Found this in another post in the AFSDK Section ... It was the problem.
If you are including a reference to the PISDK within your own code, you must make sure that you use the PISDK supplied interops (OSIsoft.PISDK, OISsoft.PISDKCommon) and not the automatically generated interops. If you are using VS2010 and compiling to .NET 4.0, you must also ensure the "Embed Interop Types" setting to False for the PISDK reference libraries.
Glad that you managed to find the problem for your issue, and thanks for closing the loop on this.