Returning an error message REST API with Xamarin

Question

I am developing (at least attempting anyways...) a xamarin android app with connection to API. Below is a basic example of what I am trying to do with a Post. I am not sure how to return the error message (not just code) back to client? I am open to a better way of doing this as long as a complete example can be provided.

API code:

[HttpPost("Consume")]//*
public IActionResult ConsumeInventory([FromBody] Part part)
{
    try
    {
        _repo.ConsumeInventory(part);
    }
    catch (Exception ex)
    {
        //ModelState.AddModelError("Error", ex.Message);
        return NotFound("Not Enough Inventory");
    }

    return Ok(part);
}

Client side call:

public async Task<HttpResponseMessage> UpdatePartAsync(Part part, string post_url)
{
    string jsonString = JsonConvert.SerializeObject(part);
    client.DefaultRequestHeaders.Accept.Add(new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"));

    var response = await client.PostAsync("invcount/" + post_url, new StringContent(jsonString, Encoding.UTF8, "application/json"));

    if (response.StatusCode == HttpStatusCode.NotFound)
    {...//Would like to display "Not enough Inventory" in here}

    return response;        
}

Mando · Accepted Answer · 2018-01-30 21:00:10Z

Even when the API call is unsuccessful, you can read the content of the response by using HttpResponseMessage.Content.ReadAsStringAsync()

if (response.StatusCode == HttpStatusCode.NotFound)
{
    // Would like to display "Not enough Inventory" in here}
    var resposeContent = response.Content?.ReadAsStringAsync();
    var result = JsonConvert.DeserializeObject<ServerResponse>(resposeContent);

    // this guy will have your human-readable description or in your case, you can display the whole `result` string
    Debug.WriteLine(result.Description); 
}

And define you ServerResponse class as you designed your API. I regularly inherit all my API responses from the same base class so on the client side I can parse them first to check for custom directives from server (custom codes which are not always errors) message:

public class ServerResponse
{
     public string Code {get;set;}
     public string Description {get;set;}
}

ps: beware, HttpResponseMessage.Content could be null in certain cases.

EvZ · Accepted Answer · 2018-01-30 20:47:11Z

Your question has nothing to do with Xamarin. In this case Xamarin app is just a front-end that consumes your API.

Generally speaking, you should generalise your error responses and if needed log the exception to DB for a further analysis. JSON example:

{
  "Code": 1000,
  "Description": "Human readable description",
  "Id": "6a154fd3-4142-4a9c-80b5-c635e84123fa"
}

Code for internal error code in the system and Id for identifying the stack trace in the DB. So your end-user will get a nice human readable description and an Id that he could use to contact the support.

Technically, use a ExceptionFilterAttribute in order to catch and return a generalised error response to your clients. Reading Global Error Handling in ASP.NET Web API 2 may also give you a better overview.

In your client app after you get a response from the API you can check if the response has a success status code:

if (!response.IsSuccessStatusCode)
{
    // Deserialize error message
}

P.S.: Stackoverflow is not a coding service, so there will be no a complete example. Hope you will get some ideas from my answer on how to handle it in a better way.

Good luck!

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