Single Instance WinForms Example

Setup steps:

First decide which form in your project will receive the single instance signals. This does not need to be your main interface, but will need to have an active instance throughout your app's lifetime. Closing this form will shut down your program. In this example we'll use a form named MainForm.

1. Add SingleAppContext.cs to your WinForms project
2. Add System.Runtime.Remoting to your project References.
3. Edit MainForm.cs (not MainForm.Designer.cs) and change public partial class MainForm : Form to public partial class MainForm : SingleInstanceWinform.FormSingleInstance
4. Add a SignalExternalCommandLineArgs override to the form code, example below.
5. Change the Application.Run in Program.cs as per below.

Using ApplicationContext

The default for a WinForm is to run a form via Application.Run(new Form1());. To make Single Instance work we need to switch to an ApplicationContext startup. Change the Run line to:

Application.Run(new SingleInstanceWinform.SingleAppContext(new MainForm(), "SingleInstanceWinform"));

The 2nd paramater, "SingleInstanceWinform", is a unique identifer for your application. It is used to create an IPC channel, so choose this name such that it will not collide with any other apps you might run.

Handling the Signal

When a second instance of your program is started, your first instance will recieve a signal that includes any command line arguments that were passed. To handle this signal we need to add an override for the SignalExternalCommandLineArgs method to the form.

To simlpy bring the form to the front:

public override bool SignalExternalCommandLineArgs(IList<string> args)
{
    //Method is called from a different thread so Invoke on the form's UI thread to interact with it
    this.Invoke((Action)delegate
    {
        if (this.WindowState == FormWindowState.Minimized)
            this.WindowState = FormWindowState.Normal;
        this.Activate();
    });
    return true;
}

Returning false from this method will allow another instance of the application to be started, but this will break the IPC channel in the current implementation.

License

Microsoft Public License (MS-PL)

This is based heavily on SingleInstance.cs from Microsoft which is also MS-PL.