Today I face this problem I was unable to debug my code and I spent several hours to solve it.
I got this exception when I put debug point and it displays this message on runtime.
Anyhow I found two solution
[assembly:System.Diagnostics.Debuggable(true,true)] attribute at assembly level.
I declare this attribute in assemblyinfo.cs
second solution is I change my webservice project from website to web application and everything is going smoothly :) Happy!!