how do I download a large file (via HTTP) in .NET

I need to download a LARGE file (2GB) over HTTP in a C# console app. Problem is, after about 1.2GB, the app runs out of memory.

Here’s the code I’m using:

WebClient request = new WebClient();
request.Credentials = new NetworkCredential(username, password);
byte[] fileData = request.DownloadData(baseURL + fName);

As you can see… I’m reading the file directly into memory. I’m pretty sure I could solve this if I were to read the data back from HTTP in chunks and write it to a file on disk.

Does anyone know how I could do this?

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You need to get the response stream and then read in blocks, writing each block to a file to allow memory to be reused.

As you have written it, the whole response, all 2GB, needs to be in memory. Even on a 64bit system that will hit the 2GB limit for a single .NET object.

Update: easier option. Get WebClient to do the work for you: with its DownloadFile method which will put the data directly into a file.

The WebClient class is the one for simplified scenarios. Once you get past simple scenarios (and you have), you’ll have to fall back a bit and use WebRequest.

With WebRequest, you’ll have access to the response stream, and you’ll be able to loop over it, reading a bit and writing a bit, until you’re done.


public void MyDownloadFile(Uri url, string outputFilePath)
    const int BUFFER_SIZE = 16 * 1024;
    using (var outputFileStream = File.Create(outputFilePath, BUFFER_SIZE))
        var req = WebRequest.Create(url);
        using (var response = req.GetResponse())
            using (var responseStream = response.GetResponseStream())
                var buffer = new byte[BUFFER_SIZE];
                int bytesRead;
                    bytesRead = responseStream.Read(buffer, 0, BUFFER_SIZE);
                    outputFileStream.Write(buffer, 0, bytesRead);
                } while (bytesRead > 0);

Note that if WebClient.DownloadFile works, then I’d call it the best solution. I wrote the above before the “DownloadFile” answer was posted. I also wrote it way too early in the morning, so a grain of salt (and testing) may be required.

If you use WebClient.DownloadFile you could save it directly into a file.

i would use something like this

WebClient.OpenRead returns a Stream, just use Read to loop over the contents, so the data is not buffered in memory but can be written in blocks to a file.