Different assemblies with the same assembly name, namespace and class? Use an extern alias (which is how different versions of the same assembly can be used if necessary): https://docs.microsoft.com/en-us/dotnet/csharp/language-refe...

Different assemblies with different assembly names but same namespace and class? Just use the fully qualified type name (and alias it for a shorter name).

Single assembly/project and namespace with the same class in multiple files? If you're just spreading out the code then use partial classes, otherwise why would you define the same type twice?

C# (and other languages) that use virtual namespaces can do everything that file-based namespaces can do, but also support many more scenarios.