I come from a C# background but we have an app that is written a long time ago and is a vbp project file
What development enviroment will open that file - I know cls
This is a Visual Basic project file for vb6. Visual Studio should be able to open this file but you will need the VB6 IDE.
Update: As of Visual Studio 2010, VB6 projects are no longer supported. VB6 is still supported by Microsoft, but you will need the specific IDE for it to perform project updates. As of 9 February 2018, this can be found on the VB6 support site.