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API controllers deal with data, controllers deal with the API controller and the view.

WCF is used for developing SOAP-based services whereas Web API is used for both SOAP-based and RESTful services. WCF does not offer any support for MVC features whereas Web API supports MVC features

As the name implies, attribute routing uses attributes to define routes. Attribute routing gives you more control over the URIs in your web API.

ASP .NET MVC is used to create web applications that return both views and data but ASP .NET Web API is used to create full-blown HTTP services with an easy and simple way that returns only data

OAuth is about Authorization while OpenID is Authentication.

When it comes to errors, study and know the error codes. Set break points when de-bugging.