HttpClient post json,可以是数组

var json = JsonConvert.SerializeObject(obj);
StringContent theContent = new StringContent(json, Encoding.UTF8, "application/json");
var r = client.PostAsync(url, theContent).Result.Content.ReadAsStringAsync().Result;

原文：http://www.cnblogs.com/czcz1024/p/4994325.html