This error occurs when your referenced projects or referenced lib is mixed mode and built upon different dotnet frame work version.
For this you have to set target framework version same for all project and if you are using some lib then it will be better to build in current dotnet version or you can add following configuration in app.config file.
< startup useLegacyV2RuntimeActivationPolicy=”true”>
< supportedRuntime version=”v4.0″/>
< requiredRuntime version=”v4.0.20506″/>
You have to change supportedRuntime and requiredRuntime if you are facing this problem rather than version 4.0.