异步调用API并解析JSON

我来自iOS（Swift）背景。在我的一个Swift应用程序中，我有一个调用API的类。我正试图将它移植到C#（Windows窗体应用程序），但遇到了几个障碍。首先是Swift代码。没什么新奇的。一个方法执行POST请求以登录API，另一个函数执行GET方法以检索用户配置文件的JSON响应。这两种方法都是异步的。

import Foundation
class API {
    private let session = NSURLSession.sharedSession()
    private let baseURL = "https://www.example.com/api/"
    func login(userID userID: String, password: String, completion: (error: NSError?) -> ()) {
        let url = NSURL(string: baseURL + "login")!
        let params = ["username": userID, "password": password]
        let request = NSMutableURLRequest(URL: url)
        request.HTTPMethod = "POST"
        request.encodeParameters(params) // encodeParameters is an extension method
        session.dataTaskWithRequest(request, completionHandler: { data, response, error in
            if let httpResponse = response as? NSHTTPURLResponse {
                if httpResponse.statusCode != 200 {
                    completion(error: error)
                } else {
                    completion(error: nil)
                }
            }
        }).resume()
    }
    func fetchUser(completion: (user: User?, error: NSError?) -> ()) {
        let url = NSURL(string: baseURL + "profile")!
        let request = NSURLRequest(URL: url)
        session.dataTaskWithRequest(request, completionHandler: { data, response, error in
            if let error = error {
                completion(user: nil, error: error)
            } else {
                // Parsing JSON
                var jsonDict = [String: AnyObject]()
                do {
                    jsonDict = try NSJSONSerialization.JSONObjectWithData(data, options: []) as! [String: AnyObject]
                } catch {
                    print("Error occurred parsing data: '(error)")
                    completion(user: nil, error: error)
                }
                let user = User()
                user.name = jsonDict["name"] as! String
                user.age = jsonDict["age"] as! Int
                completion(user: user, error: nil)
            }
        }).resume()
    }
}

以下是我将其转换为C#的尝试。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Net.Http;
using System.Runtime.Serialization.Json;
using System.Text;
using System.Xml.Linq;
using System.Xml.XPath;
namespace MyTrayApp
{
    public partial class Form1 : Form
    {
        private string baseURL = "https://www.example.com/api/";

        public Form1()
        {
            InitializeComponent();
        }
        private async void Form1_Load(object sender, EventArgs e)
        {
            await login("myusername", "mypassword");
            await fetchUser();
        }
        async Task login(string userID, string password)
        {
            using (var client = new HttpClient())
            {
                client.BaseAddress = new Uri(baseURL);
                var parameters = new Dictionary<string, string>
                {
                    { "username", userID },
                    { "password", password }
                };
                var encodedParameters = new FormUrlEncodedContent(parameters);
                var response = await client.PostAsync("login", encodedParameters);
                string responseString = await response.Content.ReadAsStringAsync();
                //Console.WriteLine(responseString);
            }
        }
        async Task fetchUser()
        {
            using (var client = new HttpClient())
            {
                client.BaseAddress = new Uri(baseURL);
                var response = await client.GetAsync("profile");
                response.EnsureSuccessStatusCode();
                var responseString = await response.Content.ReadAsStringAsync();
                var jsonReader = JsonReaderWriterFactory.CreateJsonReader(Encoding.UTF8.GetBytes(responseString.ToCharArray()), new System.Xml.XmlDictionaryReaderQuotas());
                var root = XElement.Load(jsonReader);
                Console.WriteLine(root.XPathSelectElement("//name").Value);
                //Console.WriteLine(responseString);
            }
        }
    }
}

这些都是我遇到的问题。

1. 在我的Swift方法中，它们有完成处理程序。我怎样才能在C#中做同样的事情
2. 在Swift中，您可以获得一个NSData对象，并将其传递给NSJSONSerialization以创建一个JSON对象。在我当前的实现中，我在XElement.Load(jsonReader);处得到一个XML异常。我甚至不确定这样做是否正确。我在SO上找到了很多不同的解决方案。但有些是针对Metro应用程序的，有些是针对网络的——这太让人应接不暇了。此外，大多数解决方案都是使用像JSON.NET这样的第三方库。我正试图在没有第三方程序库的情况下实现这一点