Message Analyzer: Error loading VAPR.OPN


After installing and updating Microsoft Message Analyzer, it complains about an error in VAPR.OPN:

“…VAPR.opn(166,46-166,49):  Invalid literal 2.0 : Input string was not in a correct format.”


There is a reference to line 166, character 46-49 in the VAPR.opn file. This particular opn is a parser for the App-V protocol. If we open the file and look at its contents at the specified location it does indeed contain the text “2.0”. As far as I can tell, the code in question (2.0 as decimal) tries to convert the string 2.0 to a decimal value and fails. In a broader context, the line appears to try a version check of the protocol as line 16 refers to the protocol as MS-VAPR version 2.0.

Sadly I do not have the time required to learn the PEF OPN language structure, but it resembles C#. In C# the code “as decimal” fails to compile since decimal is not a nullable type. The “as” keyword tries to cast a value into a specific type and returns null if the conversion fails. In C#. OPN seems to have a different approach. Just as an experiment, I tried replacing 2.0 with just 2, and voila, the OPN “compiles” and the error message goes away. I do not have any APP-V captures to test, so I cannot guarantee that this will actually work.

Possible solution

  • Open your VAPR.OPN file and navigate to line 166, column 46
  • image
  • Replace “2.0” with just “2”
  • Save the file
  • Restart Message Analyzer


Just remove the VAPR.OPN file. You will not receive the error message, and your ability to parse the MS-VAPR protocol will also vanish. Though you did not really have that ability anyway as the parser did not compile.