My C# project---a plugin for Rhino---shows the following strange behavior: after a while, it randomly stops compiling, with a compile-time error (shown in the editor too) on the call to Socket.Receive(). The message (in french) translates to: A reference to the type 'Span<>' declares that it is defined in 'mscorlib', but cannot be found.
I can go to the definition of that symbol in Visual Studio (F12). It shows the decompiled version of the Socket DLL. However, it is not the same one works fine and when it no longer works. When it works, the decompiled version shows no usage of the Span<> type; when it no longer works, the decompiled version shows it used at various places, including in some overloads of Socket.Receive(). But the type Span<> is not defined anywhere. I did not include System.Memory in my .csproj and I don't think I need to; I'm using net48. FWIW, the decompiled version is full of this error wherever Span<> appears:
//IL_001b: Unknown result type (might be due to invalid IL or missing references)
I could fix the problem once by reinstalling Visual Studio from scratch. That seems like an expensive workaround if it needs to be repeated every few days... I am absolutely sure that it stopped working very suddenly, while I was working on the project, and not after any installing or uninstalling anything. Version control says the .csproj and .sln files are unchanged. I did add a new unrelated .cs file recently, so it may be related to that, though a mostly empty one so far. (For some reason the list of .cs files does not show up in the the .csproj file, unlike what I'm used to in different projects.)
What I would like to understand is why Visual Studio suddenly picks up a different DLL for the Socket class. How can I even know where that file is? The decompiled version starts with // emplacement inconnu, i.e. unknown place.