在C#中，可以使用HttpClient類和Task類來實現多線程下載。以下是一個簡單的示例代碼，演示如何使用多線程下載多個文件：

using System;
using System.Net.Http;
using System.Threading.Tasks;

class Program
{
    static async Task Main(string[] args)
    {
        string[] urls = { "url1", "url2", "url3" }; // 定義要下載的文件URL

        var tasks = new Task[urls.Length];

        for (int i = 0; i < urls.Length; i++)
        {
            tasks[i] = DownloadFile(urls[i]);
        }

        await Task.WhenAll(tasks);

        Console.WriteLine("All files downloaded successfully.");
    }

    static async Task DownloadFile(string url)
    {
        using (var client = new HttpClient())
        {
            var response = await client.GetAsync(url);

            if (response.IsSuccessStatusCode)
            {
                var fileName = url.Substring(url.LastIndexOf('/') + 1);

                using (var fileStream = System.IO.File.Create(fileName))
                {
                    await response.Content.CopyToAsync(fileStream);
                }

                Console.WriteLine($"File {fileName} downloaded successfully.");
            }
            else
            {
                Console.WriteLine($"Failed to download file from {url}. Status code: {response.StatusCode}");
            }
        }
    }
}

在上面的代碼中，我們通過Task類創建了多個下載文件的任務，并使用Task.WhenAll方法等待所有任務完成。每個下載任務都使用HttpClient類來發送HTTP請求并下載文件。當所有文件下載完成后，會輸出"All files downloaded successfully."。