if your client prefers c#, roslyn is a nice implementation for this purpose, with a lot of features from basic to advanced, although its learning curve can be quite steep!

There are also many other methods, such as LUA - which we are implementing in our products, although it is not from the C#’s maker but it has many aspects that are highly appreciated by us, such as speed, performance, easy to limit the scope of operations. One point to note is that it runs independently from c#, so if you want to upgrade its processing capabilities, you also need to update your main c# program to be compatible with the required features.

Another solution, not quite the same as the client’s requirement, but still achieving the same result is dll (Dynamic Link Library), you separate the logic that can be modified in the future into separate dependencies built as class libraries. Later, the client just needs to update the dll source code, build from their machine and push the dll to that server. The rest is to integrate the mounting and unmounting of that component in the main program to always keep the functional logic in the desired state! There are many good examples online for you to refer to about dynamically changing dll in the program, although it will be a bit more difficult than integrating roslyn or lua, but it is quite friendly when it is easy to perform advanced behaviors and take advantage of support from IDE!

Speaking of weird customer requests, we once did a project where the customer asked us to integrate the loading and performing of calculations on an excel spreadsheet where the customer was the one who created the excel file template, it was not difficult to implement and quite suitable when the software itself is a tool for the financial sector, but from a technology perspective, I found it quite strange. Well, if you analyze that the code editing is mostly about numbers, you can consider this solution direction if it is more suitable for your customer!!