download online image asynchronously in .Net

I found the following example to download web page(string) asynchounously:

let getImage (imageUrl:string) =
    async {
        try 
            let req = WebRequest.Create(imageUrl) :?> HttpWebRequest
            req.UserAgent <- "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322)";
            req.Method <- "GET";
            req.AllowAutoRedire开发者_高级运维ct <- true;
            req.MaximumAutomaticRedirections <- 4;
            let! response1 = req.AsyncGetResponse()
            let response = response1 :?> HttpWebResponse
            use stream = response.GetResponseStream()
            use streamreader = new System.IO.StreamReader(stream)
            return! streamreader.AsyncReadToEnd() // .ReadToEnd()
        with
            _ -> return "" // if there's any exception, just return an empty string

    }

It returns a string. However, I need to download an online image(an array of bytes) asynchournously.

Anyone could give me some hint?

Maybe you can use my answer to another question (here). It asks the google chart API for an image (an URL) and converts the bytes to a Bitmap.

The relevant code:

async {
    let req = HttpWebRequest.Create(..URI HERE..)
    let! response = req.AsyncGetResponse()
    return new Bitmap(response.GetResponseStream())
} |> Async.RunSynchronously

Use http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.begingetresponse.aspx

Then you just GetResponseStream and read the bytes.

You can read the bytes directly from the stream. StreamReader works on strings. It actually inherits TextReader. Alternativly look at using a BinaryReader which works on bytes.

(Since you tagged it as C# I'll write my code sample in C#.)

Stream s = response.GetResponseStream()
MemoryStream memStream = new MemoryStream();
int bytesRead;
byte[] buffer = new byte[0x1000];
for (bytesRead = s.Read(buffer, 0, buffer.Length); bytesRead > 0; bytesRead = s.Read(buffer, 0, buffer.Length))
{
    memStream.Write(buffer, 0, bytesRead);
}
s.Close();
return memStream.ToArray();

You can convert stream to array of bytes like this (C#)

byte[] buffer = new byte[16 * 1024];
using (MemoryStream ms = new MemoryStream(stream))
{
    int read;
    while ((read = input.Read(buffer, 0, buffer.Length)) > 0)
    {
        ms.Write(buffer, 0, read);
    }

    var bytes = ms.ToArray();
}