Reasons you should prefer a single class / interface / record / enum / struct per file in C# projects.
Congitive load
Multiple types per file increases the cognitive overhead of remembering where everything lives.
When working on a project, having to remember where each thing is hidden and how things are organised is extra mental load that can be eliminated by following the simple rule of “every class gets a file” regardless of how trivial or short they may be.
Reduced dependence on IDE support
IDEs that can jump to a definition are not the only tool we use to inspect and navigate code.
By hiding multiple types in a file where the name doesn’t match you make it harder to operate in a codebase without a full IDE.
You might be an IDE-only developer but not everyone operates that way, and you might be missing out on some other excellent tools out there.
Git merge conflict avoidance
You are more likely to run in to merge conflicts if you put types all together in one file as multiple developers work on a project. Especially when things get moved / renamed / refactored.
Simple refactoring
Putting multiple things in a file breaks refactoring tool flows (resharper) which assume they can rename files to match types.
Can be applied consistently
Having multiple types per file requires you to make constant judgement calls about what goes together in one file and what is separate. Different developers will inevitably make different judgement calls.
This problem can be entirely eliminated by sticking to class-per-file.
It is impossible to come up with a concrete rule for exactly what goes together in a file and what is separate, whereas the rule of one-class-per-file is trivial to apply consistently.
Avoid need for additional naming
If you have one class in a file you can name the file the same as the class. If you have two types in a file what do you name the file?
If you have two types in a file you now have to come up with a third name that covers both things, or use the name of only one of the things, which is then misleading / surprising. By having a class per file you eliminate this entirely.
There's an updated version of this on my blog, see the "originally published at" link at the top of the post.
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