Asp.Net MVC Consuming a web api from a Class Library

Web API methods are called over HTTP, so calling a Web API service requires it to be hosted somewhere and called using a suitable client as detailed in @David's link. You can self-host Web API, so in theory you could have an assembly local to an MVC application which contained a class which set up and then called a self-hosted Web API service.

You would inject the Web API service behind an interface, something like this:

public interface IProductsService
{
    IEnumerable<Product> GetAllProducts();
}

...implemented something like this:

public class SelfHostedWebApiProductsService
{
    public SelfHostedWebApiProductsService()
    {
        // Set up a self-hosted Web API service
    }

    public IEnumerable<Product> GetAllProducts()
    {
        // Call your self-hosted WebApi to get the products
    }
}

Configure your DI container to use SelfHostedWebApiProductsService for the IProductsService interface, and away you go. This article details how to set up and call a self-hosted Web API.

As the SelfHostedWebApiProductsService sets up the self-hosted Web API in its constructor - a relatively expensive operation - you might want to consider giving this class a singleton lifetime in your DI container.