public MainWindow()
    {
        InitializeComponent();
        this.DataContext = new ViewModel(System.Windows.Window wnd);
    }

public ViewModel(System.Windows.Window wnd)
    {
        InitCBViewer(wnd);
    }

    private void InitCBViewer(System.Windows.Window wnd)
    {
        WindowInteropHelper wih = new WindowInteropHelper(wnd);
        hWndSource = HwndSource.FromHwnd(wih.Handle);

        hWndSource.AddHook(this.WinProc);   // start processing window messages
        hWndNextViewer = Win32.SetClipboardViewer(hWndSource.Handle);   // set this window as a viewer

    }

private void InitCBViewer()
    {
        WindowInteropHelper wih = new WindowInteropHelper(this);
        hWndSource = HwndSource.FromHwnd(wih.Handle);

        hWndSource.AddHook(this.WinProc);   // start processing window messages
        hWndNextViewer = Win32.SetClipboardViewer(hWndSource.Handle);   // set this window as a viewer

    }

    private void CloseCBViewer()
    {
        // remove this window from the clipboard viewer chain
        Win32.ChangeClipboardChain(hWndSource.Handle, hWndNextViewer);

        hWndNextViewer = IntPtr.Zero;
        hWndSource.RemoveHook(this.WinProc);
    }

    private IntPtr WinProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
    {
        switch (msg)
        {
            case Win32.WM_CHANGECBCHAIN:
                if (wParam == hWndNextViewer)
                {
                    // clipboard viewer chain changed, need to fix it.
                    hWndNextViewer = lParam;
                }
                else if (hWndNextViewer != IntPtr.Zero)
                {
                    // pass the message to the next viewer.
                    Win32.SendMessage(hWndNextViewer, msg, wParam, lParam);
                }
                break;

            case Win32.WM_DRAWCLIPBOARD:
                // clipboard content changed
                this.DisplayContent();            // calls a function to do something with the data on the clipboard
                // pass the message to the next viewer.
                Win32.SendMessage(hWndNextViewer, msg, wParam, lParam);
                break;
        }

        return IntPtr.Zero;
    }

internal static class Win32
{
    /// <summary>
    /// The WM_DRAWCLIPBOARD message notifies a clipboard viewer window that
    /// the content of the clipboard has changed.
    /// </summary>
    internal const int WM_DRAWCLIPBOARD = 0x0308;

    /// <summary>
    /// A clipboard viewer window receives the WM_CHANGECBCHAIN message when
    /// another window is removing itself from the clipboard viewer chain.
    /// </summary>
    internal const int WM_CHANGECBCHAIN = 0x030D;

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    internal static extern IntPtr SetClipboardViewer(IntPtr hWndNewViewer);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    internal static extern bool ChangeClipboardChain(IntPtr hWndRemove, IntPtr hWndNewNext);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    internal static extern IntPtr SendMessage(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);
}