ASP.NET Web API – How to Always Return JSON

All you have to do is to remove JSON media formatters.

// Identify JSON formatters in global config.
var jsonMediaTypeFormatters = GlobalConfiguration.Configuration.Formatters
    .Where(x => x.SupportedMediaTypes
    .Any(y => y.MediaType.Equals("application/json", StringComparison.InvariantCultureIgnoreCase)))
    .ToList();

// Remove formatters from global config.
foreach (var formatter in jsonMediaTypeFormatters)
{
    GlobalConfiguration.Configuration.Formatters.Remove(formatter);
}

There are a few alternatives. The simplest one is to have your method return a HttpResponseMessage, and create that response with a StringContent based on your string, something similar to the code below:

public HttpResponseMessage Get()
{
    string yourJson = GetJsonFromSomewhere();
    var response = this.Request.CreateResponse(HttpStatusCode.OK);
    response.Content = new StringContent(yourJson, Encoding.UTF8, "application/json");
    return response;
}

And checking null or empty JSON string

public HttpResponseMessage Get()
{
    string yourJson = GetJsonFromSomewhere();
    if (!string.IsNullOrEmpty(yourJson))
    {
        var response = this.Request.CreateResponse(HttpStatusCode.OK);
        response.Content = new StringContent(yourJson, Encoding.UTF8, "application/json");
        return response;
    }
    throw new HttpResponseException(HttpStatusCode.NotFound);
}