Hi Andy,
I have good news, as I can now create a C# DLL directly with Visual Studio 2019 and have the external functions work in Xojo 2020 r1.
Here are the instructions on how to create a CSharp and Xojo file:
[Create a CSharp DLL ](https://scispec.ca/index.php/blog/55-create-a-csharp-dll-for-xojo
Here is the Github project with all the files:
CSharpDLLXojo
Here is a running screen grab:
Let me know if it works on your computer