Skip to main content

MVVM anti-pattern: explicitly using data context in View code behind

I believe explicitly using the data context in the code behind of the view (custom, user control etc) in any MVVM application is an anti-pattern. The view has no need to explicitly access the data context it is there purely for binding concerns. The following screen shot illustrates what I mean:
This is an anti-pattern because the view knows about the view model - as you can see the data context is being cast to the DataContextViewModel. The view (user control) is explicitly coupled to the view model and has reduced the cohesiveness of the view - it can now only be bound to instance of DataContextViewModel. Now you could argue if it was interface this would make is more useful but this doesn't reduce the tight coupling it just gives the illusion.

I guess the reason why this is common is because it makes the XAML appear very clean and simple, but this is also not ideal because it's not obvious what's being bound between the view model and the view:
How should this have been done?

By defining a Dependancy Property on the view (user control) and then binding this in the XAML to the FirstNames property of the view model, then there is no need to access the DataContext in the code behind of the view:
The XAML has now changed to be more declarative, which is a good thing:)


  1. Can you imagine doing this for each control inside an View...the ViewModel is a part of presentation layer and should be treated as such.


Post a Comment

Popular posts from this blog

WPF tips & tricks: Dispatcher thread performance

Not blogged for an age, and I received an email last week which provoked me back to life. It was a job spec for a WPF contract where they want help sorting out the performance of their app especially around grids and tabular data. I thought I'd shared some tips & tricks I've picked up along the way, these aren't probably going to solve any issues you might be having directly, but they might point you in the right direction when trying to find and resolve performance issues with a WPF app. First off, performance is something you shouldn't try and improve without evidence, and this means having evidence proving you've improved the performance - before & after metrics for example. Without this you're basically pissing into the wind, which can be fun from a developer point of view but bad for a project :) So, what do I mean by ' Dispatcher thread performance '? The 'dispatcher thread' or the 'UI thread' is probably the most

Showing a message box from a ViewModel in MVVM

I was doing a code review with a client last week for a WPF app using MVVM and they asked ' How can I show a message from the ViewModel? '. What follows is how I would (and have) solved the problem in the past. When I hear the words ' show a message... ' I instantly think you mean show a transient modal message box that requires the user input before continuing ' with something else ' - once the user has interacted with the message box it will disappear. The following solution only applies to this scenario. The first solution is the easiest but is very wrong from a separation perspective. It violates the ideas behind the Model-View-Controller pattern because it places View concerns inside the ViewModel - the ViewModel now knows about the type of the View and specifically it knows how to show a message box window: The second approach addresses this concern by introducing the idea of messaging\events between the ViewModel and the View. In the example below

Implementing a busy indicator using a visual overlay in MVVM

This is a technique we use at work to lock the UI whilst some long running process is happening - preventing the user clicking on stuff whilst it's retrieving or rendering data. Now we could have done this by launching a child dialog window but that feels rather out of date and clumsy, we wanted a more modern pattern similar to the way <div> overlays are done on the web. Imagine we have the following simple WPF app and when 'Click' is pressed a busy waiting overlay is shown for the duration entered into the text box. What I'm interested in here is not the actual UI element of the busy indicator but how I go about getting this to show & hide from when using MVVM. The actual UI elements are the standard Busy Indicator coming from the WPF Toolkit : The XAML behind this window is very simple, the important part is the ViewHost. As you can see the ViewHost uses a ContentPresenter element which is bound to the view model, IMainViewModel, it contains 3 child v