Category Archives: SDK 2.2

Saving Images to a Blob Container with Azure SDK 2.2 and VS2013

On the face of it the below is quite an obscure post but since this has bitten me and image upload is so common I figure it will bite others too. And to be fair I have called this blog Azure From The Trenches so inevitably it’s going to get a bit grubby every now and then!

I’ve been quite happily working on my companion application for this blog over the last few evenings and tonight ported it all over to VS2013, Azure SDK 2.2 and the various NuGet updates that have been released alongside all that and hit an odd problem – a simple image upload that worked previously no longer does.

Part of my application uploads images into blob storage via Web API and I’d been using code that looks somewhat like the below:

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Image myImage = new Image(); // paraphrasing here
using(CloudBlobStream uploadStream = blobRef.OpenWrite())
{
    myImage.Save(uploadStream, ImageFormat.Png);
}

This has been working with no issues at all (on my laptop, desktop and in Azure) until I ran the app after the upgrade this evening and then I began to get a NotSupportedException with the following root cause:

Microsoft.WindowsAzure.Storage.Blob.BlobWriteStreamBase.get_Length() is not supported

Cracking out dotPeek to take a look at the storage client code in the latest NuGet package (2.1.0.3) reveals that yes, getting the length of a stream on a block blob (the kind I am using) is unsupported:

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public override long Length
{
  get
  {
    if (this.pageBlob != null)
      return this.pageBlobSize;
    else
      throw new NotSupportedException();
  }
}

However this is the exact same implementation on the previous version of the client and so it’s not that – and it makes sense right? You can’t really expect to get the length of a stream that is writing to a remote server.

This led me off into System.Drawing. I’ll spare you the grisly details but if your Image class doesn’t have raw data, and chances are you don’t, and you call Save without an EncoderParameters parameter (and even sometimes when you do) then a native method called GdipSaveImageToStream is called and this expects a COM stream. Your .net stream is converted to a COM stream using a class called ComStreamFromDataStream and unfortunately this class has a nasty habit of calling get_Length() on your .net stream.

And this is what causes the crash.

It’s easy to fix: grab yourself a byte array or memory stream first and upload that. I’ve wrapped this into my application framework with an UploadImage method but essentially this does the following:

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Image myImage = new Image(); // paraphrasing here
using (MemoryStream ms = new MemoryStream())
{
    image.Save(ms, ImageFormat.Png);
    ms.Position = 0;
    blobRef.UploadFromStream(ms);
}

I’m still none the wiser as to why this has become a problem now but I guess with the move to VS2013 there is a lot of change and the layers of abstraction are now so deep that without spending hours reading Microsoft code I’m never going to know.