web 上读取图片，并转化为指定格式

public static string ObtainBase64FromWeb(string domain, string path)
{
    string url = "https://" + domain + path;
    System.Net.WebRequest webReq = System.Net.WebRequest.Create(url);
    System.Net.WebResponse webRes = webReq.GetResponse();
    System.IO.Stream webStream = webRes.GetResponseStream();
    byte[] buf;

    // 网络流不支持查找操作，通过 response 长度， 将其转化为 byte[] 进行转换
    using (BinaryReader br = new BinaryReader(webStream))
    {
        int len = (int)(webRes.ContentLength);
        buf = br.ReadBytes(len);
        br.Close();
    }
    webStream.Close();
    webRes.Close();

    string base64_string = Convert.ToBase64String(buf, 0, buf.Length);
    return base64_string;
}