Setting Up Test Data for Blend

One of the big challenges in using Blend to set up an application that relies heavily on dynamic data is that without realistic data available at design-time, what you see isn’t what you get at run-time. Blend includes some basic built in data generation that can be useful for simple data structures but this won’t really help much if you have conditional layout that is expecting specific values.

If all you need is a generic set of data to fill out an ItemsControl you can use Blend to generate design-time data for itself. Select the control to generate data for and open the data binding dialog for the ItemsSource property (either directly or through the “Bind ItemsSource to Data…” context menu option). Locate the collection to bind to and click the “Define Data Template” button at the bottom. This dialog can set up a basic data template (that will probably need some tweaking later) and more importantly includes the “Generate sample data” checkbox at the bottom. Selecting this adds a new d:UseSampleData=”True” setting on the ItemsControl (along with the d and mc xmlns’s used by Blend) which signals Blend to create data for the types of the items in the bound collection. Unfortunately, the values generated are pretty limited. Strings are fruits, ints are small values, and enums are chosen at random (in the latest 2.0 SP1 version; earlier versions didn’t do enums).

When the built in generation isn’t enough test data can be made available in a number of ways but there are some challenges to getting it to show up. In most cases the key to getting Blend specific data relies in some way on a check of the DesignerProperties.IsInDesignMode Attached Property. This value is false at run-time and true at design-time in Blend. It can be checked with a call to DesignerProperties.GetIsInDesignMode, passing a DependencyObject instance or statically with the more verbose (bool)DependencyPropertyDescriptor.FromProperty(DesignerProperties.IsInDesignModeProperty, typeof(DependencyObject)).Metadata.DefaultValue.

Since Blend skips the constructor and many initialization event handlers declared in the code-behind of the view being loaded, anything that is hooked up in those methods will not be called in Blend. This can make it tricky to even get to code that can check IsInDesignMode and generate your data. To ensure code gets called it needs to somehow be referenced from XAML. A few ways to do this:

  • A property assignment of a new object with generation code in the constructor

    <local:ViewModel />

  • A Binding to a property that includes generation code

DataContext=”{Binding RelativeSource={RelativeSource Self}, Path=ViewModel}”

  • An assignment to a custom attached property that includes generation code in its set method

For any of these methods you should either use an IsInDesignMode check or make sure that everything being run will work at both Blend design-time and run-time. See my post on debugging Blend errors for more on Blend unsafe code. The simplest way to actually get the test data from your code to Blend is by assigning it to a DataContext property of some element (as in the examples above). If you plan ahead you should be able to get both your run-time and design-time data through the same DataContext properties which will keep your Binding behaviors consistent.

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