在C#中下載文件有多種方法：

1. 使用WebClient類：可以使用WebClient類的DownloadFile方法來下載文件。以下是一個示例：

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：可以使用HttpClient類的GetAsync方法來下載文件。以下是一個示例：

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

using (HttpClient client = new HttpClient())
{
    HttpResponseMessage response = await client.GetAsync(url);
    response.EnsureSuccessStatusCode();

    using (Stream contentStream = await response.Content.ReadAsStreamAsync(),
                  stream = new FileStream(savePath, FileMode.Create, FileAccess.Write, FileShare.None, 8192, true))
    {
        await contentStream.CopyToAsync(stream);
    }
}

3. 使用WebRequest和WebResponse類：可以使用WebRequest類發送HTTP請求，并使用WebResponse類獲取響應數據。以下是一個示例：

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

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

using (Stream responseStream = response.GetResponseStream(),
              stream = new FileStream(savePath, FileMode.Create, FileAccess.Write, FileShare.None, 8192, true))
{
    byte[] buffer = new byte[8192];
    int bytesRead;
    while ((bytesRead = responseStream.Read(buffer, 0, buffer.Length)) > 0)
    {
        stream.Write(buffer, 0, bytesRead);
    }
}

以上是三種常用的下載文件的方法，你可以根據自己的需求選擇適合的方法。