A simple POST request to Web API does not hit the API at all

From my MVC application, I am trying to make a POST request to these sample end-points (actions) in an API controller named MembershipController:

[HttpPost]
public string GetFoo([FromBody]string foo)
{
    return string.Concat("This is foo: ", foo);
}

[HttpPost]
public string GetBar([FromBody]int bar)
{
    return string.Concat("This is bar: ", bar.ToString());
}

[HttpPost]
public IUser CreateNew([FromBody]NewUserAccountInfo newUserAccountInfo)
{
    return new User();
}

Here's the client code:

var num = new WebAPIClient().PostAsXmlAsync<int, string>("api/membership/GetBar", 4).Result;

And here's the code for my WebAPIClient class:

public class WebAPIClient
{
    private string _baseUri = null;

    public WebAPIClient()
    {
        // TO DO: Make this configurable
        _baseUri = "http://localhost:54488/";
    }

    public async Task<R> PostAsXmlAsync<T, R>(string uri, T value)
    {
        using (var client = new HttpClient())
        {
            client.BaseAddress = new Uri(_baseUri);

            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));

            var requestUri = new Uri(client.BaseAddress, uri);

            var response = await client.PostAsXmlAsync<T>(requestUri, value);

            response.EnsureSuccessStatusCode();

            var taskOfR = await response.Content.ReadAsAsync<R>();

            return taskOfR;
        }
    }
}

I have the following default route defined for the Web API:

config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{action}/{id}",
            defaults: new { id = RouteParameter.Optional }

UPDATE My code breaks into the debugger until the time the PostAsXmlAsync method on the System.Net.HttpClient code is called. However, no request shows up in Fiddler.

However, if I try to compose a POST request in Fiddler or try to fire a GET request via the browser to one of the API end-points, the POST request composed via Fiddler tells me that I am not sending any data and that I must. The browser sent GET request rightly tells me that the action does not support a GET request.

It just seems like the System.Net.HttpClient class is not sending the POST request properly.

One of the most usual problems is that you don't use the appropriate attribute.

Take into account that there are attributes for ASP.NET MVC and ASP.NET Web API with the same name, but which live in different namespaces:

• For Web API you must use the one in System.Web.Http
• For MVC, the one in System.Web.MVc

This is a very very usual error, and it affects to allkind of things that exist for both MVC and Web API. So you must be very careful when using something which can exists in bith worlds (for example filters, attributes, or dependency injection registration).