class MyApplication 
{ 
    public static void Main() 
    { 
        if ( InstanceExists() )
        { 
            return; 
        }

        Console.WriteLine( “Application is running… Press enter to exit” );
        Console.ReadLine();
    } 
     
    static bool InstanceExists() 
    { 
        bool createdNew; 
        Mutex mutex = new Mutex( false, "My Mutex Name", out createdNew ); 
        return createdNew;
    } 
}

• MSDN (Windows Forms FAQ - Controls and Components)

Form FindOpenForm( string key )
        {
            foreach (Form frm in Application.OpenForms)
            {
                foreach (object obj in frm.GetType().GetCustomAttributes(false))
                {
                    if (obj is FormKeyAttribute)
                    {
                        FormKeyAttribute fk = obj as FormKeyAttribute;
                        if (fk.Key == FormKeys.VoucherInfo)
                            return frm;
                    }
                }
            }
            return null;
        }