Sunday, 31 August 2014

Using Custom Model Binders in ASP.Net MVC

I answered a question on Reddit this week from someone starting out in MVC who had read an incorrect article about model binding which was mostly correct, but made using custom binders look like they require more code than they actually do, so I thought it was worth a post to clear that up.

What is (Custom) Model Binding?

Model Binding is the process through which MVC takes a form post and maps all of the form values in to a custom object, allowing you to have a POST action method which takes in a ViewModel and have it automagically populated for you. Custom Model Binders allow you to insert your own binders for particular scenarios where the default binding won't quite cut it.

Creating our custom binder

We have the following typical example ViewModel:
    public class MyViewModel
        public string MyStringProperty { get; set; }
It's just a class, nothing special about it at all. Now we want to manually handle the binding of this model because we want to add some text to the end of MyStringProperty when it gets bound. This is unlikely to be something you would want to do in real life, but we're just proving the point here.
This is our binder:
    public class MyViewModelBinder:IModelBinder
        protected System.Collections.Specialized.NameValueCollection Form { get; set; }

        private void Initialise(ControllerContext controllerContext, ModelBindingContext bindingContext)
            Form = controllerContext.HttpContext.Request.Form;

        public object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
            Initialise(controllerContext, bindingContext);
            var msp = Form["MyStringProperty"];
            return new MyViewModel {MyStringProperty = msp + " from my custom binder"};
Model Binders need to implement IModelBinder and have a BindModel method. This gives you access to the controllerContext from which you can access HttpContext and the bindingContext, which admittedly I have never had to use.
In our binder, we just manually pick up the MyStringProperty value from the form, add it to a new instance of our object and return it, adding our incredibly important piece of text to the end of the retrieved value.

Using our Custom Binder

There are 2 ways we can use our custom binder, which one we use depends on each scenario. If we need to override the binding of a class for a particular Action method, we can use the ModelBinder attribute on the relevant parameter of the Action Method:
        public ActionResult Index([ModelBinder(typeof(MyViewModelBinder))]MyViewModel model)
            return View(model);
This will apply our Custom Binder to this property (MyViewModel) for this action only, no other actions or controllers will be affected.
Alternatively, if we want to apply our custom binder to MyViewModel globally within the application, we can add the following line to Application_Start in global.asax.cs:
        protected void Application_Start()

            ModelBinders.Binders[typeof(MyViewModel)] = new MyViewModelBinder();
Using this method, everywhere a parameter of type MyViewModel is encountered on an ActionResult, our custom binder will be invoked instead of the standard one. Because this applies globally, we do not need the ModelBinder attribute on our Action Method so the overridden behaviour is completely transparent to the controller, promoting code reuse and keeping model binding logic where it belongs.

Wednesday, 6 August 2014

API Head-to-head: AWS S3 Vs Windows Azure Table Storage

Recently, I was experimenting with using S3 as a tertiary backup for my photos, an honour which eventually went to Azure as it was cheaper and I am more familiar with the Azure APIs as I use them in my day job.

I thought I’d take a deeper look at both APIs and see how they compare. I’ll go through some standard operations, comparing the amount of code required to perform the operation.

If you want a comparison of features, there are plenty of blog posts on the subject, just Bingle It

All the code in this test is being run in Linqpad, using the AWS SDK for .Net and Windows Azure Storage Nuget packages.

Create the client

Both Azure and S3 have the concept of a client, this represents the service itself and is where you provide credentials for accessing the service.


var account = Microsoft.WindowsAzure.Storage.CloudStorageAccount.Parse("connectionstring");
var client = account.CreateCloudBlobClient();


var client = AWSClientFactory.CreateAmazonS3Client("accessKey", "secret",RegionEndpoint.EUWest1);
S3 wins on lines of code but I don’t like having to declare the datacenter the account is in. In my opinion, the application shouldn’t be aware of this. 1 point to Azure.

Creating a container

This is a folder, Azure refers to is a container, S3 calls it a bucket.


var container = client.GetContainerReference("test-container");


 client.PutBucket(new PutBucketRequest { BucketName = "my-testing-bucket-123456", UseClientRegion = true});
catch (AmazonS3Exception ex)
 if(ex.ErrorCode != "BucketAlreadyOwnedByYou") {

S3 loses big time on simplicity here. To my knowledge, this is the only way to do a blind create of a container, that is creating it without knowing up front if it already exists. Azure makes this trivial with CreateIfNotExists. 2 points to Azure.

Uploading a file


var container = client.GetContainerReference("test-container");
var blob = container.GetBlockBlobReference("testfile");


var putObjectRequest = new PutObjectRequest {BucketName = "my-testing-bucket-123456", FilePath = @"M:\testfile.txt", Key = "testfile", GenerateMD5Digest = true, Timeout=-1};
var upload = client.PutObject(putObjectRequest);
They’re pretty much equal here, but the S3 code is more verbose. I like the idea of getting a reference to a blob while not knowing if it actually exists or not.

List Blobs


var container = client.GetContainerReference("test-container");
var blobs = container.ListBlobs(null, true, BlobListingDetails.Metadata);
blobs.OfType().Select (cbb => cbb.Name).Dump();


var listRequest = new ListObjectsRequest(){ BucketName = "my-testing-bucket-123456"};
client.ListObjects(listRequest).S3Objects.Select (so => so.Key).Dump();

In terms of complexity, they’re pretty even here too. Azure has one more line but it’s not a difficult one. Notice that whereas with Azure, we get a reference to a container and then perform operations against that reference, with AWS all requests are individual so you end up having to explicitly tell the client for every operation what the bucket name is. Point to Azure.

Deleting a Blob


var dblob = container.GetBlockBlobReference("testfile");


var delRequest = new DeleteObjectRequest(){ BucketName = "my-testing-bucket-123456", Key="testfile"};
Neither code is particularly complicated here, but I prefer Azure’s simplicity with the container and blob reference model so point Azure.

Delete a Container


var container = client.GetContainerReference("test-container");


var delBucket = new DeleteBucketRequest(){ BucketName = "my-testing-bucket-123456"};
Again, pretty equal. To micro-analyse the lines, you could say that for Azure, you’ve got one potentially reusable line, and one throw-away line. With S3, they’re both throw away. But in reality, unless you’re doing thousands of consecutive operations, it doesn’t really matter.


In terms of complexity, Azure’s and S3’s APIs are pretty much equal, but it’s easy to see where they each have their uses. Azure’s API is a much thicker abstraction over REST, whereas the S3 API is such a thin-veneer that you could imagine a home-grown API not turning out that differently (but most likely not as reliable).

In my mind, if you’re doing lots of operations against lots of different blobs and containers then S3’s API is more suitable as each operation is self-contained and there are no references to containers or blobs hanging around.

If you’re doing operations which share common elements, such as performing numerous operations on a blob or working with lots of blobs within a few containers, Azure’s API seems better suited as you create the references and then reuse them, reducing the amount of repeated code.

Bonus Section

If you could be bothered to read past my conclusion, congratulations on your determination! The comparative speed of Azure and AWS has been done to death, but I couldn’t resist getting my own stats.

These are ridiculously simple stats, essentially Stopwatch calls wrapped around the code in this post. The file I am uploading is only 6k. The simple reason for this is that everyone tests how these services handle lots of large objects, but no one seems to cover the probably more common scenario of users uploading very small files. The average size is probably higher than 6kb, but this is what I’ve got hanging around so this is what I’m using.

So here are my extremely simple and probably not at all reliable benchmarks.

Operation S3 Azure
Create Container573279
Upload 6Kb file9955
List Blobs (1)41103
Delete Blob5545
Delete Container22138
All times are in milliseconds. I’ve got to admit; I was expecting a more even spread here. Azure is significantly faster creating and deleting containers and uploading the file. It is also faster at deleting a blob, but the difference is insignificant. S3 wins significantly listing blobs.

Not covered in this post: Both APIs also have the Begin/End style of async operations and Azure has the bonus of async operations based on the async/await pattern, I may do another post on that in the future.

TL;DR; Azure's API is in my opinion a better abstraction and it's faster for most operations.