I recently created a WCF service (dll) and a service host (exe). I know my WCF service is working correctly since I am able to successfully add the service to WcfTestClient.
I just ran into this issue and checked all of the above answers to make sure I wasn't missing anything obvious. Well, I had a semi-obvious issue. My casing of my classname in code and the classname I used in the configuration file didn't match.
For example: if the class name is CalculatorService and the configuration file refers to Calculatorservice ... you will get this error.