/// <summary>
        /// Enables an external user-interface handler. This external UI handler is called before the
        /// normal internal user-interface handler. The external UI handler has the option to suppress
        /// the internal UI by returning a non-zero value to indicate that it has handled the messages.
        /// </summary>
        /// <param name="uiHandler">A callback delegate that handles the UI messages</param>
        /// <param name="messageFilter">Specifies which messages to handle using the external message handler.
        /// If the external handler returns a non-zero result, then that message will not be sent to the UI,
        /// instead the message will be logged if logging has been enabled.</param>
        /// <returns>The previously set external handler, or null if there was no previously set handler</returns>
        /// <remarks><p>
        /// To restore the previous UI handler, a second call is made to SetExternalUI using the
        /// ExternalUIHandler returned by the first call to SetExternalUI and specifying
        /// <see cref="InstallLogModes.None"/> as the message filter.
        /// </p><p>
        /// The external user interface handler does not have full control over the external user
        /// interface unless <see cref="SetInternalUI(InstallUIOptions)"/> is called with the uiLevel parameter set to
        /// <see cref="InstallUIOptions.Silent"/>. If SetInternalUI is not called, the internal user
        /// interface level defaults to <see cref="InstallUIOptions.Basic"/>. As a result, any message not
        /// handled by the external user interface handler is handled by Windows Installer. The initial
        /// "Preparing to install..." dialog always appears even if the external user interface
        /// handler handles all messages.
        /// </p><p>
        /// SetExternalUI should only be called from a bootstrapping application. You cannot call
        /// it from a custom action
        /// </p><p>
        /// Win32 MSI API:
        /// <a href="http://msdn.microsoft.com/library/en-us/msi/setup/msisetexternalui.asp">MsiSetExternalUI</a>
        /// </p></remarks>
        public static ExternalUIHandler SetExternalUI(ExternalUIHandler uiHandler, InstallLogModes messageFilter)
        {
            NativeExternalUIHandler nativeHandler = null;

            if (uiHandler != null)
            {
                nativeHandler = new ExternalUIProxy(uiHandler).ProxyHandler;
                Installer.externalUIHandlers.Add(nativeHandler);
            }
            NativeExternalUIHandler oldNativeHandler = NativeMethods.MsiSetExternalUI(nativeHandler, (uint)messageFilter, IntPtr.Zero);

            if (oldNativeHandler != null && oldNativeHandler.Target is ExternalUIProxy)
            {
                Installer.externalUIHandlers.Remove(oldNativeHandler);
                return(((ExternalUIProxy)oldNativeHandler.Target).Handler);
            }
            else
            {
                return(null);
            }
        }
 /// <summary>
 /// Enables an external user-interface handler. This external UI handler is called before the
 /// normal internal user-interface handler. The external UI handler has the option to suppress
 /// the internal UI by returning a non-zero value to indicate that it has handled the messages.
 /// </summary>
 /// <param name="uiHandler">A callback delegate that handles the UI messages</param>
 /// <param name="messageFilter">Specifies which messages to handle using the external message handler.
 /// If the external handler returns a non-zero result, then that message will not be sent to the UI,
 /// instead the message will be logged if logging has been enabled.</param>
 /// <returns>The previously set external handler, or null if there was no previously set handler</returns>
 /// <remarks><p>
 /// To restore the previous UI handler, a second call is made to SetExternalUI using the
 /// ExternalUIHandler returned by the first call to SetExternalUI and specifying
 /// <see cref="InstallLogModes.None"/> as the message filter.
 /// </p><p>
 /// The external user interface handler does not have full control over the external user
 /// interface unless <see cref="SetInternalUI(InstallUIOptions)"/> is called with the uiLevel parameter set to
 /// <see cref="InstallUIOptions.Silent"/>. If SetInternalUI is not called, the internal user
 /// interface level defaults to <see cref="InstallUIOptions.Basic"/>. As a result, any message not
 /// handled by the external user interface handler is handled by Windows Installer. The initial
 /// "Preparing to install..." dialog always appears even if the external user interface
 /// handler handles all messages.
 /// </p><p>
 /// SetExternalUI should only be called from a bootstrapping application. You cannot call
 /// it from a custom action
 /// </p><p>
 /// Win32 MSI API:
 /// <a href="http://msdn.microsoft.com/library/en-us/msi/setup/msisetexternalui.asp">MsiSetExternalUI</a>
 /// </p></remarks>
 public static ExternalUIHandler SetExternalUI(ExternalUIHandler uiHandler, InstallLogModes messageFilter)
 {
     NativeExternalUIHandler nativeHandler = null;
     if (uiHandler != null)
     {
         nativeHandler = new ExternalUIProxy(uiHandler).ProxyHandler;
         Installer.externalUIHandlers.Add(nativeHandler);
     }
     NativeExternalUIHandler oldNativeHandler = NativeMethods.MsiSetExternalUI(nativeHandler, (uint) messageFilter, IntPtr.Zero);
     if (oldNativeHandler != null && oldNativeHandler.Target is ExternalUIProxy)
     {
         Installer.externalUIHandlers.Remove(oldNativeHandler);
         return ((ExternalUIProxy) oldNativeHandler.Target).Handler;
     }
     else
     {
         return null;
     }
 }