ASP.NET WebApi unit testing with Request.CreateResponse

You could set up the controller object for testability like this:

var config = new HttpConfiguration();
var request = new HttpRequestMessage(HttpMethod.Post, "http://localhost/api/products");
var route = config.Routes.MapHttpRoute("DefaultApi", "api/{controller}/{id}");
var routeData = new HttpRouteData(route, new HttpRouteValueDictionary { { "controller", "products" } });

controller.ControllerContext = new HttpControllerContext(config, routeData, request);
controller.Request = request;
controller.Request.Properties[HttpPropertyKeys.HttpConfigurationKey] = config;

Copied from Peter Provost's comprehensive blog post on Unit Testing ASP.NET Web API.

Another way to solve this is to do the following:

controller.Request = new HttpRequestMessage();
controller.Request.Properties.Add(HttpPropertyKeys.HttpConfigurationKey, 
                                  new HttpConfiguration());

If you are upgrading to webapi 5.0, then you'll need to change this to:

controller.Request = new HttpRequestMessage();
controller.Request.SetConfiguration(new HttpConfiguration());

The reason why you need to do this is because you have to have Request populated on the controller otherwise the extension methods on Request won't work. You also have to have an HttpConfiguration set on the Request otherwise routing and other parts of the pipeline won't function correctly.

For Web API 2, you can simply add

controller.Request = new HttpRequestMessage();
controller.Configuration = new HttpConfiguration();

Like so

[TestMethod]
public void GetReturnsProduct()
{
    // Arrange
    var controller = new ProductsController(repository);
    controller.Request = new HttpRequestMessage();
    controller.Configuration = new HttpConfiguration();

    // Act
    var response = controller.Get(10);

    // Assert
    Product product;
    Assert.IsTrue(response.TryGetContentValue<Product>(out product));
    Assert.AreEqual(10, product.Id);
}

It's important to set Request and Configuration on the controller. Otherwise, the test will fail with an ArgumentNullException or InvalidOperationException.

See here for more info.