboardViewerNext = SetClipboardViewer(this.Handle);
Responding to the WM_DRAWCLIPBOARD message
After calling SetWindowLong the WindowProc method will receive the WM_DRAWCLIPBOARD
message when something is copied to the Clipboard.
protected override void WndProc(ref Message m)
{
switch ((Win32.Msgs)m.Msg)
{
case Win32.Msgs.WM_DRAWCLIPBOARD:
This is the place that I do my processing of the Clipboard data. The .NET
Framework has the Clipboard, DataObject and DataFormats classes
for working with the Clipboard. Clipboard.GetDataObject gets the current
contents of the Clipboard as an IDataObject.
IDataObject iData = new DataObject();
iData = Clipboard.GetDataObject();
As the Clipboard can contain different types of data, use the DataObject.GetDataPresent
method to check for a certain type of data. It returns true if the contents of
the Clipboard can be converted to the specified type. The DataFormats class
contains constants that define the names of some of the more common Clipboard
formats.
if (iData.GetDataPresent(DataFormats.Rtf))
{
// ...
Once I have determined that the format is available then the DataObject.GetData
method can be used to get the Clipboard content in the required format. The
following example gets the contents in Rich Text Format (RTF).
ctlClipboardText.Rtf = (string)iData.GetData(DataFormats.Rtf);
After performing any processing that the application requires the next window
in the Clipboard Viewer chain must be notified of the Clipboard change. This is
done by calling the Windows API function SendMessage. The
same parameters that were passed to the WndProc method are passed
to SendMessage to be used by the next Clipboard Viewer in the
chain.
SendMessage(_ClipboardViewerNext, m.Msg, m.WParam, m.LParam);
When overriding WndProc I must pass any unhandled messages to the
base class by calling base.WndProc
protected override void WndProc(ref Message m)
{
switch ((Win32.Msgs)m.Msg)
{
case Win32.Msgs.WM_DRAWCLIPBOARD:
// ... process Clipboard
default:
// unhandled window message
base.WndProc(ref m);
break;
The WM_CHANGECBCHAIN Message
The WM_CHANGECBCHAIN message is sent when a window is being
removed from the chain of Clipboard Viewers. Note that the message is only sent
to the first window in the Clipboard Viewer chain. The first window and
subsequent windows in the chain must use SendMessage to pass the
message along the chain.
If wParam is equal to the handle of the next Clipboard Viewer then
the next Clipboard Viewer is being removed and I need to update the pointer
that refers to the next window in the chain. m.LParam is a pointer
to the Clipboard Viewer in the chain after the one being removed.
if (m.WParam == _ClipboardViewerNext)
{
_ClipboardViewerNext = m.LParam;
}
else
{
SendMessage(_ClipboardViewerNext, m.Msg, m.WParam, m.LParam);
}
This message can be seen by starting multiple copies of the Clipboard Viewer
and watching the Debug output.
Unregistering the Application as a Clipboard Viewer
When the application closes I make sure I call ChangeClipboardChain to
remove the application from the Clipboard chain.
ChangeClipboardChain(this.Handle, _ClipboardViewerNext);
And Finally...
There are a few tricks to using the SetClipboardViewer function
but it allows your application to be notified of Clipboard events.
If you have any comments or feedback please send them via email: Ross Donald
(ross @ radsoftware.com.au)