Opening multiple document windows with previous instance

• Download demo project - 12 Kb
• Download source - 27 Kb

Introduction

Programs like Microsoft Word used to use MDI for displaying multiple windows, but users would often "lose" windows behind each other. Instead, each document which is opened is now placed in its own window, and displayed in the task bar. However, only one instance handles all the windows. This code sample shows how to setup your own application with this functionality. In particular, when the application is called with a file name as a command line argument, it checks for a previous instance. If found, it uses interprocess communication, to pass the file name to the running instance, then quits. The running application then opens a new window for the new file name.

Background

The interprocess communication uses code from the following article:

• vbAccelerator Simple InterProcess Communication

Please read for more details on the interprocess communication code.

Using the code

SingleInstance Project complies to SingleInstance.exe, which is the sample application. TestHarness Project compiles to TestHarness.exe, which can be used to call SingleInstance.exe from a windows form with various command line arguments.

There are various ways to make the main form invisible. Setting Visible = false is not enough, as when the form is activated, it will become visible automatically. Another trick is to set the form's Opacity = 0, which prevent the form being "flashed" when it starts up. You can also position the form so it out of the bounds of the screen.

Note that we must use an invisible form so we can receive window messages. I couldn't discover an easy way of starting a message handling loop without including a form.

FrmMain.cs Make Invisible
public FrmMain()
{
  InitializeComponent();

  // Do everything possible to keep this window hidden
  // note: form.Opacity = 0 set in designer will keep
  // the window from flashing during startup
  this.SetBounds( -1000, -1000, 100, 100 );
  this.Visible = false;
  initialize();
}

If we are the first instance, start FrmMain. If there is a previous instance running, send it the file name which should be opened, and quit this instance.

FrmMain.cs Main method
static void Main(string[] args) 
{
  // look for previous instances of this application running
  Process[] processes = Process.GetProcessesByName(
    Process.GetCurrentProcess().ProcessName);
  int length = processes.Length;

  // get first parameter as a file name
  string fileName = string.Empty;
  if ( args.Length > 0 )
    fileName = args[0];

  // if no other instance running, start new instance
  if ( length == 1 )
  {
    FrmMain frmMain = new FrmMain();
    Form form = new FrmChild(fileName);
    frmMain.AddForm( form );
    Application.Run(frmMain);
    return;
  }
  // if another process exists, send file name as message and quit
  if ( length > 1 )
  {
    // Create a new instance of the class:
    CopyData copyData = new CopyData();
            
    // Create the named channels to send and receive on.  The name
    // is arbitary; it can be anything you want.
    copyData.Channels.Add("HcClientFrmMain");

    // send fileName
    copyData.Channels["HcClientFrmMain"].Send(fileName);
        
    // quit
    return;
  }
}

During initialization, setup interprocess communication class, and get ready to handle receive message event.

FrmMain.cs Initialization
private void initialize()
{
  forms = new ArrayList();

  // Create a new instance of the class:
  copyData = new CopyData();
            
  // Assign the handle:
  copyData.AssignHandle(this.Handle);

  // Create the named channels to send and receive on.  The name
  // is arbitary; it can be anything you want.
  // copyData.Channels.Add("HcClientFrmMain");

  // Hook up event notifications whenever a message is received:
  copyData.DataReceived += new DataReceivedEventHandler(copyData_DataReceived);
}

When we (the previous instance) receive the file name, open a new window using the file name. For demonstration purposes, the file name is just appended to the child window title.

FrmMain.cs Data Received Event Handler
private void copyData_DataReceived(object sender, DataReceivedEventArgs e)
{            
  // Display the data in the logging list box:
  if (e.ChannelName.Equals("HcClientFrmMain"))
  {
    string fileName = (string) e.Data;
    FrmChild form = new FrmChild( fileName );
    AddForm( form );
  }
}

Note that FrmMain keeps track of the open child forms, and when the last form closes, FrmMain closes.

FrmMain.cs AddForm
public void AddForm( Form form )
{
  form.Closed += new EventHandler(OnFormClosed);
  forms.Add ( form );
  form.Show();
}

Points of Interest

Note that CopyData.Channels[x].Send( parameter ) can send any object which can be serialized, not just a string. Internally, it sends a WM_COPYDATA message between the applications. This communication only works for processes running locally.