可以使用以下代码示例来解决该问题。

使用HttpClient下载并保存文件：

using (var client = new HttpClient())
{
    var result = await client.GetAsync(url);
    var contentStream = await result.Content.ReadAsStreamAsync();
    using (var fileStream = new FileStream("file.txt", FileMode.Create))
    {
        await contentStream.CopyToAsync(fileStream);
    }
}

如果您需要从流中下载文件并返回到客户端，可以使用以下代码示例：

[HttpGet]
public async Task DownloadFile()
{
    var client = new HttpClient();
    var response = await client.GetAsync("https://example.com/file.txt");
    var stream = await response.Content.ReadAsStreamAsync();
    return File(stream, response.Content.Headers.ContentType.MediaType, "file.txt");
}