Copy MemoryStream to FileStream and save the file?

问题

I don't understand what I'm doing wrong here. I generate couple of memory streams and in debug-mode I see that they are populated. But when I try to copy MemoryStream to FileStream in order to save the file fileStream is not populated and file is 0bytes long (empty).

Here is my code

if (file.ContentLength > 0)
{
    var bytes = ImageUploader.FilestreamToBytes(file); // bytes is populated

    using (var inStream = new MemoryStream(bytes)) // inStream is populated
    {
        using (var outStream = new MemoryStream())
        {
            using (var imageFactory = new ImageFactory())
            {
                imageFactory.Load(inStream)
                            .Resize(new Size(320, 0))
                            .Format(ImageFormat.Jpeg)
                            .Quality(70)
                            .Save(outStream);
            }

            // outStream is populated here

            var fileName = "test.jpg";

            using (var fileStream = new FileStream(Server.MapPath("~/content/u/") + fileName, FileMode.CreateNew, FileAccess.ReadWrite))
            {
                outStream.CopyTo(fileStream); // fileStream is not populated
            }
        }
    }
}

回答1:

You need to reset the position of the stream before copying.

outStream.Position = 0;
outStream.CopyTo(fileStream);

You used the outStream when saving the file using the imageFactory. That function populated the outStream. While populating the outStream the position is set to the end of the populated area. That is so that when you keep on writing bytes to the steam, it doesn't override existing bytes. But then to read it (for copy purposes) you need to set the position to the start so you can start reading at the start.

回答2:

If your objective is simply to dump the memory stream to a physical file (e.g. to look at the contents) - it can be done in one move:

memoryStream.Position = 0;
System.IO.File.WriteAllBytes(@"C:\\filename", memoryStream.ToArray());

回答3:

Another alternative to CopyTo is WriteTo.

Advantage:

No need to reset Position.

Usage:

outStream.WriteTo(fileStream);

Function Description:

Writes the entire contents of this memory stream to another stream.

来源：https://stackoverflow.com/questions/18766055/copy-memorystream-to-filestream-and-save-the-file

标签

file-upload

filestream

memorystream