C# POST 表单发送文件

表单提交协议规定：
要先将 HTTP 要求的 Content-Type 设为 multipart/form-data，而且要设定一个 boundary 参数，
这个参数是由应用程序自行产生，它会用来识别每一份资料的边界 (boundary)，
用以产生多重信息部份 (message part)。
而 HTTP 服务器可以抓取 HTTP POST 的信息，

基本内容：
1. 每个信息部份都要用 --[BOUNDARY_NAME] 来包装，以分隔出信息的每个部份，而最后要再加上一个 --[BOUNDARY_NAME] 来表示结束。
2. 每个信息部份都要有一个 Content-Disposition: form-data; name=""，而 name 设定的就是 HTTP POST 的键值 (key)。
3. 声明区和值区中间要隔两个新行符号（\r\n）。
4. 中间可以夹入二进制资料，但二进制资料必须要格式化为二进制字符串。
5. 若要设定不同信息段的资料型别 (Content-Type)，则要在信息段内的声明区设定。

两个form内容模板
boundary = "----" + DateTime.Now.Ticks.ToString("x");//程序生成
1.文本内容
"\r\n--" + boundary +
"\r\nContent-Disposition: form-data; name=\"键\"; filename=\"文件名\"" +
"\r\nContent-Type: application/octet-stream" +
"\r\n\r\n";
2.文件内容
"\r\n--" + boundary +
"\r\nContent-Disposition: form-data; name=\"键\"" +
"\r\n\r\n内容";

代码

//1.表示一个表单项的对象
/// <summary>
/// 表单数据项
/// </summary>
public class FormItemModel
{
    /// <summary>
    /// 表单键，request["key"]
    /// </summary>
    public string Key { set; get; }
    /// <summary>
    /// 表单值,上传文件时忽略，request["key"].value
    /// </summary>
    public string Value { set; get; }
    /// <summary>
    /// 是否是文件
    /// </summary>
    public bool IsFile
    {
        get
        {
            if (FileContent==null || FileContent.Length == )
                return false;

            if (FileContent != null && FileContent.Length >  && string.IsNullOrWhiteSpace(FileName))
                throw new Exception("上传文件时 FileName 属性值不能为空");
            return true;
        }
    }
    /// <summary>
    /// 上传的文件名
    /// </summary>
    public string FileName { set; get; }
    /// <summary>
    /// 上传的文件内容
    /// </summary>
    public Stream FileContent { set; get; }
}

//2.提交表单主逻辑实现
/// <summary>
/// 使用Post方法获取字符串结果
/// </summary>
/// <param name="url"></param>
/// <param name="formItems">Post表单内容</param>
/// <param name="cookieContainer"></param>
/// <param name="timeOut">默认20秒</param>
/// <param name="encoding">响应内容的编码类型（默认utf-8）</param>
/// <returns></returns>
public static string PostForm(string url, List<FormItemModel> formItems, CookieContainer cookieContainer = null, string refererUrl = null, Encoding encoding = null,int timeOut = )
{
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
    #region 初始化请求对象
    request.Method = "POST";
    request.Timeout = timeOut;
    request.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8";
    request.KeepAlive = true;
    request.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.57 Safari/537.36";
    if (!string.IsNullOrEmpty(refererUrl))
        request.Referer = refererUrl;
    if (cookieContainer != null)
        request.CookieContainer = cookieContainer;
    #endregion

    string boundary = "----" + DateTime.Now.Ticks.ToString("x");//分隔符
    request.ContentType = string.Format("multipart/form-data; boundary={0}", boundary);
    //请求流
    var postStream = new MemoryStream();
    #region 处理Form表单请求内容
    //是否用Form上传文件
    var formUploadFile = formItems != null && formItems.Count > ;
    if (formUploadFile)
    {
        //文件数据模板
        string fileFormdataTemplate =
            "\r\n--" + boundary +
            "\r\nContent-Disposition: form-data; name=\"{0}\"; filename=\"{1}\"" +
            "\r\nContent-Type: application/octet-stream" +
            "\r\n\r\n";
        //文本数据模板
        string dataFormdataTemplate =
            "\r\n--" + boundary +
            "\r\nContent-Disposition: form-data; name=\"{0}\"" +
            "\r\n\r\n{1}";
        foreach (var item in formItems)
        {
            string formdata = null;
            if (item.IsFile)
            {
                //上传文件
                formdata = string.Format(
                    fileFormdataTemplate,
                    item.Key, //表单键
                    item.FileName);
            }
            else
            {
                //上传文本
                formdata = string.Format(
                    dataFormdataTemplate,
                    item.Key,
                    item.Value);
            }

            //统一处理
            byte[] formdataBytes = null;
            //第一行不需要换行
            if (postStream.Length == )
                formdataBytes = Encoding.UTF8.GetBytes(formdata.Substring(, formdata.Length - ));
            else
                formdataBytes = Encoding.UTF8.GetBytes(formdata);
            postStream.Write(formdataBytes, , formdataBytes.Length);

            //写入文件内容
            if (item.FileContent != null && item.FileContent.Length>)
            {
                using (var stream = item.FileContent)
                {
                    byte[] buffer = new byte[];
                    int bytesRead = ;
                    while ((bytesRead = stream.Read(buffer, , buffer.Length)) != )
                    {
                        postStream.Write(buffer, , bytesRead);
                    }
                }
            }
        }
        //结尾
        var footer = Encoding.UTF8.GetBytes("\r\n--" + boundary + "--\r\n");
        postStream.Write(footer, , footer.Length);

    }
    else
    {
        request.ContentType = "application/x-www-form-urlencoded";
    }
    #endregion

    request.ContentLength = postStream.Length;

    #region 输入二进制流
    if (postStream != null)
    {
        postStream.Position = ;
        //直接写入流
        Stream requestStream = request.GetRequestStream();

        byte[] buffer = new byte[];
        int bytesRead = ;
        while ((bytesRead = postStream.Read(buffer, , buffer.Length)) != )
        {
            requestStream.Write(buffer, , bytesRead);
        }

        ////debug
        //postStream.Seek(0, SeekOrigin.Begin);
        //StreamReader sr = new StreamReader(postStream);
        //var postStr = sr.ReadToEnd();
        postStream.Close();//关闭文件访问
    }
    #endregion

    HttpWebResponse response = (HttpWebResponse)request.GetResponse();
    if (cookieContainer != null)
    {
        response.Cookies = cookieContainer.GetCookies(response.ResponseUri);
    }

    using (Stream responseStream = response.GetResponseStream())
    {
        using (StreamReader myStreamReader = new StreamReader(responseStream, encoding ?? Encoding.UTF8))
        {
            string retString = myStreamReader.ReadToEnd();
            return retString;
        }
    }
}

//3.调用模拟post表单

var url = "http://127.0.0.1/testformdata.aspx?aa=1&bb=2&ccc=3";
var log1=@"D:\temp\log1.txt";
var log2 = @"D:\temp\log2.txt";
var formDatas = new List<Grass.Net.FormItemModel>();
//添加文件
formDatas.Add(new Grass.Net.FormItemModel()
{
    Key="log1",
    Value="",
    FileName = "log1.txt",
    FileContent=File.OpenRead(log1)
});
formDatas.Add(new Grass.Net.FormItemModel()
{
    Key = "log2",
    Value = "",
    FileName = "log2.txt",
    FileContent = File.OpenRead(log2)
});
//添加文本
formDatas.Add(new Grass.Net.FormItemModel()
{
    Key = "id",
    Value = "id-test-id-test-id-test-id-test-id-test-"
});
formDatas.Add(new Grass.Net.FormItemModel()
{
    Key = "name",
    Value = "name-test-name-test-name-test-name-test-name-test-"
});
//提交表单
var result = PostForm(url, formDatas);

https://blog.csdn.net/xxj_jing/article/details/50221113