Client3.cs

```
using System;
using System.IO;
using System.Net;
using System.Security.Cryptography;
using System.Text;

/// <summary>请求体签名客户端。</summary>
public sealed class Client3
{

    /// <summary>服务器源。</summary>
    /// <value>http://api.group.mes/open/</value>
    public string Source { get; private set; }

    /// <summary>令牌。</summary>
    public string Token { get; private set; }

    /// <summary>密钥。</summary>
    public string Secret { get; private set; }

    /// <summary>构造函数，创建客户端实例。</summary>
    public Client3(string source, string token, string secret)
    {
        if (string.IsNullOrEmpty(source)) throw new ArgumentException("未指定服务器源。");
        if (string.IsNullOrEmpty(token)) throw new ArgumentException("未指定令牌。");
        if (string.IsNullOrEmpty(secret)) throw new ArgumentException("未指定密钥。");

        Source = source;
        Token = token;
        Secret = secret;
    }

    /// <summary>发送 POST 请求。</summary>
    /// <param name="func">要调用的方法。</param>
    /// <param name="json">请求体 JSON 字符串。</param>
    /// <returns>HTTP 响应体</returns>
    /// <exception cref="ArgumentNullException"></exception>
    /// <exception cref="Exception"></exception>
    public string Post(string func, string json)
    {
        // 请求体
        var body = Encoding.UTF8.GetBytes(json ?? "");

        // 计算签名
        var stamp = Convert.ToInt64((DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc)).TotalMilliseconds).ToString();
        var sha256 = Sha256(body);
        var sample = Secret + "\n" + stamp + "\n" + sha256;
        var signature = Sha256(Encoding.UTF8.GetBytes(sample));

        // 发起 HTTP 请求
        var url = Source + func;
        var request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "POST";
        request.ContentType = "application/json";
        request.ContentLength = body.Length;
        request.Headers.Add("Token", Token);
        request.Headers.Add("Stamp", stamp);
        request.Headers.Add("Signature", signature);
        request.GetRequestStream().Write(body, 0, body.Length);
        var response = request.GetResponse() as HttpWebResponse;
        using (var memory = new MemoryStream())
        {
            var stream = response.GetResponseStream();
            var buffer = new byte[4096];
            while (true)
            {
                var count = stream.Read(buffer, 0, buffer.Length);
                if (count < 1) break;
                memory.Write(buffer, 0, count);
            }
            return Encoding.UTF8.GetString(memory.ToArray());
        }
    }

    /// <summary>计算 SHA256，获取哈希的小写十六进制。</summary>
    /// <exception cref="ArgumentException"></exception>
    static string Sha256(byte[] bytes)
    {
        using (var provider = new SHA256CryptoServiceProvider())
        {
            var hash = provider.ComputeHash(bytes);
            provider.Clear();

            const string hex = "0123456789abcdef";
            var chars = new char[hash.Length * 2];
            for (var i = 0; i < hash.Length; i++)
            {
                var b = hash[i];
                var offset = i * 2;
                chars[offset] = hex[b / 16];
                chars[offset + 1] = hex[b % 16];
            }
            return new string(chars);
        }
    }

}
```

Program.cs

```
using System;

class Program
{

    static void Main(string[] args)
    {
        // 准备客户端
        var client3 = new Client3("http://127.0.0.1:8033/open/", "token3", "secret3");

        // 发起接口请求
        var response = client3.Post("checked", "");
        Console.WriteLine(response);
    }

}

```