HOWTO: Programmatically Close a Separate Application

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 5.0

SUMMARY

This article shows you how use API functions to programmatically close a single instance of most Windows programs through your Visual Basic program. You can use this feature to start a program, run the program without any user interaction, and shut down the program when all the program tasks are completed.

MORE INFORMATION

To programmatically close a single instance of a Windows program, you begin by getting the handle to the Window that you want to close with the FindWindow function. The FindWindow API function returns the handle of a top-level window whose class name and window name matches the string parameters. This function returns the handle of the window as a long value if it is successful and a null if it fails. To use this function, you must supply two parameters:

• lpClassName - a pointer to a null-terminated string that specifies the class name or is an atom that identifies the class-name string. In this application, you can pass vbNullString.
• lpWindowName - a pointer to a null-terminated string that specifies the window name (the window's title).

• hWnd - the handle of the window you want to close. This long value is available as the result of the FindWindow function.
• uInt - the message to post to the message queue. In this application, you would use the WM_CLOSE message.
• wParam - the first message parameter. For this message, you should pass a null value.
• lParam - the second message parameter. For this message, you should pass a null value.

• hHandle - the handle of the object you want to monitor.
• dwMilliseconds - specifies the time-out interval, in milliseconds. If you set this parameter to INFINITE, the function's time-out interval never elapses.

If the handle still exists, then you can use the TerminateProcess function to stop the handle process. However, this extreme approach is not recommended because while you may have stopped the handle process, the DLL's used by the handle may still reside in memory, causing a memory leak. Use the TerminateProcess function with caution.

The next section shows how to create a sample program that demonstrates how to close a running application from a Visual Basic program. The program starts and stops the Windows calculator program.

Create the Sample Project

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default.

2. Add two CommandButtons to Form1.

3. Copy the following code to the Code window of the Form1 form:

         Option Explicit
   

         Private Declare Function WaitForSingleObject Lib "kernel32" _
            (ByVal hHandle As Long, _
            ByVal dwMilliseconds As Long) As Long
   
         Private Declare Function FindWindow Lib "user32" _
            Alias "FindWindowA" _
            (ByVal lpClassName As String, _
            ByVal lpWindowName As String) As Long
   
         Private Declare Function PostMessage Lib "user32" _
            Alias "PostMessageA" _
            (ByVal hwnd As Long, _
            ByVal wMsg As Long, _
            ByVal wParam As Long, _
            ByVal lParam As Long) As Long
   
         Private Declare Function IsWindow Lib "user32" _
            (ByVal hwnd As Long) As Long
   
         'Constants used by the API functions
         Const WM_CLOSE = &H10
         Const INFINITE = &HFFFFFFFF
   
         Private Sub Form_Load()
            Command1.Caption = "Start the Calculator"
            Command2.Caption = "Close the Calculator"
         End Sub
   
         Private Sub Command1_Click()
         'Starts the Windows Calculator
            Shell "calc.exe", vbNormalNoFocus
         End Sub
   
         Private Sub Command2_Click()
         'Closes the Windows Calculator
            Dim hWindow As Long
            Dim lngResult As Long
            Dim lngReturnValue As Long
   
            hWindow = FindWindow(vbNullString, "Calculator")
            lngReturnValue = PostMessage(hWindow, WM_CLOSE, vbNull, vbNull)
            lngResult = WaitForSingleObject(hWindow, INFINITE)
   
            'Does the handle still exist?
            If IsWindow(hWindow) = 0 Then
               'The handle still exists. Use the TerminateProcess function
               'to close all related processes to this handle. See the
               'article for more information.
               MsgBox "Handle still exists."
            Else
               'Handle does not exist.
               MsgBox "Program closed."
            End If
         End Sub
   
   

4. Press the F5 key to run the program. Click Start the Calculator to start the Windows calculator program. Click Close the Calculator to close the Windows calculator program.

REFERENCES

For additional information, please see the following articles in the Microsoft Knowledge Base:

   ARTICLE-ID: Q129797
   TITLE     : HOWTO: Launch a Win32 Application from Visual Basic

   ARTICLE-ID: Q129796
   TITLE     : HOWTO: 32-Bit App Can Determine When a Shelled Process Ends

   ARTICLE-ID: Q90749
   TITLE     : HOWTO: Implement a "Kill" Operation in Windows NT

Visual Basic 5.30 Programmer's Guide to the Win32 API by Dan Appleman