How do you catch the VBScript runtime error: "Object required: 'Session' -- Developing vbs script for MSI, want to develop on cmd line where Session isn't defined.

Question

Writing a script for a custom msi installer action. When my script is invoked by an Installer you can get Installer properties via Session.Property("PropName")

If you don't invoke via installer you get a runtime exception. I want to make my script so I can develop and debug w/o the Installer so how do I catch this error?

I want to do something like:

if Session != null 
  setting=Session.Property("prop1")
else 
  setting="SomeOtherSetting"
end if

Answer 1

Are you looking for the VBScript syntax to check for null?

How about this:

If (IsNull(Session)) Then
  setting=Session.Property("prop1")
Else 
  setting="SomeOtherSetting"
End If

Answer 2

The problem is that the Session object isn't defined outside of MSI scripts, so any reference to its properties or methods will raise an exception. To check if an object or a variable is defined, you can use the IsEmpty function:

If Not IsEmpty(Session) Then
  setting = Session.Property("prop1")
Else
  setting = "SomeOtherSetting"
End If

Another possible solution is using the On Error Resume Next statement to catch exceptions caused by referencing the properties and methods of the Session object:

On Error Resume Next

setting = Session.Property("prop1")

If Err.Number <> 0 Then
  setting = "SomeOtherSetting"
End If