How to check if System.Net.WebClient.DownloadData is downloading a binary file?

Question

I am trying to use WebClient to download a file from web using a WinForms application. However, I really only want to download HTML file. Any other type I will want to ignore.

I checked the WebResponse.ContentType, but its value is always null.

Anyone have any idea what could be the cause?

Answer 1

Given your update, you can do this by changing the .Method in GetWebRequest:

using System;
using System.Net;
static class Program
{
    static void Main()
    {
        using (MyClient client = new MyClient())
        {
            client.HeadOnly = true;
            string uri = "http://www.google.com";
            byte[] body = client.DownloadData(uri); // note should be 0-length
            string type = client.ResponseHeaders["content-type"];
            client.HeadOnly = false;
            // check 'tis not binary... we'll use text/, but could
            // check for text/html
            if (type.StartsWith(@"text/"))
            {
                string text = client.DownloadString(uri);
                Console.WriteLine(text);
            }
        }
    }

}

class MyClient : WebClient
{
    public bool HeadOnly { get; set; }
    protected override WebRequest GetWebRequest(Uri address)
    {
        WebRequest req = base.GetWebRequest(address);
        if (HeadOnly && req.Method == "GET")
        {
            req.Method = "HEAD";
        }
        return req;
    }
}

Alternatively, you can check the header when overriding GetWebRespons(), perhaps throwing an exception if it isn't what you wanted:

protected override WebResponse GetWebResponse(WebRequest request)
{
    WebResponse resp = base.GetWebResponse(request);
    string type = resp.Headers["content-type"];
    // do something with type
    return resp;
}

Answer 2

I'm not sure the cause, but perhaps you hadn't downloaded anything yet. This is the lazy way to get the content type of a remote file/page (I haven't checked if this is efficient on the wire. For all I know, it may download huge chunks of content)

        Stream connection = new MemoryStream(""); // Just a placeholder
        WebClient wc = new WebClient();
        string contentType;
        try
        {
            connection = wc.OpenRead(current.Url);
            contentType = wc.ResponseHeaders["content-type"];
        }
        catch (Exception)
        {
            // 404 or what have you
        }
        finally
        {
            connection.Close();
        }