通过 HttpClient 的 ContentLength 很多时候都可以拿到下载的内容的长度，通过 ReadAsync 可以返回当前读到的长度，将读取到的长度加起来就是已经下载的长度

看起来很简单，于是直接给代码

private static async Task DownloadFile(string url, FileInfo file)
{
var httpClient = new HttpClient();
var response = await httpClient.GetAsync(url);

try
{
var n = response.Content.Headers.ContentLength;
var stream = await response.Content.ReadAsStreamAsync();
using(var fileStream = file.Create())
using (stream)
{
byte[] buffer = new byte[1024];
var readLength = 0;
int length;
while ((length = await stream.ReadAsync(buffer, 0, buffer.Length)) != 0)
{
readLength += length;

Console.WriteLine("下载进度" + ((double)readLength) / n * 100);

// 写入到文件
fileStream.Write(buffer, 0, length);
}
}

}
catch (Exception e)
{
}
}

如果不是需要获取进度，那么最简单的方法是

var stream = await response.Content.ReadAsStreamAsync();
using(var fileStream = file.Create())
using (stream)
{
await stream.CopyToAsync(fileStream);
}