在C#中，有多種方法可以下載文件。下面是其中幾種常見的方法：

1. 使用WebClient類：

string url = "http://example.com/file.txt";
string savePath = "C:\\downloads\\file.txt";

using (WebClient client = new WebClient())
{
    client.DownloadFile(url, savePath);
}

2. 使用HttpClient類：

string url = "http://example.com/file.txt";
string savePath = "C:\\downloads\\file.txt";

using (HttpClient client = new HttpClient())
{
    using (HttpResponseMessage response = await client.GetAsync(url))
    {
        using (Stream stream = await response.Content.ReadAsStreamAsync())
        {
            using (FileStream fileStream = new FileStream(savePath, FileMode.Create))
            {
                await stream.CopyToAsync(fileStream);
            }
        }
    }
}

3. 使用WebRequest和WebResponse類：

string url = "http://example.com/file.txt";
string savePath = "C:\\downloads\\file.txt";

WebRequest request = WebRequest.Create(url);
WebResponse response = request.GetResponse();

using (Stream stream = response.GetResponseStream())
{
    using (FileStream fileStream = new FileStream(savePath, FileMode.Create))
    {
        stream.CopyTo(fileStream);
    }
}

response.Close();

請注意，以上代碼示例中的URL和保存路徑應根據實際情況進行更改。另外，下載文件時可能需要處理一些異常情況，例如網絡連接失敗或文件無法訪問等。