Writing Code for Design Time
Ensure that your code runs at design time, as well as run time. If your code runs at design time, do not assume that Application..::.Current is your application. For example, when you are using Expression Blend, Current is Expression Blend. At design time, MainWindow is not your application's main window. Typical operations that cause a custom control to fail at design time include the following.
There are two approaches to writing code for design time. The first approach is to write defensive code by checking the input parameters to classes, such as value converters. The second approach is to check whether design mode is active by calling the GetIsInDesignMode method.
Checking input parameters for some implementations is necessary because the design environment provides different types for some inputs than those provided by the runtime environment.
Style selectors and value converters usually require one of these approaches to run correctly at design time.
Value Converters
Your custom IValueConverter implementations should check for null and for the expected type in the first parameter of the Convert method. The following XAML shows a binding to Application..::.Current that fails at design time if the value converter is not implemented correctly.
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<ComboBox.IsEnabled>
<MultiBinding Converter="{StaticResource specialFeaturesConverter}">
<Binding Path="CurrentUser.Rating" Source="{x:Static Application.Current}"/>
<Binding Path="CurrentUser.MemberSince" Source="{x:Static Application.Current}"/>
</MultiBinding>
</ComboBox.IsEnabled>
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The binding raises an exception at design time because Application..::.Current refers to the designer application instead of your application. To prevent the exception, the value converter must check its input parameters or check for design mode.
The following code example shows how to check input parameters in a value converter that returns true if two input parameters satisfy particular business logic.
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public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
// Check the values array for correct parameters.
// Designers may send null or unexpected values.
if (values == null || values.Length < 2) return false;
if (!(values[0] is int)) return false;
if (!(values[1] is DateTime)) return false;
int rating = (int)values[0];
DateTime date = (DateTime)values[1];
// If the user has a good rating (10+) and has been a member for
// more than a year, special features are available.
if((rating >= 10) &&
(date.Date < (DateTime.Now.Date - new TimeSpan(365, 0, 0, 0))))
{
return true;
}
return false;
}
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The second approach to writing code for design time is to check whether design mode is active. The following code example shows a design-mode check instead of the parameter check shown previously.
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public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
// Check for design mode.
if ((bool)(DesignerProperties.IsInDesignModeProperty.GetMetadata(typeof(DependencyObject)).DefaultValue))
{
return false;
}
int rating = (int)values[0];
DateTime date = (DateTime)values[1];
// If the user has a good rating (10+) and has been a member for
// more than a year, special features are available.
if((rating >= 10) &&
(date.Date < (DateTime.Now.Date - new TimeSpan(365, 0, 0, 0))))
{
return true;
}
return false;
}
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Style Selectors
Your custom style selectors also must be implemented to run in design mode. The following XAML shows a custom template selector that uses Application..::.MainWindow at run time to determine which resource is returned as a DataTemplate. At design time, this resource may not be available, so the SelectTemplate override returns null at design time.
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<local:TaskListDataTemplateSelector x:Key="myDataTemplateSelector"/>
<ListBox Width="400" Margin="10"
ItemsSource="{Binding Source={StaticResource myTodoList}}"
ItemTemplateSelector="{StaticResource myDataTemplateSelector}"
HorizontalContentAlignment="Stretch"
IsSynchronizedWithCurrentItem="True"/>
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The following code shows the implementation of the style selector.
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public class TaskListDataTemplateSelector : DataTemplateSelector
{
public override DataTemplate SelectTemplate(
object item,
DependencyObject container)
{
if (item != null && item is Task)
{
Task taskitem = item as Task;
Window window = Application.Current.MainWindow;
// To run in design mode, either test for the correct window class
// or test for design mode.
if (window.GetType() == typeof(Window1))
// Or check for design mode:
//if (!DesignerProperties.GetIsInDesignMode(window))
{
if (taskitem.Priority == 1)
return window.FindResource("importantTaskTemplate") as DataTemplate;
else
return window.FindResource("myTaskTemplate") as DataTemplate;
}
}
return null;
}
}
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