Return empty json on null in WebAPI

You can use a HttpMessageHandler to perform behaviour on all requests. The example below is one way to do it. Be warned though, I whipped this up very quickly and it probably has a bunch of edge case bugs, but it should give you the idea of how it can be done.

  public class NullJsonHandler : DelegatingHandler
    {
        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {

            var response = await base.SendAsync(request, cancellationToken);
            if (response.Content == null)
            {
                response.Content = new StringContent("{}");
            } else if (response.Content is ObjectContent)
            {
                var objectContent = (ObjectContent) response.Content;
                if (objectContent.Value == null)
                {
                    response.Content = new StringContent("{}");
                }

            }
            return response;
        }
    }

You can enable this handler by doing,

config.MessageHandlers.Add(new NullJsonHandler());

Thanks to Darrel Miller, I for now use this solution.

WebApi messes with StringContent "{}" again in some environment, so serialize through HttpContent.

/// <summary>
/// Sends HTTP content as JSON
/// </summary>
/// <remarks>Thanks to Darrel Miller</remarks>
/// <seealso cref="http://www.bizcoder.com/returning-raw-json-content-from-asp-net-web-api"/>
public class JsonContent : HttpContent
{
    private readonly JToken jToken;

    public JsonContent(String json) { jToken = JObject.Parse(json); }

    public JsonContent(JToken value)
    {
        jToken = value;
        Headers.ContentType = new MediaTypeHeaderValue("application/json");
    }

    protected override Task SerializeToStreamAsync(Stream stream, TransportContext context)
    {
        var jw = new JsonTextWriter(new StreamWriter(stream))
        {
            Formatting = Formatting.Indented
        };
        jToken.WriteTo(jw);
        jw.Flush();
        return Task.FromResult<object>(null);
    }

    protected override bool TryComputeLength(out long length)
    {
        length = -1;
        return false;
    }
}

Derived from OkResult to take advantage Ok() in ApiController

public class OkJsonPatchResult : OkResult
{
    readonly MediaTypeWithQualityHeaderValue acceptJson = new MediaTypeWithQualityHeaderValue("application/json");

    public OkJsonPatchResult(HttpRequestMessage request) : base(request) { }
    public OkJsonPatchResult(ApiController controller) : base(controller) { }

    public override Task<HttpResponseMessage> ExecuteAsync(CancellationToken cancellationToken)
    {
        var accept = Request.Headers.Accept;
        var jsonFormat = accept.Any(h => h.Equals(acceptJson));

        if (jsonFormat)
        {
            return Task.FromResult(ExecuteResult());
        }
        else
        {
            return base.ExecuteAsync(cancellationToken);
        }
    }

    public HttpResponseMessage ExecuteResult()
    {
        return new HttpResponseMessage(HttpStatusCode.OK)
        {
            Content = new JsonContent("{}"),
            RequestMessage = Request
        };
    }
}

Override Ok() in ApiController

public class BaseApiController : ApiController
{
    protected override OkResult Ok()
    {
        return new OkJsonPatchResult(this);
    }
}

Maybe better solution is using Custom Message Handler.

A delegating handler can also skip the inner handler and directly create the response.

Custom Message Handler:

public class NullJsonHandler : DelegatingHandler
    {
        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {

            var updatedResponse = new HttpResponseMessage(HttpStatusCode.OK)
            {
                Content = null
            };

            var response = await base.SendAsync(request, cancellationToken);

            if (response.Content == null)
            {
                response.Content = new StringContent("{}");
            }

            else if (response.Content is ObjectContent)
            {

                var contents = await response.Content.ReadAsStringAsync();

                if (contents.Contains("null"))
                {
                    contents = contents.Replace("null", "{}");
                }

                updatedResponse.Content = new StringContent(contents,Encoding.UTF8,"application/json");

            }

            var tsc = new TaskCompletionSource<HttpResponseMessage>();
            tsc.SetResult(updatedResponse);   
            return await tsc.Task;
        }
    }

Register the Handler:

In Global.asax file inside Application_Start() method register your Handler by adding below code.

GlobalConfiguration.Configuration.MessageHandlers.Add(new NullJsonHandler());

Now all the Asp.NET Web API Response which contains null will be replaced with empty Json body {}.

References:
- https://stackoverflow.com/a/22764608/2218697
- https://learn.microsoft.com/en-us/aspnet/web-api/overview/advanced/http-message-handlers