"Missing compiler required member" error being thrown multiple times with almost no changes to code

Question

Today after deploying some changes to a C# MVC site that I run, I went back to make some more modifications and came across this error:

Missing compiler required member System.Runtime.CompilerServices.ExtensionAttribute..ctor

The error is a bit vague (other than it's description, obviously) as it doesn't give me a file, line, or column to reference, only the project. Also, it throws the error a total of 20 times. I only made three changes to the code between the time I deployed (it was completely functional at that time) and now. I reverted my changes and it is still throwing the same error which makes no sense to me.

I haven't found a lot of information on this error on SO or Google, other than this guys solution and a couple references to some Mono project errors (I'm not using Mono). The solution the guy above gives requires adding a class definition that will allow the compiler to resolve the reference. I don't particularly want to do this because I haven't needed to do it up until this point and it will just muddy my code.

Just curious if anyone has run across this before. Thanks in advance!

Answer 1

In my case it was because the project was not referencing Microsoft.CSharp. Once I added a reference to that assembly, it compiled just fine.

Answer 2

I don't know if anyone else has experienced this, but I suddenly ran into this error after adding some code utilizing dynamic types and incorporating WebAPI into a project that originated as a TypeScript application in VS2013. Simply adding a reference to Microsoft.CSharp resolved my issue.

Hope this helps someone else.