The sample API code provided by PayPal has not been updated in some time. When it comes to recent versions of Microsoft’s .NET platform, not only is PayPal missing some DLLs needed to use it ‘out of the box’, the sample functions are actually wrong!
We discovered the problem while working on a custom built .NET web application for the Council of Public Relations Firms that includes integrated eCommerce. Thankfully, with some sleuthing and tinkering, we were able to update the necessary DLLs and correct the functions.