web 上讀取圖片，並轉化爲指定格式

一、 轉換爲 base64

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;

    // 網絡流不支持查找操作，通過 response 長度， 將其轉化爲 byte[] 進行轉換
    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;
}