Writing a Desktop Sharing Application
Windows Desktop Sharing (WDS) API provides a rich set of API to share your complete desktop or just individual applications. Below is a step-by-step guide to write a sharing application. I have chosen C#/.Net to make things simpler and we will be using Visual Studio 2005 as the IDE.
Windows Desktop Sharing API Introduction:
Windows Desktop Sharing API allows a Windows Desktop Session to be shared across multiple viewers. More information on the API can be found on MSDN at http://msdn2.microsoft.com/en-us/library/aa373852.aspx. You could also refer to the Windows Desktop Sharing API Introduction blog. Windows Meeting Space & Remote Assistance use WDS API for collaboration and assistance scenarios. Similarly, you can write applications to achieve your sharing, collaboration, assistance, administration and deployment scenarios.
Object Model of the API: API currently is published as an in-proc COM DLL (RdpEncom.dll) and is available in Vista. There are 2 primary objects that can be created:
- 1. IRDPSRAPISharingSession - COM object that enables sharing of desktop
- 2. IRDPSRAPIViewer - ActiveX control that can be embedded into a host window for viewing the sharing session.
Other objects can be created or queried from these 2 primary objects. There is also an event sink interface (IRDPSessionEvents) that the API would use to report events to the application. Applications should sink appropriate events as needed.
Desktop Sharing Application: Writing a desktop sharing application involves writing 2 different applications - one for sharing and the other for viewing. These 2 applications can be written as one application working in different modes similar to Windows Meeting Space.
Sharer Application: You can choose to create either a console based application or windows based application. It is nice to have a GUI interface to control the viewers, start/stop sharing etc., so let's build a windows application. To create a sharer application you will need to first add a reference to RdpComApi 1.0 Type Library (rdpencom.dll in %windows%\system32 directory) to your project.
Starting the sharing Session: To start the sharer, first create an instance of RdpSession Class and then subscribe to the events published by the WDS API. For this you will need to declare an event handler of the corresponding delegate signature. Here is an example of declaring and defining an event handler:
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private void OnAttendeeConnected(object pObjAttendee)
{
IRDPSRAPIAttendee pAttendee = pObjAttendee as IRDPSRAPIAttendee;
pAttendee.ControlLevel = CTRL_LEVEL.CTRL_LEVEL_VIEW;
LogTextBox.Text += ("Attendee Connected: " + pAttendee.RemoteName + Environment.NewLine);
}
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Above is the event handler for the delegate:
void _IRDPSessionEvents_OnAttendeeConnectedEventHandler(object pAttendee);
This event is fired by the API when an attendee connects to the sharing session and will pass in the attendee instance to the event handler. Use that attendee instance to query information and also to set control level of the attendee. Control levels define the level of information that an attendee gets. In this implementation of event handler, attendee is given view control which means that attendee can only see the sharing session but cannot interact with it. We also print out some message to a text box so that the user who is running the sharing app can see the status of attendees connecting and disconnecting. Similarly we add event handlers for attendee disconnected and control level change requests.
After subscribing to the events that we are interested in, start the sharing session by calling Open() method on the RdpSession object.
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// Create a new RdpSession instance
m_pRdpSession = new RDPSession();
// Subscribe to events
m_pRdpSession.OnAttendeeConnected += new _IRDPSessionEvents_OnAttendeeConnectedEventHandler(OnAttendeeConnected);
m_pRdpSession.OnAttendeeDisconnected += new _IRDPSessionEvents_OnAttendeeDisconnectedEventHandler(OnAttendeeDisconnected);
m_pRdpSession.OnControlLevelChangeRequest += new _IRDPSessionEvents_OnControlLevelChangeRequestEventHandler(OnControlLevelChangeRequest);
// Start the Sharing Session
m_pRdpSession.Open();
LogTextBox.Text += "Presentation Started. Your Desktop is being shared." + Environment.NewLine;
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Creating an Invitation: Create an invitation that can be sent to attendees so that they can use the invitation to connect to the sharing session. The invitation can be sent to the attendees using any mechanism desired. For example, Remote assistance allows to save the invitation and send it by email or to send through IM. Windows Meeting space uses "People Near Me" feature to identify and then send the invitations. In this implementation, let's take a simple approach and save it to a file. Here is the corresponding code:
// Create invitation.
IRDPSRAPIInvitation pInvitation = m_pRdpSession.Invitations.CreateInvitation("WinPresenter","PresentationGroup","",5);
string invitationString = pInvitation.ConnectionString;
// Save Connection String to File
WriteToFile(invitationString);
Stopping the Sharing Session: To stop the sharing session, call close method on the RdpSession class instance. Here is the code:
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m_pRdpSession.Close();
LogTextBox.Text += "Presentation Stopped." + Environment.NewLine;
Marshal.ReleaseComObject(m_pRdpSession);
m_pRdpSession = null;
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You may notice that I am calling ReleaseComObject. Reason being that once Close() is called then that RdpSession class instance cannot be used anymore. We will need to create another instance of RdpSession class to restart sharing.
Viewer Application: You will need to create a windows application and then add RdpViewer Class as an ActiveX control. Adding ActiveX controls to Windows Forms is a good resource to learn about adding ActiveX controls in Visual Studio IDE. Visual Studio will create an instance of AxRdpViewer and you can resize the ActiveX control to fit to your needs on the Windows Form. You can also choose to create this ActiveX control dynamically and place it in the host Form.
Connecting to the Sharing Session: First subscribe to the events that you are interested in. The easiest way in Visual Studio to add event handlers to an event is to click on the Events lightning bolt in the property browser of ActiveX control, then double-click on any events you wish to handle. An empty method signature and the appropriate event hooking and unhooking code are then emitted for you. Subscribe to OnConnectionEstablished, OnConnectionTerminated, OnConnectionFailed and OnError events at a minimum so that you can report status and errors, if there are any. Here is some code for OnError event handler method:
private void OnError(object sender, _IRDPSessionEvents_OnErrorEvent e)
{
int ErrorCode = (int)e.errorInfo;
LogTextBox.Text += ("Error 0x" + ErrorCode.ToString("X") + Environment.NewLine);
}
To connect to the Sharing Session, you will need an invitation. Since we saved the invitation created in the Sharing Application as a file, let's just read the file into a string. Then call Connect method with the invitation file. Please note that we are passing empty string as password. Since we created the invitation on the sharing application with empty string as the password this would work. This is not advised if you are writing a commercial application because anyone can connect to your Sharing Session if they can get hold of the invitation.
Here is the code to connect to the Sharing Session:
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string ConnectionString = ReadFromFile();
if (ConnectionString != null)
{
pRdpViewer.Connect(ConnectionString, "Viewer1", "");
} |
Once the connection is succesfully established with sharing session, API will fire OnConnectionEstablished event or OnConnectionFailed event if the connection fails.
Disconnecting from the Sharing Session: To disconnect from the sharing session, call Disconnect method on the AxRdpViewer class instance. Here is the code:
pRdpViewer.Disconnect();
API will fire OnConnectionTerminated event once the viewer is disconnected from the sharing session.
Wrapping up:
This is all you need to do to share your desktop. Obviously you will need to do much more if you want to have control on who is connected, display a nice GUI for attendee information, kick attendees etc. This will give you a good start and explore the API provided by WDS API to achieve your other scenarios.
Sample Application:
WinPresenter is a sample Desktop Sharing Suite of applications encompassing both Sharing and Viewing application. You should be able to find source code including the snippets above. I left out error checking, printing messages to user etc., so please modify it to your needs.
Usage of Sample Applications:
Sharer Application: Run winsharer.exe. Invitation file will be saved to the current directory with file name inv.xml
Viewer application: Run winviewer.exe %path to invitation file%. Either specify the directory where you have run the winsharer or copy inv.xml to some other location and specify the path. If there is no path specified it will try to read inv.xml from the current directory.
