What is the best way to make a single instance app

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Closed 11 years ago.

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What is the correct way to create a single instance application?
Prevent multiple instances of a given app in .NET?

Do I check whether another process with the same name exists? (What if the user doesn't have permission to do that?)

Write a file to disk and delete it before exiting? (what about abnormal termination?)

What would be a best practice to do this?

Answer 1

Generally, a named mutex works best. Here's a link to en example: http://www.iridescence.no/post/CreatingaSingleInstanceApplicationinC.aspx

Answer 2

There are several ways to do this, all of which have their advantages and disadvantages, and googling will give you a lot of discussion about it.

The methods you describe have the weaknesses you mention. Also you need to consider how unique you want your instance to be:

• per WindowStation

• per machine

I have a class that can be used to force a single instance per WindowStation for Windows Forms applications. Basically it uses P/Invoke to do the following during startup, protecting against a race condition using a Mutex:

• Define a fixed unique string to identify your app (e.g. a GUID)

• Obtain a Mutex whose name is derived from your unique string.

• Call EnumWindows using P/Invoke to enumerate all main windows.

• In the EnumWindows callback, call GetProp using P/Invoke on each window in turn, passing the window handle and your unique value as arguments. If GetProp detects a window that has your unique key (i.e. another instance of your application), activate it (PostMessage(...SC_RESTORE...), SetForegroundWindow, SetFocus), then exit your new instance.

• If you enumerate all windows without finding a previous instance, call SetProp using P/Invoke to set your unique value for your application's main form window handle.

• Release the mutex and continue starting up.

In my use case, this solution is better than simply holding a mutex for the lifetime of the process, because it gives a way to activate the previous instance if the user starts a second instance rather than just exiting. Using GetProp/SetProp gives you a way of identifying your process instance without relying on the process name.

Answer 3

I depends on the technique you're using. There are quite a few solutions, some more suitable for WPF, some for WinForms:

• WindowsFormsApplicationsBase (my preferred way)
• With a Mutex (bit more complex)
• Checking for a process with the same details (I dislike this one)

More on WindowsFormsApplicationsBase:
http://www.openwinforms.com/single_instance_application.html

More on Mutex and checking a process:
http://www.ravinderweb.com/page/Single-Instance-of-the-Application-using-C-Trick-and-Mutex.aspx

I wouldn't use a File, it's prone to errors and not having File access might even cause Exceptions in your program even though it's not a crucial part of your application.

Answer 4

You can use a Mutex.

bool firstInstance = true;
using (Mutex mutex = new Mutex(true, "MyApplicationName", out firstInstance))
{
    if (firstInstance)
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new MainForm());
    }
    else
    {
        // Another instance loaded
    }
}

Answer 5

You can use the Process class's GetProcessesByName method to look for another running process with the same name, and exit if you find one.