How do I unit test web api action method when it returns IHttpActionResult?

This is the accepted answer by Kiran Challa, adapted for NUnit;

var valuesController = controller;
// if your action returns: NotFound()
IHttpActionResult actionResult = valuesController.Get(10);
var notFoundRes = actionResult as NotFoundResult;
Assert.IsNotNull(notFoundRes);

// if your action returns: Ok()
actionResult = valuesController.Get(11);
var posRes = actionResult as OkResult;
Assert.IsNotNull(posRes);

// if your action was returning data in the body like: Ok<string>("data: 12")
actionResult = valuesController.Get(12);
var conNegResult = actionResult as OkNegotiatedContentResult<string>;
Assert.IsNotNull(conNegResult);
Assert.AreEqual("data: 12", conNegResult.Content);

// if your action was returning data in the body like: Content<string>(HttpStatusCode.Accepted, "some updated data");
actionResult = valuesController.Get(13);
var negResult = actionResult as NegotiatedContentResult<string>;
Assert.IsNotNull(negResult);
Assert.AreEqual(HttpStatusCode.Accepted, negResult.StatusCode);
Assert.AreEqual("some updated data", negResult.Content);

https://docs.microsoft.com/en-us/aspnet/web-api/overview/testing-and-debugging/unit-testing-controllers-in-web-api#testing-actions-that-return-ihttpactionresult

Assert.IsInstanceOfType(httpActionResult, typeof(OkResult));

Time to resurrect a dead question

The current answers all rely on casting the response object to a known type. Unfortunately, the responses do not seem to have a useable hierarchy or implicit conversion path for this to work without intimate knowledge of the controller implementation. Consider the following:

public class MixedCodeStandardController : ApiController {

    public readonly object _data = new Object();

    public IHttpActionResult Get() {
        return Ok(_data);
    }

    public IHttpActionResult Get(int id) {
        return Content(HttpStatusCode.Success, _data);
    }
}

Testing the class:

var testController = new MixedCodeStandardController();

var getResult = testController.Get();
var posRes = getResult as OkNegotiatedContentResult<object>;
Assert.IsType<OkNegotiatedContentResult<object>>(getResult);
Assert.AreEqual(HttpStatusCode.Success, posRes.StatusCode);
Assert.AreEqual(testController._data, posRes.Content);

var idResult = testController.Get(1);
var oddRes = getResult as OkNegotiatedContentResult<object>; // oddRes is null
Assert.IsType<OkNegotiatedContentResult<object>>(idResult); // throws failed assertion
Assert.AreEqual(HttpStatusCode.Success, oddRes.StatusCode); // throws for null ref
Assert.AreEqual(testController._data, oddRes.Content); // throws for null ref

From outside the black box, the response stream is essentially the same. The test must know how the controller implemented the return call to test it in this way.

Instead, use the HttpResponseMessage object from the IHttpActionResult returned. This ensures the test can be consistent, even when the controller code may not be:

var testController = new MixedCodeStandardController();

var getResult = testController.Get();
var getResponse = getResult.ExecuteAsync(CancellationToken.None).Result;
Assert.IsTrue(getResponse.IsSuccessStatusCode);
Assert.AreEqual(HttpStatusCode.Success, getResponse.StatusCode);

var idResult = testController.Get(1);
var idResponse = idResult.ExecuteAsync(CancellationToken.None).Result;
Assert.IsTrue(idResponse.IsSuccessStatusCode);
Assert.AreEqual(HttpStatusCode.Success, idResponse.StatusCode);

Here Ok() is just a helper for the type OkResult which sets the response status to be HttpStatusCode.Ok...so you can just check if the instance of your action result is an OkResult...some examples(written in XUnit):

// if your action returns: NotFound()
IHttpActionResult actionResult = valuesController.Get(10);
Assert.IsType<NotFoundResult>(actionResult);

// if your action returns: Ok()
actionResult = valuesController.Get(11);
Assert.IsType<OkResult>(actionResult);

// if your action was returning data in the body like: Ok<string>("data: 12")
actionResult = valuesController.Get(12);
OkNegotiatedContentResult<string> conNegResult = Assert.IsType<OkNegotiatedContentResult<string>>(actionResult);
Assert.Equal("data: 12", conNegResult.Content);

// if your action was returning data in the body like: Content<string>(HttpStatusCode.Accepted, "some updated data");
actionResult = valuesController.Get(13);
NegotiatedContentResult<string> negResult = Assert.IsType<NegotiatedContentResult<string>>(actionResult);
Assert.Equal(HttpStatusCode.Accepted, negResult.StatusCode);
Assert.Equal("some updated data", negResult.Content);