Converting the content of HttpResponseMessage to object

So, first to address the you need Newtonsoft.Json comments, I really haven't felt the need yet. I've found the built in support to work well so far (using the APIError Json in my original question:

[DataContract]
internal class APIError
{
    [DataMember (Name = "status")]
    public int StatusCode { get; set; }
    [DataMember (Name = "code")]
    public int ErrorCode { get; set; }
}

I have also defined a JsonHelper class to (de)serialize:

public class JsonHelper
{
    public static T fromJson<T> (string json)
    {
        var bytes = Encoding.Unicode.GetBytes (json);

        using (MemoryStream mst = new MemoryStream(bytes))
        {
            var serializer = new DataContractJsonSerializer (typeof (T));
            return (T)serializer.ReadObject (mst);
        }
    }

    public static string toJson (object instance)
    {
        using (MemoryStream mst = new MemoryStream())
        {
            var serializer = new DataContractJsonSerializer (instance.GetType());
            serializer.WriteObject (mst, instance);
            mst.Position = 0;

            using (StreamReader r = new StreamReader(mst))
            {
                return r.ReadToEnd();
            }
        }
    }
}

The above bits I already had working. As for a single method that would handle each request execution based on the type of result expected while it makes it easier to change how I handle things (like errors, etc), it also adds to the complexity and thus readability of my code. I ended up creating separate methods (all variants of the Execute method in the original question:

// execute and return response.StatusCode
private static async Task<HttpStatusCode> ExecuteForStatusCode (HttpRequestMessage request, bool authenticate = true)
// execute and return response without processing
private static async Task<HttpResponseMessage> ExecuteForRawResponse(HttpRequestMessage request, bool authenticate = true)
// execute and return response.IsSuccessStatusCode
private static async Task<Boolean> ExecuteForBoolean (HttpRequestMessage request, bool authenticate = true)
// execute and extract JSON payload from response content and convert to RESULT 
private static async Task<RESULT> Execute<RESULT>(HttpRequestMessage request, bool authenticate = true)

I can move the unauthorized responses (which my current code isn't handling right now anyway) into a new method CheckResponse that will (for example) log the user out if a 401 is received.

i would use a Deserializer.

HttpResponseMessage response = await client.GetAsync("your http here");
            var responseString = await response.Content.ReadAsStringAsync();
[Your Class] object= JsonConvert.DeserializeObject<[Your Class]>(responseString.Body.ToString());

Here is an example of how I've done it using MVC API 2 as backend. My backend returns a json result if the credentials are correct. UserCredentials class is the exact same model as the json result. You will have to use System.Net.Http.Formatting which can be found in the Microsoft.AspNet.WebApi.Client NugetPackage

public static async Task<UserCredentials> Login(string username, string password)
{
    string baseAddress = "127.0.0.1/";
    HttpClient client = new HttpClient();

    var authorizationHeader = Convert.ToBase64String(Encoding.UTF8.GetBytes("xyz:secretKey"));
    client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", authorizationHeader);



    var form = new Dictionary<string, string>
    {
        { "grant_type", "password" },
        { "username", username },
        { "password", password },
    };

    var Response = await client.PostAsync(baseAddress + "oauth/token", new FormUrlEncodedContent(form));
    if (Response.StatusCode == HttpStatusCode.OK)
    {
        return await Response.Content.ReadAsAsync<UserCredentials>(new[] { new JsonMediaTypeFormatter() });
    }
    else
    {
        return null;
    }
}

and you also need Newtonsoft.Json package.

public class UserCredentials
    {
        [JsonProperty("access_token")]
        public string AccessToken { get; set; }

        [JsonProperty("token_type")]
        public string TokenType { get; set; }

        [JsonProperty("expires_in")]
        public int ExpiresIn { get; set; }

        //more properties...
    }