feat(wpcarro/blog): Create short post about C#

Inspired by Profpatsch's "Notes", here's a short post about a thing that
happened today...

Note: I'd like a way to convert `git log --format=%b` into blog
posts (maybe). Maybe some commit metadata like `POST=true`

Change-Id: I492c48c81891d9f64da8c8149d0d2bfa864ed731
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5889
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>

users/wpcarro/website/blog/posts.nix

@@ -29,4 +29,11 @@
     content = ./posts/auto-reboot-nixos.md;
     draft = false;
   }
+  {
+    key = "csharp-unused-variables";
+    title = "Unused Variables Broke Prod";
+    date = 1655840877;
+    content = ./posts/csharp-unused-variables.md;
+    draft = false;
+  }
 ]

users/wpcarro/website/blog/posts/csharp-unused-variables.md

@@ -0,0 +1,40 @@
+# Unused Variables Broke Prod
+
+**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:
+
+```diff
++    <!-- 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.