.NET HttpClient add query string and JSON body to POST

I can suggest that you use RestSharp for this purpose. It's basically a wrapper of the HttpWebRequest that does exactly what you want: makes it easy to compose url and body parameters and deserialize the result back.

Example from the site:

var client = new RestClient("http://example.com");

var request = new RestRequest("resource/{id}", Method.POST);
request.AddParameter("name", "value"); // adds to POST or URL querystring based on Method
request.AddUrlSegment("id", "123"); // replaces matching token in request.Resource

// easily add HTTP Headers
request.AddHeader("header", "value");

// add files to upload (works with compatible verbs)
request.AddFile(path);

// execute the request
IRestResponse response = client.Execute(request);

I ended up finding Microsoft.AspNetCore.WebUtilities.QueryHelpers.AddQueryString() that was what I needed. This allowed me to add the query string parameters without having to build the string manually (and worry about escaping characters and such).

Note: I'm using ASP.NET Core, but the same method is also available through Microsoft.Owin.Infrastructure.WebUtilities.AddQueryString()

New code:

// Query string parameters
var queryString = new Dictionary<string, string>()
{
    { "foo", "bar" }
};

// Create json for body
var content = new JObject(json);

// Create HttpClient
var client = new HttpClient();
client.BaseAddress = new Uri("https://api.baseaddress.com/");

// This is the missing piece
var requestUri = QueryHelpers.AddQueryString("something", queryString);

var request = new HttpRequestMessage(HttpMethod.Post, requestUri);
// Setup header(s)
request.Headers.Add("Accept", "application/json");
// Add body content
request.Content = new StringContent(
    content.ToString(),
    Encoding.UTF8,
    "application/json"
);

// Send the request
client.SendAsync(request);