在C#中實現文件上傳可以通過使用HttpWebRequest類來發送HTTP POST請求，并在請求中包含要上傳的文件。以下是一個示例代碼，演示如何使用C#實現文件上傳：

using System;
using System.IO;
using System.Net;

class Program
{
    static void Main(string[] args)
    {
        string url = "http://example.com/upload";
        string filePath = "path/to/file.txt";

        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "POST";

        string boundary = "---------------------------" + DateTime.Now.Ticks.ToString("x");
        request.ContentType = "multipart/form-data; boundary=" + boundary;

        using (Stream requestStream = request.GetRequestStream())
        {
            using (StreamWriter writer = new StreamWriter(requestStream))
            {
                writer.WriteLine("--" + boundary);
                writer.WriteLine("Content-Disposition: form-data; name=\"file\"; filename=\"" + Path.GetFileName(filePath) + "\"");
                writer.WriteLine("Content-Type: application/octet-stream");
                writer.WriteLine();

                using (FileStream fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read))
                {
                    byte[] buffer = new byte[4096];
                    int bytesRead;
                    while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0)
                    {
                        requestStream.Write(buffer, 0, bytesRead);
                    }
                }

                writer.WriteLine();
                writer.WriteLine("--" + boundary + "--");
            }
        }

        using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
        {
            using (Stream responseStream = response.GetResponseStream())
            {
                using (StreamReader reader = new StreamReader(responseStream))
                {
                    string responseText = reader.ReadToEnd();
                    Console.WriteLine(responseText);
                }
            }
        }
    }
}

在上述代碼中，首先創建一個HttpWebRequest對象，并設置請求的方法為POST。然后設置請求的Content-Type為multipart/form-data，并定義一個boundary用于分隔請求的不同部分。接著將文件內容寫入請求的流中，并在結束時添加boundary標記。最后發送請求并讀取響應內容。

請注意，以上示例中使用了HttpWebRequest類來實現文件上傳，也可以考慮使用更方便的第三方庫，如HttpClient來實現文件上傳。