C# classes in separate files?

While the one class per file policy is strictly enforced in Java, it's not required by C#. However, it's generally a good idea.

I typically break this rule if I have a very small helper class that is only used by the main class, but I prefer to do that as a nested inner class for clarity's sake.

You can however, split a single class into multiple files using the partial keyword. This is useful for separating your code from wizard-generated code.

Files are cheap, you aren't doing anyone a favor by consolidating many classes into single files.

In Visual Studio, renaming the file in Solution Explorer will rename the class and all references to that class in your project. Even if you rarely use that feature, the cheapness of files and the ease of managing them mean the benefit is infinitely valuable, when divided by its cost.

As others have said, one file per type in general - although where others have made the public/private distinction, I'd just say "one top-level file per type" (so even top-level internal types get their own files).

I have one exception to this, which is less relevant with the advent of the Func and Action delegate types in .NET 3.5: if I'm defining several delegate types in a project, I often bunch them together in a file called Delegates.cs.

There are other very occasional exceptions too - I recently used partial classes to make several autogenerated classes implement the same interface. They already defined the appropriate methods, so it was just a case of writing:

public partial class MessageDescriptor : IDescriptor<MessageDescriptorProto> {}
public partial class FileDescriptor : IDescriptor<FileDescriptorProto> {}

etc. Putting all of those into their own files would have been slightly silly.

One thing to bear in mind with all of this: using ReSharper makes it easier to get to your classes whether they're in sensibly named files or not. That's not to say that organising them properly isn't a good thing anyway; it's more to reinforce the notion that ReSharper rocks :)