ASP.NET WebApi and Partial Responses

Question

I have a ASP.NET WebApi project that I am working on. The boss would like the returns to support "partial response", meaning that though the data model might contain 50 fields, the client should be able to request specific fields for the response. The reason being that if they are implementing for example a list they simply don't need the overhead of all 50 fields, they might just want the First Name, Last Name and Id to generate the list. Thus far I have implemented a solution by using a custom Contract Resolver (DynamicContractResolver) such that when a request comes in I am peeking into it through a filter (FieldListFilter) in the OnActionExecuting method and determining if a field named "FieldList" is present and then if it is I am replacing the current ContractResolver with a new instance of my DynamicContractResolver and I pass the fieldlist to the constructor.

Some sample code

DynamicContractResolver.cs

protected override IList<JsonProperty> CreateProperties(Type type, Newtonsoft.Json.MemberSerialization memberSerialization)
    {
        List<String> fieldList = ConvertFieldStringToList();

        IList<JsonProperty> properties = base.CreateProperties(type, memberSerialization);
        if (fieldList.Count == 0)
        {
            return properties;
        }
        // If we have fields, check that FieldList is one of them.
        if (!fieldList.Contains("FieldList"))
            // If not then add it, FieldList must ALWAYS be a part of any non null field list.
            fieldList.Add("FieldList");
        if (!fieldList.Contains("Data"))
            fieldList.Add("Data");
        if (!fieldList.Contains("FilterText"))
            fieldList.Add("FilterText");
        if (!fieldList.Contains("PageNumber"))
            fieldList.Add("PageNumber");
        if (!fieldList.Contains("RecordsReturned"))
            fieldList.Add("RecordsReturned");
        if (!fieldList.Contains("RecordsFound"))
            fieldList.Add("RecordsFound");
        for (int ctr = properties.Count-1; ctr >= 0; ctr--)
        {
            foreach (string field in fieldList)
            {
                if (field.Trim() == properties[ctr].PropertyName)
                {
                    goto Found;
                }
            }
            System.Diagnostics.Debug.WriteLine("Remove Property at Index " + ctr + " Named: " + properties[ctr].PropertyName);
            properties.RemoveAt(ctr);
        // Exit point for the inner foreach.  Nothing to do here.
        Found: { }
        }
        return properties;
    }

FieldListFilter.cs

public override void OnActionExecuting(System.Web.Http.Controllers.HttpActionContext actionContext)
    {
        if (!actionContext.ModelState.IsValid)
        {
            throw new HttpResponseException(HttpStatusCode.BadRequest);
        }
        // We need to determine if there is a FieldList property of the model that is being used.
        // First get a reference to the model.
        var modelObject = actionContext.ActionArguments.FirstOrDefault().Value;
        string fieldList = string.Empty;
        try
        {
            // Using reflection, attempt to get the value of the FieldList property
            var fieldListTemp = modelObject.GetType().GetProperty("FieldList").GetValue(modelObject);
            // If it is null then use an empty string
            if (fieldListTemp != null)
            {
                fieldList = fieldListTemp.ToString();
            }
        }
        catch (Exception)
        {
            fieldList = string.Empty;
        }

        // Update the global ContractResolver with the fieldList value but for efficiency only do it if they are not the same as the current ContractResolver.
        if (((DynamicContractResolver)GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings.ContractResolver).FieldList != fieldList)
        {
            GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings.ContractResolver = new DynamicContractResolver(fieldList);
        }
    }

I can then send a request with the json content payload looking as such:

{
  "FieldList":"NameFirst,NameLast,Id",
  "Data":[
    {
      "Id":1234
    },
    {
      "Id":1235
    }
  ]
}

and I will receive a response like so:

{
  "FieldList":"NameFirst,NameLast,Id",
  "Data":[
    {
      "NameFirst":"Brian",
      "NameLast":"Mueller",
      "Id":1234
    },
    {
      "NameFirst":"Brian",
      "NameLast":"Mueller",
      "Id":1235
    }
  ]
}

I believe that using the ContractResolver might run into threading issues. If I change it for one request is it going to be valid for all requests thereafter until someone changes it on another request (seems so through testing) If that is the case, then I don't see the usefulness for my purpose.

In summary, I am looking for a way to have dynamic data models such that the output from a request is configurable by the client on a request by request basis. Google implements this in their web api and they call it "partial response" and it works great. My implementation works, to a point but I fear that it will be broken for multiple simultaneous requests.

Suggestions? Tips?

Answer 1

A simpler solution that may work.

Create a model class with all 50 members with nullable types. Assign values to the requested members. Just return the result in the normal way.

In your WebApiConfig.Register() you must set the null value handling.

   config.Formatters.JsonFormatter.SerializerSettings =
        new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore };

Answer 2

You must not touch the configuration. You need the contract resolver on per-request basis. You can use it in your action method like this.

public class MyController : ApiController
{
    public HttpResponseMessage Get()
    {
        var formatter = new JsonMediaTypeFormatter();
        formatter.SerializerSettings.ContractResolver = 
              new DynamicContractResolver(new List<string>()
                       {"Id", "LastName"}); // you will get this from your filter

        var dto = new MyDto()
              { FirstName = "Captain", LastName = "Cool", Id = 8 };

        return new HttpResponseMessage()
        {
            Content = new ObjectContent<MyDto>(dto, formatter)
        };
        // What goes out is {"LastName":"Cool","Id":8}
    }
}

By doing this, you are locking yourself into JSON content type for response messages but you have already made that decision by using a Json.NET specific feature. Also, note you are creating a new JsonMediaTypeFormatter. So, anything you configure to the one in the configuration such as media type mapping is not going to be available with this approach though.