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;

有简单一点的方法

client.PostAsJsonAsync(url, new {});

需要引入System.Net.Http.Formatting.dll