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Checking previous instance (single instance) of an C# windows application, also setting MDI child form to a MDI parent

By Rehan Ahmad Abbasi on Sep 04, 2008
In this article I will describe how to check previous instance (single instance) of an C# windows application and also whether a MDI child is set to a MDI parent


This article will describe two things:

  1. How to check previous instance of a C# windows application.

  2. How to check that whether a MDI child is set to a mdiparent, and if it is already loaded then just activate it instead of creating a new instance and setting it as MDI child.

This code is written in C#. This article will especially help those who were VB 6.0 programmer and recently shifted to .NET.

Check the Previous Instance of a Windows Application 

using System.Diagnostics;


      private static bool getPrevInstance()


            //get the name of current process, i,e the process

            //name of this current application


            string currPrsName = Process.GetCurrentProcess()



            //Get the name of all processes having the

            //same name as this process name

            Process[] allProcessWithThisName

                         = Process.GetProcessesByName(currPrsName);


            //if more than one process is running return true.

            //which means already previous instance of the application

            //is running

            if (allProcessWithThisName.Length > 1)


                MessageBox.Show("Already Running");

                return true; // Yes Previous Instance Exist




                return false; //No Prev Instance Running





Instead of the above method of checking by process name  we can also use mutex,but this method is easy as compared to the mutex which requires a knowledge of threading.

This method will work fine unless and untill you change the process name.

Check Whether a MDI Child is Set to a mdiparent


The below code will check that whether a MDI child is set to a mdi parent and if it is already loaded and a MDI child. Then just activate it instead of creating a new instance and setting it as MDI child.


Suppose I have a windows form named as frmModule and I want to set it as a mdi child to MDI Form. I am calling function ActivateThisChill() with the name of the form as a parameter to this function.



          if (ActivateThisChild("frmModule") == false)


                frmModule newMDIChild = new frmModule();

                newMDIChild.Text = "Module";

                newMDIChild.MdiParent = this;



            notifyIcon1.Visible = true;

            notifyIcon1.BalloonTipTitle = " Project Tracker Suite (PTS) ";

            notifyIcon1.BalloonTipText = " PTS: Modules ";






      //This function will return true or false.

      //false means this form was not previously set

      //as mdi child hence needs to creat a instance

      //and set it as a mdi child.

     private Boolean ActivateThisChild(String formName)


            int i;

            Boolean formSetToMdi = false;

            for (i = 0; i < this.MdiChildren.Length; i++) 

             // loop for all the mdi children


                if (this.MdiChildren[i].Name == formName)

                  // find the Mdi child with the same name as your form


                    // if found just activate it.


                    formSetToMdi = true;




            if (i == 0 || formSetToMdi == false)

                // if the given form not found as mdi child return false.

                return false;



                return true;





The other way around is to use static property in your each form and check in your MDI parent that whether the static property of that particular form is assigned a value or not and then take your actions accordingly.