Is is possible to export functions from a C# DLL like in VS C++?

Unmanaged Exports => https://sites.google.com/site/robertgiesecke/Home/uploads/unmanagedexports

DLLExport => https://github.com/3F/DllExport

How does it work?

Create a new classlibrary or proceed with an existing one. Then add the UnmanagedExports Nuget package.

This is pretty much all setup that is required.

Now you can write any kind of static method, decorate it with [DllExport] and use it from native code.
It works just like DllImport, so you can customize the marshalling of parameters/result with MarshalAsAttribute.

During compilation, my task will modify the IL to add the required exports...

I've seen people do this before, but it required ildasm, adding the MSIL .export directive, and then reassembling. A program named dll_tool can do these steps for you.

If you want to build a mixed-mode DLL with both native and managed exports, you should be using C++/CLI, which is specially designed for this purpose.

Yes, it is possible to export functions from a C# dll in much the same way that C++ does it! You need a little help from an add-in Unmanaged Exports (DllExport for .Net) that facilitates this process, or from a similar method such as Exporting functions in C#/VB.NET to native code.

Please see Code to Export C# DLL to Metatrader Build 600+ for a working example using Robert Giesecke's C# Project Template for Unmanaged Exports to export a C# dll to a legacy application (Metatrader) that has a great deal of similarity to C++.

Additionally, you might find Native and .NET Interopability interesting though it is mostly geared toward accessing native code from within .NET rather than the other way around.