使用 WebRequest 发送文件

以向微信接口上传媒体文件为例子，通过拼接 multipart/form-data 格式完成向请求附加文件。

public string UploadMedia(){
    var accessToken = "";
    var type = "image";
    string filePath = "\path\to\file.png";
    string fileName = "app.png";

    var apiUrl = $"https://api.weixin.qq.com/cgi-bin/media/upload?access_token={accessToken}&type={type}";
    var boundary = "----------------------------" + DateTime.Now.Ticks.ToString("x");

    var fileInfoPart = $"Content-Disposition: form-data; name=\"file\"; filename=\"{fileName}\"\r\nContent-Type: image/png";
    var startBuffer = Encoding.UTF8.GetBytes($"--{boundary}\r\n{fileInfoPart}\r\n\r\n");

    var fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read);
    var fileBuffer = new BinaryReader(fileStream).ReadBytes((int)fileStream.Length);

    var endBoundaryBuffer = Encoding.UTF8.GetBytes($"\r\n--{boundary}--\r\n");

    var request = WebRequest.Create(apiUrl);
    request.Method = "POST";
    request.Timeout = 100000;
    request.ContentType = "multipart/form-data; boundary=" + boundary;
    request.ContentLength = startBuffer.Length + fileBuffer.Length + endBoundaryBuffer.Length;

    var requestStream = request.GetRequestStream();
    requestStream.Write(startBuffer, 0, startBuffer.Length);
    requestStream.Write(fileBuffer, 0, fileBuffer.Length);
    requestStream.Write(endBoundaryBuffer, 0, endBoundaryBuffer.Length);
    requestStream.Close();

    var response = request.GetResponse();
    using (var httpStreamReader = new StreamReader(response.GetResponseStream(), Encoding.GetEncoding("utf-8")))
    {
        var responseContent = httpStreamReader.ReadToEnd();
        return Success(responseContent);
    }
}