If you are writing MsBuild scripts that are expected to work consistently in a number of different environments it quickly becomes clear that encoding pre-requisites in the script and anticipating configuration errors across these environments can be a huge time saver.
It Works On My Machine
Installations, configurations, folders, environment variables can all differ on individual machines depending on their use: CI servers, plain build servers, developer's local machines etc. etc.
Property Diagnostics
Providing diagnostics for a correctly configured PropertyGroup value is easy using the Error task with a condition checking that the value is/is not empty:
Usually, some other part of the script or a calling process will fill in the blank value and all your script is trying to do is make sure there is some kind of sensible value in there.
Item Diagnostics
What about if we want to do a similar validation against an ItemGroup? We can rely on a similar technique and force the ItemGroup list into a delimited list using the "@" operator and test this value in the Error task.
Wix
Here's how I try to ensure Wix is installed on a machine using this technique.
The two FindWixCompiler values are include to support either a 32-bit or 64-bit machines.