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.

C# Source Code: RAD_ClipboardMonitorExampleSrc.zip (~50 KB)
VB.NET Example: ClipboardMonitor_VB.txt

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.