C# HttpWebRequest发送post form-data

可以使用 `HttpWebRequest` 类来发送带有 `form-data` 的 POST 请求。

以下是一个示例代码：

public static string HttpPostForm(string url, Dictionary<string, object> formData, string boundary)
{
    string result = string.Empty;
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
    request.Method = "POST";
    request.ContentType = "multipart/form-data; boundary=" + boundary;

    using (Stream requestStream = request.GetRequestStream())
    {
        // 添加表单数据
        foreach (var item in formData)
        {
            if (item.Value is byte[])
            {
                byte[] fileData = (byte[])item.Value;
                string fileName = item.Key;
                string fileContentType = "application/octet-stream";

                // 写入文件数据
                WriteFileData(requestStream, boundary, fileName, fileContentType, fileData);
            }
            else
            {
                // 写入表单数据
                WriteFormData(requestStream, boundary, item.Key, item.Value.ToString());
            }
        }

        // 写入结束标识
        WriteEndBoundary(requestStream, boundary);
    }

    // 发送请求
    using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
    {
        using (Stream responseStream = response.GetResponseStream())
        {
            using (StreamReader reader = new StreamReader(responseStream, Encoding.UTF8))
            {
                result = reader.ReadToEnd();
            }
        }
    }
    return result;
}

private static void WriteFormData(Stream stream, string boundary, string key, string value)
{
    string formDataTemplate = "--{0}\r\nContent-Disposition: form-data; name=\"{1}\"\r\n\r\n{2}\r\n";
    string formData = string.Format(formDataTemplate, boundary, key, value);
    byte[] formDataBytes = Encoding.UTF8.GetBytes(formData);
    stream.Write(formDataBytes, 0, formDataBytes.Length);
}

private static void WriteFileData(Stream stream, string boundary, string fileName, string fileContentType, byte[] fileData)
{
    string fileDataTemplate = "--{0}\r\nContent-Disposition: form-data; name=\"{1}\"; filename=\"{2}\"\r\nContent-Type: {3}\r\n\r\n";
    string fileDataHeader = string.Format(fileDataTemplate, boundary, "file", fileName, fileContentType);
    byte[] fileDataHeaderBytes = Encoding.UTF8.GetBytes(fileDataHeader);
    stream.Write(fileDataHeaderBytes, 0, fileDataHeaderBytes.Length);
    stream.Write(fileData, 0, fileData.Length);
    byte[] fileDataEndBytes = Encoding.UTF8.GetBytes("\r\n");
    stream.Write(fileDataEndBytes, 0, fileDataEndBytes.Length);
}

private static void WriteEndBoundary(Stream stream, string boundary)
{
    string endBoundaryTemplate = "--{0}--\r\n";
    string endBoundary = string.Format(endBoundaryTemplate, boundary);
    byte[] endBoundaryBytes = Encoding.UTF8.GetBytes(endBoundary);
    stream.Write(endBoundaryBytes, 0, endBoundaryBytes.Length);
}

其中，`HttpPostForm` 方法用于发送 POST 请求，接收一个 url、一个表单数据字典、一个边界字符串作为参数。在方法内部，我们首先设置请求方法和请求头，然后通过 `GetRequestStream` 方法获取请求流。在请求流中，我们遍历表单数据字典，将表单数据和文件数据分别写入请求流中，最后写入结束标识。最后，我们通过 `GetResponse` 方法获取响应，并将响应内容读取出来。