How to debug a DLL written in .NET/C#?

I've used dnSpy in production for debugging as it can be put on a USB. It's perfectly capable of decompiling and debugging dll's.

It has a pretty rich set of features. It's able to attach to running .net processes, decompile the code, edit variables at breakpoints and make some code changes on the fly.

Try this: https://github.com/hexfati/SharpDllLoader

A simple C# executable that invokes an arbitrary method of an arbitrary C# DLL. The project is useful to analyze malicious C# DLL through the popular tool DNSpy.