REtful POST ：要写入流的字节超过指定的 Content-Length 字节大小

抛出此错误

Bytes to be written to the stream exceed 
the Content-Length bytes size specified. 

当我运行以下代码时：

var request = (HttpWebRequest)WebRequest.Create(url);
request.Method = "POST";
request.ContentType = "application/json";
request.ContentLength = Encoding.UTF8.GetByteCount(json);
using (var webStream = request.GetRequestStream())
using (var requestWriter = new StreamWriter(webStream, System.Text.Encoding.UTF8))
{
    requestWriter.Write(json);
}

我读到，当方法为 HEAD 或 GET 时可能会发生该错误，但这里是 POST。

知道那里出了什么问题吗？

问题是你先写UTF-8 BOM，因为Encoding.UTF8默认情况下会这样做。简短但完整的示例：

using System;
using System.IO;
using System.Text;

class Test
{
    static void Main()
    {
        string text = "text";
        var encoding = Encoding.UTF8;
        Console.WriteLine(encoding.GetByteCount(text));
        using (var stream = new MemoryStream())
        {
            using (var writer = new StreamWriter(stream, encoding))
            {
                writer.Write(text);
            }
            Console.WriteLine(BitConverter.ToString(stream.ToArray()));
        }
    }
}

Output:

4
EF-BB-BF-74-65-78-74

最简单的修复是either将前导码大小添加到内容长度中，or使用没有 BOM 的编码：

Encoding utf8NoBom = new UTF8Encoding(false);

用它代替Encoding.UTF8，一切都应该很好。