How to Avoid Multiple Instances of Windows Form in C#

How to avoid multiple instances of windows form in c#

implement the Singleton pattern

an example: CodeProject: Simple Singleton Forms (ok, it's in VB.NET, but just to give you a clue)

Prevent multiple instances of a given app in .NET?

Use Mutex. One of the examples above using GetProcessByName has many caveats. Here is a good article on the subject:

http://odetocode.com/Blogs/scott/archive/2004/08/20/401.aspx

[STAThread]
static void Main()
{
using(Mutex mutex = new Mutex(false, "Global\\" + appGuid))
{
if(!mutex.WaitOne(0, false))
{
MessageBox.Show("Instance already running");
return;
}

Application.Run(new Form1());
}
}

private static string appGuid = "c0a76b5a-12ab-45c5-b9d9-d693faa6e7b9";

What is the safest way to prevent multiple instances of a program?

The safest way is to use the built-in support in .NET, WindowsFormsApplicationBase.IsSingleInstance property. Hard to guess if it is appropriate, you didn't make much effort describing your exact needs. And no, nothing changed in the past 5 years. – Hans Passant Jan 7 at 0:38

This was the best answer but Hans didn't submit it as an answer.

preventing multiple instance of one form from displaying

Use a Singleton:

class HelpForm : Form
{
private static HelpForm _instance;
public static HelpForm GetInstance()
{
if(_instance == null) _instance = new HelpForm();
return _instance;
}
}

.......

private void heToolStripMenuItem_Click(object sender, EventArgs e)
{
HelpForm form = HelpForm.GetInstance();
if(!form.Visible)
{
form.Show();
}
else
{
form.BringToFront();
}
}

How to stop a form opening multiple times when a button is clicked

Keep the same instance, don't go creating a new one every time.

private Login _passForm = new Login();

private void button3_Click(object sender, EventArgs e)
{
if (!_passForm.Visible)
{
_passForm.Show();
}
else
{
_passForm.BringToFront();
}
}

How can I prevent launching my app multiple times?

At program startup check if same process is already running:

using System.Diagnostics;

static void Main(string[] args)
{
String thisprocessname = Process.GetCurrentProcess().ProcessName;

if (Process.GetProcesses().Count(p => p.ProcessName == thisprocessname) > 1)
return;
}

Prevent form from showing multiple times

You are most likely doing something like this:

void button1_OnClick(object sender, EventArgs e) {
var newForm = new MyForm();
newForm.Show();
}

So you are showing a new instance of the form every time it is clicked. You want to do something like this:

MyForm _form = new MyForm();

void button1_OnClick(object sender, EventArgs e) {
_form.Show();
}

Here you have just one instance of the form you wish to show, and just Show() it.

Prevent my windows application to run multiple times

You can use a named mutex for that purpose. A named(!) mutex is a system-wide synchronization object. I use the following class (slightly simplified) in my projects. It creates an initially unowned mutex in the constructor and stores it in a member field during the object lifetime.

public class SingleInstance : IDisposable
{
private System.Threading.Mutex _mutex;

// Private default constructor to suppress uncontrolled instantiation.
private SingleInstance(){}

public SingleInstance(string mutexName)
{
if(string.IsNullOrWhiteSpace(mutexName))
throw new ArgumentNullException("mutexName");

_mutex = new Mutex(false, mutexName);
}

~SingleInstance()
{
Dispose(false);
}

public bool IsRunning
{
get
{
// requests ownership of the mutex and returns true if succeeded
return !_mutex.WaitOne(1, true);
}
}

public void Dispose()
{
GC.SuppressFinalize(this);
Dispose(true);
}

protected virtual void Dispose(bool disposing)
{
try
{
if(_mutex != null)
_mutex.Close();
}
catch(Exception ex)
{
Debug.WriteLine(ex);
}
finally
{
_mutex = null;
}
}
}

This example shows, how to use it in a program.

static class Program
{
static SingleInstance _myInstance = null;

[STAThread]
static void Main()
{
// ...

try
{
// Create and keep instance reference until program exit
_myInstance = new SingleInstance("MyUniqueProgramName");

// By calling this property, this program instance requests ownership
// of the wrapped named mutex. The first program instance gets and keeps it
// until program exit. All other program instances cannot take mutex
// ownership and exit here.
if(_myInstance.IsRunning)
{
// You can show a message box, switch to the other program instance etc. here

// Exit the program, another program instance is already running
return;
}

// Run your app

}
finally
{
// Dispose the wrapper object and release mutex ownership, if owned
_myInstance.Dispose();
}
}
}


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