PRB: No Trappable Error on Shell to Nonexistent .BAT File

• Microsoft Visual Basic for Windows, versions 2.0 and 3.0, Standard and Professional Editions
• Microsoft Visual Basic Programming System for Windows, version 1.0

SYMPTOMS

When you execute the Shell command to run a batch (.BAT) file, no error message is generated if the batch file specified does not exist.

MORE INFORMATION

The following is from the Help topic "Shell":

   The Visual Basic Shell command runs an executable program. If the
   Shell() function successfully executes the named program, it returns
   the instance handle of the started program. If the Shell() function
   can't start the named program, an error occurs.

     Cannot find file.
     Check to make sure the path and filename are correct.

CAUSE

This is by design and is the same behavior you see when using the Windows API WinExec. "Shelling" to a batch program generates a handle to the command processor, which runs the batch program. This handle is returned to Visual Basic. When the command processor determines that there is no such batch file, it is a process independent from Visual Basic and the error is therefore not reported to Visual Basic.

Steps to Reproduce the Problem

1. Start Visual Basic; or, if Visual Basic is already running, choose New Project (ALT + F, N) from the File menu. Form1 is created by default.

2. Add a text box (Text1) to Form1 and type "Bogus.Bat" (without the quotes) in the Text property.

3. Add the following code to the Form_Click event for Form1:

         Sub Form_Click()
         Dim X As Integer
           X = Shell((Text1.Text))
           Print X, Err
           'returns a valid process handle in x and err = 0
         End Sub
   
   

4. From the Run menu, choose Start (ALT + R, S), or press the F5 key to run the program. Click the form.

Workaround

Replace the code from the above example with the following code:

      Sub Form_Click()
      Dim X As Integer

        If Len(Dir((Text1.Text))) > 0 Then
          X = Shell((Text1.Text))
          Print X, Err
          'returns a valid process handle in x and err = 0
        Else
          MsgBox "No such file exists."
        End If
      End Sub