R – Silverlight vs WPF

Here is a very good article regarding the Mutex solution. The approach described by the article is advantageous for two reasons.

First, it does not require a dependency on the Microsoft.VisualBasic assembly. If my project already had a dependency on that assembly, I would probably advocate using the approach shown in another answer. But as it is, I do not use the Microsoft.VisualBasic assembly, and I'd rather not add an unnecessary dependency to my project.

Second, the article shows how to bring the existing instance of the application to the foreground when the user tries to start another instance. That's a very nice touch that the other Mutex solutions described here do not address.

UPDATE

As of 8/1/2014, the article I linked to above is still active, but the blog hasn't been updated in a while. That makes me worry that eventually it might disappear, and with it, the advocated solution. I'm reproducing the content of the article here for posterity. The words belong solely to the blog owner at Sanity Free Coding.

Today I wanted to refactor some code that prohibited my application from running multiple instances of itself.

Previously I had use System.Diagnostics.Process to search for an instance of my myapp.exe in the process list. While this works, it brings on a lot of overhead, and I wanted something cleaner.

Knowing that I could use a mutex for this (but never having done it before) I set out to cut down my code and simplify my life.

In the class of my application main I created a static named Mutex:

static class Program
{
    static Mutex mutex = new Mutex(true, "{8F6F0AC4-B9A1-45fd-A8CF-72F04E6BDE8F}");
    [STAThread]
    ...
}

Having a named mutex allows us to stack synchronization across multiple threads and processes which is just the magic I'm looking for.

Mutex.WaitOne has an overload that specifies an amount of time for us to wait. Since we're not actually wanting to synchronizing our code (more just check if it is currently in use) we use the overload with two parameters: Mutex.WaitOne(Timespan timeout, bool exitContext). Wait one returns true if it is able to enter, and false if it wasn't. In this case, we don't want to wait at all; If our mutex is being used, skip it, and move on, so we pass in TimeSpan.Zero (wait 0 milliseconds), and set the exitContext to true so we can exit the synchronization context before we try to aquire a lock on it. Using this, we wrap our Application.Run code inside something like this:

static class Program
{
    static Mutex mutex = new Mutex(true, "{8F6F0AC4-B9A1-45fd-A8CF-72F04E6BDE8F}");
    [STAThread]
    static void Main() {
        if(mutex.WaitOne(TimeSpan.Zero, true)) {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
            mutex.ReleaseMutex();
        } else {
            MessageBox.Show("only one instance at a time");
        }
    }
}

So, if our app is running, WaitOne will return false, and we'll get a message box.

Instead of showing a message box, I opted to utilize a little Win32 to notify my running instance that someone forgot that it was already running (by bringing itself to the top of all the other windows). To achieve this I used PostMessage to broadcast a custom message to every window (the custom message was registered with RegisterWindowMessage by my running application, which means only my application knows what it is) then my second instance exits. The running application instance would receive that notification and process it. In order to do that, I overrode WndProc in my main form and listened for my custom notification. When I received that notification I set the form's TopMost property to true to bring it up on top.

Here is what I ended up with:

• Program.cs

static class Program
{
    static Mutex mutex = new Mutex(true, "{8F6F0AC4-B9A1-45fd-A8CF-72F04E6BDE8F}");
    [STAThread]
    static void Main() {
        if(mutex.WaitOne(TimeSpan.Zero, true)) {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
            mutex.ReleaseMutex();
        } else {
            // send our Win32 message to make the currently running instance
            // jump on top of all the other windows
            NativeMethods.PostMessage(
                (IntPtr)NativeMethods.HWND_BROADCAST,
                NativeMethods.WM_SHOWME,
                IntPtr.Zero,
                IntPtr.Zero);
        }
    }
}

• NativeMethods.cs

// this class just wraps some Win32 stuff that we're going to use
internal class NativeMethods
{
    public const int HWND_BROADCAST = 0xffff;
    public static readonly int WM_SHOWME = RegisterWindowMessage("WM_SHOWME");
    [DllImport("user32")]
    public static extern bool PostMessage(IntPtr hwnd, int msg, IntPtr wparam, IntPtr lparam);
    [DllImport("user32")]
    public static extern int RegisterWindowMessage(string message);
}

• Form1.cs (front side partial)

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }
    protected override void WndProc(ref Message m)
    {
        if(m.Msg == NativeMethods.WM_SHOWME) {
            ShowMe();
        }
        base.WndProc(ref m);
    }
    private void ShowMe()
    {
        if(WindowState == FormWindowState.Minimized) {
            WindowState = FormWindowState.Normal;
        }
        // get our current "TopMost" value (ours will always be false though)
        bool top = TopMost;
        // make our form jump to the top of everything
        TopMost = true;
        // set it back to whatever it was
        TopMost = top;
    }
}

Should you learn ASP.NET or Winforms first? ASP or MFC? HTML or VB? C# or VB?

Set aside the idea that there is a logical progression through what has become a highly complex interwoven set of technologies, and take a step back and ask yourself a series of questions:

• What are your goals; how do you want to balance profit against enjoyment
• Are you short term oriented or in for the long haul
• Are you the type of person who likes to get good at something and do it a lot or do you get bored once you fully understand it?

The next and hardest step is to come to accept that any advice you are given is bound to be wrong; and the longer the time horizon the more likely it is to be incorrect. If the advice is for more than six to 12 months, the probability the advice is wildly incorrect approaches 1.

I can only tell you my story, quickly. In 2000 I was happy as a consultant working profitably in C++ on Windows applications, writing about ASP.NET and WinForms. then I saw C# and the world turned upside down. I never went back.

Two years ago I had the same kind of revelation, only an order of magnitude bigger, stronger and with more conviction about Silverlight. Yes, WPF is magnificent, and it may be that I'm all wet about this, but I believe in my gut that Silverlight changes everything. There was no doubt then and there is no doubt today that Silverlight is the most important development platform for Microsoft since .NET (certainly) and possibly since the switch to C++.

In a nutshell, here is why. I don't understand where its limitations are. With most platforms I do: you can do this, but you can't do that. WPF is a pretty good case in point, as was ASP.Net and WinForms and, well really everything until now.
With Silverlight, I don't see the boundaries yet. Silverlight has already leaped off the desktop onto phones, and I don't see any reason for it to stop there. Yes, it is true, it is bound by the browser, but I see that less as a jail cell than as a tank in which Silverlight will be riding over lots of terrain (it must be very late, I should go to bed).

In any case, for now, learning Silverlight is a gas, there is a lot of material on the Silverlight.net site, and what is the very best thing about learning Silverlight is that if you don't see what you need you can holler at me and I'll make sure you get it pretty quickly.

Enjoy, good luck and the dirty little secret is you'll be fine whichever you choose. It's all just software.

-jesse

Jesse Liberty "Silverlight Geek"