Monday, 22 October 2012

MVC 3 meet Dictionary

Documenting a JsonValueProviderFactory Gotcha

About a year ago I was involved in the migration of an ASP.NET WebForms application over to MVC 3. We'd been doing a lot of AJAX-y / Single Page Application-y things in the project and had come to the conclusion that MVC might be a slightly better fit since we intended to continue down this path.

During the migration we encountered a bug in MVC 3 concerning Dictionary deserialization. This bug has subsequently tripped me up a few more times as I failed to remember the nature of the problem correctly. So I've written the issue up here as an aide to my own lamentable memory.

Before I begin I should say that the problem has been resolved in MVC 4. However given that I imagine many MVC 3 projects will not upgrade instantly there's probably some value in documenting the issue (and how to work around it). By the way, you can see my initial plea for assistance in this StackOverflow question.

The Problem

The problem is that deserialization of Dictionary objects does not behave in the expected and desired fashion. When you fire off a dictionary it arrives at your endpoint as the enormously unhelpful null. To see this for yourself you can try using this JavaScript:

With this C#:

You get a null null dictionary.


Alas, and indeed, alack...

After a long time googling around on the topic I eventually discovered, much to my surprise, that I was actually tripping over a bug in MVC 3. It was filed by Darin Dimitrov of Stack Overflow fame and I found details about it filed as an official bug here. To quote Darin:

"The System.Web.Mvc.JsonValueProviderFactory introduced in ASP.NET MVC 3 enables action methods to send and receive JSON-formatted text and to model-bind the JSON text to parameters of action methods. Unfortunately it doesn't work with dictionaries"

The Workaround

My colleague found a workaround for the issue here. There are 2 parts to this:

  1. Dictionaries in JavaScript are simple JavaScript Object Literals. In order to workaround this issue it is necessary to JSON.stringify our Dictionary / JOL before sending it to the endpoint. This is done so a string can be picked up at the endpoint.
  2. The signature of your action is switched over from a Dictionary reference to a string reference. Deserialization is then manually performed back from the string to a Dictionary within the Action itself.

I've adapted my example from earlier to demonstrate this; first the JavaScript:

Then the C#:

And now we're able to get a dictionary:


That's more like it!

Summary and a PS

So that's it; a little unglamourous but this works. I'm slightly surprised that that wasn't picked up before MVC 3 was released but at least it's been fixed for MVC 4. I look forward to this blog post being irrelevant and out of date ☺.

For what it's worth in my example above we're using the trusty old System.Web.Script.Serialization.JavaScriptSerializer to perform deserialization. My preference is actually to use JSON.Nets implementation but for the sake of simplicity I went with .NETs internal one here. To be honest, either is fine to my knowledge.

Friday, 5 October 2012

Using Web Optimization with MVC 3

A while ago I wrote about optimally serving up JavaScript in web applications. I mentioned that Microsoft had come up with a NuGet package called Microsoft ASP.NET Web Optimization which could help with that by minifying and bundling CSS and JavaScript. At the time I was wondering if I would be able to to use this package with pre-existing MVC 3 projects (given that the package had been released together with MVC 4). Happily it turns out you can. But it's not quite as straightforward as I might have liked so I've documented how to get going with this here...

Getting the Basics in Place

To keep it simple I'm going to go through taking a "vanilla" MVC 3 app and enhancing it to work with Web Optimization. To start, follow these basic steps:

  1. Open Visual Studio (bet you didn't see that coming!)
  2. Create a new MVC 3 application (I called mine "WebOptimizationWithMvc3" to demonstrate my imaginative flair). It doesn't really matter which sort of MVC 3 project you create - I chose an Intranet application but really that's by the by.
  3. Update pre-existing NuGet packages
  4. At the NuGet console type: "Install-Package Microsoft.AspNet.Web.Optimization"

Whilst the NuGet package adds the necessary references to your MVC 3 project it doesn't add the corresponding namespaces to the web.configs. To fix this manually add the following child XML element to the <namespaces> element in your root and Views web.config files:

<add namespace="System.Web.Optimization" />

This gives you access to Scripts and Styles in your views without needing the fully qualified namespace. For reasons best known to Microsoft I had to close down and restart Visual Studio before intellisense started working. You may need to do likewise.

Next up we want to get some JavaScript / CSS bundles in place. To do this, create a folder in the root of your project called "App_Start". There's nothing magical about this to my knowledge; this is just a convention that's been adopted to store all the bits of startup in one place and avoid clutterage. (I think this grew out of Nuget; see David Ebbo talking about this here.) Inside your new folder you should add a new class called BundleConfig.cs which looks like this:

The above is what you get when you create a new MVC 4 project (as it includes Web Optimization out of the box). All it does is create some JavaScript and CSS bundles relating to jQuery, jQuery UI, jQuery Validate, Modernizr and the standard site CSS. Nothing radical here but this example should give you an idea of how bundling can be configured and used. To make use of BundleConfig.cs you should modify your Global.asax.cs so it looks like this:

Once you've done this you're ready to start using Web Optimization in your MVC 3 application.

Switching over _Layout.cshtml to use Web Optimization

With a "vanilla" MVC 3 app the only use of CSS and JavaScript files is found in _Layout.cshtml. To switch over to using Web Optimization you should replace the existing _Layout.cshtml with this: (you'll see that the few differences that there are between the 2 are solely around the replacement of link / script tags with references to Scripts and Styles instead)

Do note that in the above Scripts.Render call we're rendering out 3 bundles; jQuery, jQuery UI and jQuery Validate. We're not using any of these in _Layout.cshtml but rendering these (and their associated link tags) gives us a chance to demonstrate that everything is working as expected.

In your root web.config file make sure that the following tag is in place: <compilation debug="true" targetFramework="4.0">. Then run, the generated HTML should look something like this:

This demonstrates that when the application has debug set to true you see the full scripts / links being rendered out as you would hope (to make your debugging less painful).

Now go back to your root web.config file and chance the debug tag to false: <compilation debug="false" targetFramework="4.0">. This time when you run, the generated HTML should look something like this:

This time you can see that in non-debug mode (ie how it would run in Production) minified bundles of scripts and css files are being served up instead of the raw files. And that's it; done.

Wednesday, 3 October 2012

Unit Testing and Entity Framework:
The Filth and the Fury

Just recently I've noticed that there appears to be something of a controversy around Unit Testing and Entity Framework. I first came across it as I was Googling around for useful posts on using MOQ in conjunction with EF. I've started to notice the topic more and more and as I have mixed feelings on the subject (that is to say I don't have a settled opinion) I thought I'd write about this and see if I came to any kind of conclusion...

The Setup

It started as I was working on a new project. We were using ASP.NET MVC 3 and Entity Framework with DbContext as our persistence layer. Rather than crowbarring the tests in afterwards the intention was to write tests to support the ongoing development. Not quite test driven development but certainly test supported development. (Let's not get into the internecine conflict as to whether this is black belt testable code or not - it isn't but he who pays the piper etc.) Oh and we were planning to use MOQ as our mocking library.

It was the first time I'd used DbContext rather than ObjectContext and so I thought I'd do a little research on how people were using DbContext with regards to testability. I had expected to find that there was some kind of consensus and an advised way forwards. I didn't get that at all. Instead I found a number of conflicting opinions.

Using the Repository / Unit of Work Patterns

One thread of advice that came out was that people advised using the Repository / Unit of Work patterns as wrappers when it came to making testable code. This is kind of interesting in itself as to the best of my understanding ObjectSet / ObjectContext and DbSet / DbContext are both in themselves implementations of the Repository / Unit of Work patterns. So the advice was to build a Repository / Unit of Work pattern to wrap an existing Repository / Unit of Work pattern.

Not as mad as it sounds. The reason for the extra abstraction is that ObjectContext / DbContext in the raw are not MOQ-able.

Or maybe I'm wrong, maybe you can MOQ DbContext?

No you can't. Well that's not true. You can and it's documented here but there's a "but". You need to be using Entity Frameworks Code First approach; actually coding up your DbContext yourself. Before I'd got on board the project had already begun and we were already some way down the road of using the Database First approach. So this didn't seem to be a go-er really.

The best article I found on testability and Entity Framework was this one by K. Scott Allen which essentially detailed how you could implement the Repository / Unit of Work patterns on top of ObjectSet / ObjectContext. In the end I adapted this to do the same thing sat on top of DbSet / DbContext instead.

With this in place I had me my testable code. I was quite happy with this as it seemed quite intelligible. My new approach looked similar to the existing DbSet / DbContext code and so there wasn't a great deal of re-writing to do. Sorted, right?

Here come the nagging doubts...

I did wonder, given that I found a number of articles about applying the Repository / Unit of Work patterns on top of ObjectSet / ObjectContext that there didn't seem to be many examples to do the same for DbSet / DbContext. (I did find a few examples of this but none that felt satisfactory to me for a variety of reasons.) This puzzled me.

I also started to notice that a 1 man war was being waged against the approach I was using by Ladislav Mrnka. Here are a couple of examples of his crusade:

Ladislav is quite strongly of the opinion that wrapping DbSet / DbContext (and I presume ObjectSet / ObjectContext too) in a further Repository / Unit of Work is an antipattern. To quote him: "The reason why I don’t like it is leaky abstraction in Linq-to-entities queries ... In your test you have Linq-to-Objects which is superset of Linq-to-entities and only subset of queries written in L2O is translatable to L2E". <byTheWay>It's worth looking at Jon Skeets explanation of "leaky abstractions" which he did for TekPub.</byTheWay>

As much as I didn't want to admit it - I have come to the conclusion Ladislav probably has a point for a number of reasons:

1. Just because it compiles and passes unit tests don't imagine that means it works...

Unfortunately, a LINQ query that looks right, compiles and has passing unit tests written for it doesn't necessarily work. You can take a query that fails when executed against Entity Framework and come up with test data that will pass that unit test. As Ladislav rightly points out: LINQ-to-Objects != LINQ-to-Entities.

So in this case unit tests of this sort don't provide you with any security. What you need are integration tests. Tests that run against an instance of the database and demonstrate that LINQ will actually translate queries / operations into valid SQL.

2. Complex queries

You can write some pretty complex LINQ queries if you want. This is made particularly easy if you're using comprehension syntax. Whilst these queries may be simple to write it can be uphill work to generate test data to satisfy this. So much so that at times it can feel you've made a rod for your own back using this approach.

3. Lazy Loading

By default Entity Framework employs lazy loading. This a useful approach which reduces the amount of data that is transported. Sometimes this approach forces you to specify up front if you require a particular entity through use of Include statements. This again doesn't lend itself to testing particularly well.

Where does this leave us?

Having considered all of the above for a while and tried out various different approaches I think I'm coming to the conclusion that Ladislav is probably right. Implementing the Repository / Unit of Work patterns on top of ObjectSet / ObjectContext or DbSet / DbContext doesn't seem a worthwhile effort in the end.

So what's a better idea? I think that in the name of simplicity you might as well have a simple class which wraps all of your Entity Framework code. This class could implement an interface and hence be straightforwardly MOQ-able (or alternatively all methods could be virtual and you could forego the interface). Along with this you should have integration tests in place which test the execution of the actual Entity Framework code against a test database.

Now I should say this approach is not necessarily my final opinion. It seems sensible and practical. I think it is likely to simplify the tests that are written around a project. It will certainly be more reliable than just having unit tests in place.

In terms of the project I'm working on at the moment we're kind of doing this in a halfway house sense. That is to say, we're still using our Repository / Unit of Work wrappers for DbSet / DbContext but where things move away from simple operations we're adding extra methods to our Unit of Work class or Repository classes which wrap this functionality and then testing it using our integration tests.

I'm open to the possibility that my opinion may be modified further. And I'd be very interested to know what other people think on the subject.

Update

It turns out that I'm not alone in thinking about this issue and indeed others have expressed this rather better than me - take a look at Jimmy Bogard's post for an example: http://lostechies.com/jimmybogard/2012/09/20/limiting-your-abstractions/.

Update 2

I've also recently watched the following Pluralsight course by Julie Lerman: http://pluralsight.com/training/Courses/TableOfContents/efarchitecture#efarchitecture-m3-archrepo. In this course Julie talks about different implementations of the Repository and Unit of Work patterns in conjunction with Entity Framework. Julie is in favour of using this approach but in this module she elaborates on different "flavours" of these patterns that you might want to use for different reasons (bounded contexts / reference contexts etc). She makes a compelling case and helpfully she is open enough to say that this a point of contention in the community. At the end of watching this I think I felt happy that our "halfway house" approach seems to fit and seems to work. More than anything else Julie made clear that there isn't one definitively "true" approach. Rather many different but similar approaches for achieving the same goal. Good stuff Julie!