Cannot define a class or member that utilizes 'dynamic' because the compiler required type 'System.Runtime.CompilerServices.DynamicAttribute'

I had a similar error on a project I was trying to migrate. Try re-targeting the framework of the project back a version or two, and once you find a target version where you do not get the error, re-target back to the version you originally had trouble with.

I was getting the same error on a website project targeted for 4.6.2 that was referencing some older libraries, re-targeted it back to 4.5.1 and that resolved the error. I immediately re-targeted back to 4.6.2 and did not encounter the error again.

I had this same issue, but none of the answers here solved it, but did point me in the right direction. I think I didn't update the .NET version on my machine to the newer one. After installing it, I rolled back the targeted framework (as was suggested), retargeted the newer framework, and did a few extra things also:

1. Reinstalled ALL packages by running this in the package manager:

   update-package -reinstall

I also compared my project to one that was working for 4.5 and did this:

2. Opened my project file and removed all "BCL" related elements.
3. Uninstalled all "BCL" packages in the NuGet package manager for the project (guess it's not needed moving to 4.5+...?)
4. Removed an old package version of System.Net.Http, which did not get updated for some reason (see web.config - versions should show oldVersion="0.0.0.0-4.0.0.0" newVersion="4.0.0.0").
5. Have to restart VS.
6. Rebuild solution.
7. Error is now gone. ;)

One other thing I did to resolve some other errors was to clear the component cache by deleting all files in here:

C:\Users\{USERNAME}\AppData\Local\Microsoft\VisualStudio\14.0\ComponentModelCache\

[rant] Wasted a whole day with all this. Thanks to MS for stealing one more day of my life fixing senseless issues that should never be. Every time I upgrade I always get a wonderful surprise, and a lovely game of fix the red herring. Would it not hurt to detect these conflicts for those upgrading old projects? Perhaps better error traces when things go wrong? I doubt it would be that difficult. Perhaps even something like AsmSpy, but more advanced, built in to VS? [/rant]

I got this error suddenly a couple days after updating to VS2015.3 using 'traditional' MVC app (not new .NET Core / vNext app).

Turns out this error can be a complete red herring.

Make sure:

• You haven't got any other errors at all
• You've checked the output window for any errors that don't show in the 'Output' window.
• Make sure you don't have any conflicting references

In my case I had a typescript error that was causing an error and for some reason triggering this weird 'red herring' dynamic error too.

My fix was to go into the .csproj file and update the typescript tools version to 1.8 (need to unload project and edit by hand). As I said this is unrelated to the error, but I mention the specifics because someone else may end up with this same problem.

I believe that 1.7 didn't accept 'None' for the typescript module system whereas 1.8 does, hence the error.