There seems to be an error in the spine-csharp project with the latest update. I just pulled down the code from GitHub - EsotericSoftware/spine-runtimes: 2D skeletal animation runtimes for Spine. in order to update from 3.7 to latest. We are building from source, and after updating I ran into an error in Skin.cs because it did not know what Spine.Collections was. After poking at it for a moment, I noticed there is no reference to OrderedDictionary.cs where this is defined in the spine-csharp csproj file.
I was able to fix it easily enough by simply including Collections/OrderedDictionary.cs into the project. However wanted to make sure people were aware of it.