C# windows Application

Use Command-Line Arguments

[STAThread]
       static void Main(string[] args)
       {
if(args=="meet your requirement")
{
                    Application.EnableVisualStyles();
                    Application.SetCompatibleTextRenderingDefault(false);
                    Application.Run(new frmMain());
}
}

But as per .NET frame work guide lines the child application is to be created as a class library.
It is always better to follow the standard conventions.

in the Program.cs file,give the master application's reference as
this seems similar to the following code:

Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
Application.Run(new MasterApp());

and in master application you can open other child application on any event

for example

private void btnOpen_click()
{
chileApp c=new chileApp();
this.hide();
c.show();
}

good luck...

Looks like you are looking for this work flow:
For the child exe, when the exe is launched, check for the presence of a GUID as its argument in the Main(...) method, if not found, just shut down the application. This will make sure that the child app is only launched by a process which knows the GUID. That effectively rules out direct launch (e.g., by double clicking).

Now from the master app launch the child app by providing the GUID as an argument. You can use Process class to do it.

Hope that helps.

Your language here is unclear:

You are using the word "application" to describe your two "whatevers:" to me that implies two compiled .exe files.

SAK, and others, interpret your question as meaning the second application is a "child process" of the first, where "child process" has a specific technical meaning.

I would interpret your statement as meaning that the user launches the first, primary, or main, application, and then the user, when they choose to, launches the second application:

The statement : "givingen the path of exe of the child application:" implies to me you need to launch the second application from within the first, and does not imply automatic launch when the first, primary application launches.

You can clarify this question by telling us exactly what the two applications are (both WinForms ?), and if they have any need to communicate (pass objects, or data, between them), or "interact" with each other.

And, clarify when, and how, exactly, the second application is launched.

Hint: to launch a process from within a .NET application consider the Process.Start facility:

Process.Start(SomeProgramFilePath);

This should open your second application, and give it focus: but note, if your first application has its Form 'TopMost property set to 'true; the second application may not appear in front of it.