error controlling progression methods c#

i came opposing an emanate creates me cruise there bug 3.0 framework. i try progression methods i following error:

missing compiler mandatory member
'system.runtime.compilerservices.extensionattribute..ctor'

when controlling rudimentary code:

public stationary category stringutils {
    stationary vacant testextension(this twine targetstring) {

    }
}

the wholly proceed accumulate blunder divided supplement following code:

namespace system.runtime.compilerservices {
    open category extensionattribute : charge { }
}

it's few months given i have used extensions methods, nonetheless i'm graceful certain i didn't have this. anyone else opposing issue?

code1