ACC2000: How to Determine If a Specific Windows Program Is Running

Through the use of a Windows application programming interface (API) function, Microsoft Access can determine if another program is already running. There may be times when you want only one instance of an application to run in Microsoft Windows. For example, if you add a command button to a form that starts the Windows Calculator (Calc.exe) program, you can start many instances of Calculator. This is an inefficient use of memory and system resources.

The API function used to determine whether a specific program is running is called FindWindow(). FindWindow() returns the handle of the window whose class is given by the lpClassName parameter and whose window name (or caption) is given by the lpCaption parameter. If the returned value is zero, the application is not running.

Microsoft provides programming examples for illustration only, without warranty either expressed or implied. This includes, but is not limited to, the implied warranties of merchantability or fitness for a particular purpose. This article assumes that you are familiar with the programming language that is being demonstrated and with the tools that are used to create and to debug procedures. Microsoft support engineers can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific requirements.

When you start a program from an icon or the command line, it registers the class name of its main window. The window class provides information about the name, attributes, and resources required by the program. The Microsoft Access window has a class name of "OMain." Additional command class names are provided at the end of this article.

By calling FindWindow() with a combination of a specific program's class name or the title bar caption, Access can determine whether that specific program is running.

You can determine the class name of an application by using Spy.exe, which is supplied with the Microsoft Win32 SDK.

If the window has a caption bar title, you can also use the title to locate the instance of the running application. This caption text is valid even when the application is minimized to an icon.

The following example shows three ways to determine if the Windows Calculator is running.

NOTE: You may have some Microsoft Windows API functions defined in an existing Microsoft Access library; therefore, your declarations may be duplicates. If you receive a duplicate procedure name error message, remove or comment out the declarations statement in your code.

1. In a new database, create a module and type the following lines in the Declarations section:

   Option Explicit
   
   Option Compare Database
      Declare Function FindWindow Lib "user32" Alias "FindWindowA" _
      (ByVal lpClassName As Any, ByVal lpWindowName As Any) As Long
   
   					

2. Type the following procedure:

   Function CalculatorUp ()
   
      Const lpClassName = "SciCalc"
      Const lpCaption = "Calculator"
   
      'This demonstrates three different ways to call FindWindow:
         '1. The ClassName only.
         '2. The Caption only.
         '3. Both the ClassName and the Caption
   
      MsgBox "Calculator Handle = " & FindWindow(lpClassName, _
            VBNullString)
      MsgBox "Calculator Handle = " & FindWindow(VBNullString, _
            lpCaption)
      MsgBox "Calculator Handle = " & FindWindow(lpClassName, _
            lpCaption)
   
      'This function could return the handle of a window.
      CalculatorUp = FindWindow(lpClassName, 0&)
   End Function
   
   					

3. To test this function, start Calculator, type the following line in the Immediate window, and then press ENTER:

   ?CalculatorUp()
   						

   
   Note that three message boxes open, each displaying the handle to the Calculator window. If Calculator is not running, each message box will display "0".

The following are class names of some common Windows applications:

For more information, see the Microsoft Win32 Software Development Kit.