寫了個例子程式

using System;
using System.Net.Http;
using System.Threading.Tasks;
namespace Chapter1.Threads
{
    public static class Program
    {
        public static void Main()
        {
            string result = DownloadContent().Result;
            Console.WriteLine(result);
        }
        public static async Task<string> DownloadContent()
        {
            using(HttpClient client = new HttpClient())
            {
                string result = await client.GetStringAsync("http://www.microsoft.com");
                return result;
           }
        }
    }
}
 

但是用mcs 編譯時，報錯 

async1.cs(2,18): error CS0234: The type or namespace name `Http' does not exist in the namespace `System.Net'. Are you missing `System.Net.Http' assembly reference?

解決方法：用/reference 加上依賴的DLL

mcs /reference:System.Net.Http async1.cs