Storing result of Web API call into Generic type class

Question

I have this simple model class which is defined like this:

public class CountryLanguage
{
    public string Name { get; set; }
    public string ShortName { get; set; }
    public string Description { get; set; }
}

I have this Web API which return an IEnumerable of CountryLanguage:

[HttpGet]
public IEnumerable<CountryLanguage> Get()
{        
    List<CountryLanguage> list = new List<CountryLanguage>();
    list.Add(new CountryLanguage());
    list.Add(new CountryLanguage());
    return list;
}

I have this class where I want to store the result of the Web API call:

public class ResponseResult<T>
{
    public HttpStatusCode StatusCode { get; set; }
    public string Message { get; set; }
    public T Payload { get; set; }
}

And finally, here is the code calling the Web API :

HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Get, actionToCallWithParameters);

var response = httpClient.SendAsync(request).Result;

ResponseResult<T> responseResult = new ResponseResult<T>();
responseResult.StatusCode = response.StatusCode;
responseResult.Message = response.Content.ReadAsStringAsync().Result;
responseResult.Payload = response.Content.ReadAsAsync<T>().Result;                

return responseResult;

If the web API return a CountryLanguage object, I got no problem of storing this object into my generic type property payload.

But if the Web API returns an IEnumerable, I get this error :

Cannot deserialize the current JSON array (e.g. [1,2,3]) into type 'CountryLanguage' because the type requires a JSON object (e.g. {\"name\":\"value\"}) to deserialize correctly. To fix this error either change the JSON to a JSON object (e.g. {\"name\":\"value\"}) or change the deserialized type to an array or a type that implements a collection interface (e.g. ICollection, IList) like List that can be deserialized from a JSON array. JsonArrayAttribute can also be added to the type to force it to deserialize from a JSON array.

My question is: Is it possible to "normalize" this code so I can store either an object or an IEnumerable into my payload property of type T?

Geoffry Brown · Accepted Answer · 2014-02-19 17:20:06Z

With this change I was able to fix the issue but I don't think it fully answers your question. I will give a solution to your answer below but here is what I added to fix the runtime issue and it would return your list.

var responseResult = new ResponseResult<IEnumerable<CountryLanguage>>
{
    StatusCode = response.StatusCode,
    Message = response.Content.ReadAsStringAsync().Result,
    Payload = response.Content.ReadAsAsync<IEnumerable<CountryLanguage>>().Result
 };

Basically just turn your T into IEnumerable if T is defined as CountryLanguage in the class or method then you can set it to IEnumerable but your code does not show the full class or method.

Now to your other issue, returning either object or list of objects from one generic method. The first response was right you would just need to return object and box/unbox and use reflection and other very unpleasant code which could be encapsulated but is still a bit yucky. So if your goal is to use this returned payload for things other than CountryLanguage you could try the following. Create a new class called PayloadWrapperthat contains two properties like so:

public class PayloadWrapper<T>
{
    public T Item { get; set; }
    public IEnumerable<T> Items { get; set; }
}

Then you can set on or the other in your webapi and on the client side check which is set.

public PayloadWrapper<CountryLanguage> Get()
{
    var languages = new List<CountryLanguage> {new CountryLanguage(), new CountryLanguage()};

    var payload = new PayloadWrapper<CountryLanguage>();
    payload.Items = languages;
    payload.Item = null;
    return payload;
}

And on the client:

var request = new HttpRequestMessage(HttpMethod.Get, "http://localhost:34213/api/values/");
 var httpClient = new HttpClient();
 var response = httpClient.SendAsync(request).Result;

  var responseResult = new ResponseResult<PayloadWrapper<CountryLanguage>>
  {
       StatusCode = response.StatusCode,
       Message = response.Content.ReadAsStringAsync().Result,
       Payload = response.Content.ReadAsAsync<PayloadWrapper<CountryLanguage>>().Result
   };

Your Payload is now a single object that either contains one item or a list of the same item. Can be used to send back anything that fits this pattern.

LcSalazar · Accepted Answer · 2014-02-19 15:55:40Z

0

Actually your logic is not right. If at runtime your T type is acting as the type 'CountryLanguage', your Payload property is acting as a CountryLanguage property. That's why it throws an error when you try to assign a IEnumerable value to the property, because your assigning an IEnumerable object to a non-enumerable property.
Basically, at runtime, the system is doing this:

CountryLanguage Payload = IEnumerable<CountryLanguage>(variable);

It's obviously going to throw an error.

answered Feb 19, 2014 at 15:55

LcSalazar

16.9k3 gold badges40 silver badges71 bronze badges

2 Comments

Elferone Over a year ago

Yes you are right. But can you think of any ways where I can store the result of either an object or an IEnumerable into one generic type class variable?

LcSalazar Over a year ago

Yes, if you declare the Payload with the "object" type you would store anything in there. But you would have too much trouble unboxing it to use later. If I were you, I would set the Payload as a List<T> list. Then I would make the service always return an List<CountryLanguage> object, even if only one index is filled.

Elferone · Accepted Answer · 2014-02-19 17:05:54Z

0

Ok, it is possible to store an IEnumerable into a variable of generic type T.

If I expect the returned data to be one object, the call I need to make is:

ResponseResult<CountryLanguage> response = serviceConnector.GetData <CountryLanguage>(actionToCall);

If I expect a IEnumerable from a call:

ResponseResult<IEnumerable<CountryLanguage>> response =
                serviceConnector.GetData<IEnumerable<CountryLanguage>>(actionToCall);

So my error was the fact that I was not specifying the IEnumerable type when calling the code.

answered Feb 19, 2014 at 17:05

Elferone

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