Problem: This morning we broke production because (believe it or not) an unused variable went undetected.

Solution: Consume the variable in the relevant codepath.

Further AI: Treat unused variables as errors (which will block CI).

Warning/Disclaimer

I am not a C# programmer. I know close to nothing about C#. But at $WORK, one of our codebases is written in C#, so occasionally I interface with it.

Treating Unused Variables as Errors

C# uses .csproj files to configure projects. The following changes to our .csproj file WAI'd:

+    <!-- IDE0059: Remove unnecessary value assignment -->
+    <WarningsAsErrors>IDE0059</WarningsAsErrors>
+    <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>

However, supporting this turned out to be a ~1h adventure... Why was this unexpectedly difficult? As it turns out, there are the 3x promising compiler warnings that I had to discover/try:

• CS0219: doesn't WAI (see "Note" here: https://docs.microsoft.com/en-us/dotnet/csharp/misc/cs0219)
• CA1804: silently unsupported (replaced by IDE0059)
• IDE0059: WAIs

Legend:

• CS: stands for C#
• CA: stands for Code Analysis (I think a Visual Studio concept)
• IDE: stands for IDE (I think also a Visual Studio concept)

For CA and IDE prefixed warnings, EnforceCodeStyleInBuild must also be enabled.