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我正在为一个网站在 ASP.net 中做一个网页抓取项目，因为需要 Catpcha 代码，因此我需要让用户在继续之前输入 Captcha 代码。

到目前为止，该项目运行良好，但我发现的唯一问题是有时验证码响应未完全捕获，因此将响应流转换为 Image 导致以下错误：“参数无效。”

我注意到网络浏览器没有这个问题，只要服务器没有关闭，它总是可以很好地显示验证码。

但是，这对HttpWebRequest来说没有意义，有时能得到，有时不能，请问有没有办法保证Response Stream完整？

我的代码片段如下：

public Image GetCaptchaCode()
{
    Image returnVal = null;
    Uri uri = new Uri(URL_CAPTCHA);
    HttpWebRequest request = null;
    HttpWebResponse response = null;

    try
    {
        // Get Cookies
        CookieCollection cookies = this.GetCookies();
        foreach (Cookie cookie in cookies)
        {
            Console.WriteLine(cookie.Name + ": " + cookie.Value);
        }

        // Get Catpcha
        request = (HttpWebRequest)HttpWebRequest.Create(uri);
        request.ProtocolVersion = HttpVersion.Version11;
        request.Method = WebRequestMethods.Http.Get; // use GET for loading Captcha
        request.CookieContainer = this._cookies; // Store Cookies Info
        System.Net.ServicePointManager.Expect100Continue = false;

        // Add more cookies
        if (cookies != null)
        {
            request.CookieContainer.Add(cookies);
        }

        // Handle Gzip Compression
        request.Headers.Add(HttpRequestHeader.AcceptEncoding, HEADER_TYPE);
        request.AutomaticDecompression = DecompressionMethods.GZip;
        request.Referer = URL_REFERER;
        request.UserAgent = USER_AGENT;

        // Get Response
        response = (HttpWebResponse)request.GetResponse();
        returnVal = Image.FromStream(response.GetResponseStream());

    }
    catch (Exception ex)
    {
        string errMsg = ex.Message;
    }
    finally
    {
        if (uri != null) uri = null;
        if (request != null) request = null;

        if (response != null)
        {
            response.Close();
            response = null;
        }

    }

    return returnVal;
}
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