.NET library to write Alexa skills that's interface-compatible with Amazon's AlexaSkillsKit for Java and matches that functionality:

• handles the (de)serialization of Alexa requests & responses into easy-to-use object models
• verifies authenticity of the request by validating its signature and timestamp
• code-reviewed and vetted by Amazon (Alexa skills written using this library passed certification)

Beyond the functionality in Amazon's AlexaSkillsKit for Java, AlexaSkillsKit.NET:

• performs automatic session management so you can easily build conversational Alexa apps

This library was originally developed for and is in use at https://freebusy.io

This library is available as a NuGet package at https://www.nuget.org/packages/AlexaSkillsKit.NET/

Read Getting started with Alexa App development for Amazon Echo using .NET on Windows

If your Alexa skill does any kind of I/O and assuming you're building on top of .NET Framework 4.5 it's recommended that you derive your app from the abstract SpeechletAsync and implement these methods as defined by ISpeechletAsync

public interface ISpeechletAsync
{
    Task<SpeechletResponse> OnIntentAsync(IntentRequest intentRequest, Session session);
    Task<SpeechletResponse> OnLaunchAsync(LaunchRequest launchRequest, Session session);
    Task OnSessionStartedAsync(SessionStartedRequest sessionStartedRequest, Session session);
    Task OnSessionEndedAsync(SessionEndedRequest sessionEndedRequest, Session session);
}

Or derive your app from the abstract Speechlet and implement these methods as defined by ISpeechlet.

public interface ISpeechlet
{
    SpeechletResponse OnIntent(IntentRequest intentRequest, Session session);
    SpeechletResponse OnLaunch(LaunchRequest launchRequest, Session session);
    void OnSessionStarted(SessionStartedRequest sessionStartedRequest, Session session);
    void OnSessionEnded(SessionEndedRequest sessionEndedRequest, Session session);
}

Take a look at https://github.com/AreYouFreeBusy/AlexaSkillsKit.NET/blob/master/AlexaSkillsKit.Sample/Speechlet/SampleSessionSpeechlet.cs for an example.

The Sample app is using ASP.NET 4.5 WebApi 2 so wiring-up requests & responses from the HTTP hosting environment (i.e. ASP.NET 4.5) to the "Speechlet" is just a matter of writing a 2-line ApiController like this https://github.com/AreYouFreeBusy/AlexaSkillsKit.NET/blob/master/AlexaSkillsKit.Sample/Speechlet/AlexaController.cs

Note: sample project is generated from the ASP.NET 4.5 WebApi 2 template so it includes a lot of functionality that's not directly related to Alexa Speechlets, but it does make make for a complete Web API project.

Alternatively you can host your app and the AlexaSkillsKit.NET library in any other web service framework like ServiceStack.