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Privacy policy. However, you can also declare bindings in code. This article describes how to declare bindings in both XAML and in code. Before reading this article, it's important that you're familiar with the concept and usage of markup extensions. This article doesn't cover data binding concepts.
For a discussion of data binding concepts, see Data binding overview. Binding is a markup extension. When you use the binding extension to declare a binding, the declaration consists of a series of clauses following the Binding keyword and separated by commas ,. The clauses in the binding declaration can be in any order and there are many possible combinations. When creating binding declaration strings in markup, they must be attached to the specific dependency property of a target object.
The following example shows how to bind the TextBox. Text property using the binding extension, specifying the Source and Path properties. You can specify most of the properties of the Binding class this way. For more information about the binding extension and for a list of Binding properties that cannot be set using the binding extension, see the Binding Markup Extension. NET Framework overview. Object element syntax is an alternative to creating the binding declaration.
In most cases, there's no particular advantage to using either the markup extension or the object element syntax. However, when the markup extension doesn't support your scenario, such as when your property value is of a non-string type for which no type conversion exists, you need to use the object element syntax. The previous section demonstrated how to bind with a XAML extension. The following example demonstrates doing the same binding but uses object element syntax:.
NET Framework. Another way to specify a binding is to set properties directly on a Binding object in code, and then assign the binding to a property. The following example shows how to create a Binding object in code. As you have seen in previous sections, the Binding class is the high-level class for the declaration of a binding; it provides many properties that allow you to specify the characteristics of a binding.
A related class, BindingExpression , is the underlying object that maintains the connection between the source and the target.
A binding contains all the information that can be shared across several binding expressions. A BindingExpression is an instance expression that cannot be shared and contains all the instance information of the Binding. Consider the following example, where myDataObject is an instance of the MyData class, myBinding is the source Binding object, and MyData is a defined class that contains a string property named ColorName.
You can use the same myBinding object to create other bindings. For example, you can use the myBinding object to bind the text content of a check box to ColorName. In that scenario, there will be two instances of BindingExpression sharing the myBinding object. The following articles demonstrate some of the usages of the BindingExpression class:. In the Create a binding section, the button is red because its Background property is bound to a string property with the value "Red".
This string value works because a type converter is present on the Brush type to convert the string value to a Brush. Adding this information to the figure in the Create a binding section looks like this. However, what if instead of having a property of type string your binding source object has a Color property of type Color?
In that case, in order for the binding to work you would need to first turn the Color property value into something that the Background property accepts. You would need to create a custom converter by implementing the IValueConverter interface, as in the following example.
See IValueConverter for more information. To reiterate, default conversions may be available because of type converters that are present in the type being bound to. This behavior will depend on which type converters are available in the target.
If in doubt, create your own converter. Your data should be displayed differently, depending on culture. The data being used isn't necessarily intended to change the text value of a property, but is instead intended to change some other value, such as the source for an image, or the color or style of the display text.
Converters can be used in this instance by converting the binding of a property that might not seem to be appropriate, such as binding a text field to the Background property of a table cell. More than one control or multiple properties of controls are bound to the same data.
In this case, the primary binding might just display the text, whereas other bindings handle specific display issues but still use the same binding as source information. A target property has a collection of bindings, which is termed MultiBinding.
For example, color may be computed from red, blue, and green values, which can be values from the same or different binding source objects. See MultiBinding for examples and information. A binding source object can be treated either as a single object whose properties contain data or as a data collection of polymorphic objects that are often grouped together such as the result of a query to a database. So far we've only discussed binding to single objects.
However, binding to a data collection is a common scenario. For example, a common scenario is to use an ItemsControl such as a ListBox , ListView , or TreeView to display a data collection, such as in the app shown in the What is data binding section.
Fortunately, our basic diagram still applies. If you're binding an ItemsControl to a collection, the diagram looks like this. As shown in this diagram, to bind an ItemsControl to a collection object, ItemsControl. ItemsSource property is the property to use. You can think of ItemsSource as the content of the ItemsControl.
You can enumerate over any collection that implements the IEnumerable interface. However, to set up dynamic bindings so that insertions or deletions in the collection update the UI automatically, the collection must implement the INotifyCollectionChanged interface.
This interface exposes an event that should be raised whenever the underlying collection changes. To fully support transferring data values from source objects to targets, each object in your collection that supports bindable properties must also implement the INotifyPropertyChanged interface. If you have an advanced scenario and want to implement your own collection, consider using IList , which provides a non-generic collection of objects that can be individually accessed by the index, and thus provides the best performance.
Once your ItemsControl is bound to a data collection, you may want to sort, filter, or group the data. To do that, you use collection views, which are classes that implement the ICollectionView interface. A collection view is a layer on top of a binding source collection that allows you to navigate and display the source collection based on sort, filter, and group queries, without having to change the underlying source collection itself.
A collection view also maintains a pointer to the current item in the collection. If the source collection implements the INotifyCollectionChanged interface, the changes raised by the CollectionChanged event are propagated to the views.
Because views do not change the underlying source collections, each source collection can have multiple views associated with it. For example, you may have a collection of Task objects. With the use of views, you can display that same data in different ways. For example, on the left side of your page you may want to show tasks sorted by priority, and on the right side, grouped by area.
One way to create and use a view is to instantiate the view object directly and then use it as the binding source. For example, consider the Data binding demo app shown in the What is data binding section. The app is implemented such that the ListBox binds to a view over the data collection instead of the data collection directly.
The following example is extracted from the Data binding demo app. The resource listingDataView then serves as the binding source for elements in the app, such as the ListBox. To create another view for the same collection, you can create another CollectionViewSource instance and give it a different x:Key name. The following table shows what view data types are created as the default collection view or by CollectionViewSource based on the source collection type.
Specifying a collection view as a binding source is one way to create and use a collection view. WPF also creates a default collection view for every collection used as a binding source. If you bind directly to a collection, WPF binds to its default view. This default view is shared by all bindings to the same collection, so a change made to a default view by one bound control or code such as sorting or a change to the current item pointer, discussed later is reflected in all other bindings to the same collection.
To get the default view, you use the GetDefaultView method. For an example, see Get the default view of a data collection. To improve performance, collection views for ADO. NET DataTable or DataView objects delegate sorting and filtering to the DataView , which causes sorting and filtering to be shared across all collection views of the data source.
To enable each collection view to sort and filter independently, initialize each collection view with its own DataView object. As mentioned before, views can apply a sort order to a collection. As it exists in the underlying collection, your data may or may not have a relevant, inherent order. The view over the collection allows you to impose an order, or change the default order, based on comparison criteria that you supply.
Because it's a client-based view of the data, a common scenario is that the user might want to sort columns of tabular data per the value that the column corresponds to.
Using views, this user-driven sort can be applied, again without making any changes to the underlying collection or even having to requery for the collection content. For an example, see Sort a GridView column when a header is clicked. The following example shows the sorting logic of the "Sort by category and date" CheckBox of the app UI in the What is data binding section. Views can also apply a filter to a collection, so that the view shows only a certain subset of the full collection.
You might filter on a condition in the data. If you're using one of the CollectionView classes directly instead of CollectionViewSource , you would use the Filter property to specify a callback. For an example, see Filter Data in a View. Except for the internal class that views an IEnumerable collection, all collection views support grouping , which allows the user to partition the collection in the collection view into logical groups.
The groups can be explicit, where the user supplies a list of groups, or implicit, where the groups are generated dynamically depending on the data. The following example shows the logic of the "Group by category" CheckBox.
Views also support the notion of a current item. You can navigate through the objects in a collection view. As you navigate, you're moving an item pointer that allows you to retrieve the object that exists at that particular location in the collection. For an example, see Navigate through the objects in a data CollectionView. Because WPF binds to a collection only by using a view either a view you specify, or the collection's default view , all bindings to collections have a current item pointer.
In the following example, the data context is a collection view. The first line binds to the collection. The second line binds to the current item in the collection. The third line binds to the Description property of the current item in the collection. The slash and property syntax can also be stacked to traverse a hierarchy of collections. The following example binds to the current item of a collection named Offices , which is a property of the current item of the source collection.
The current item pointer can be affected by any sorting or filtering that is applied to the collection. Sorting preserves the current item pointer on the last item selected, but the collection view is now restructured around it. Perhaps the selected item was at the beginning of the list before, but now the selected item might be somewhere in the middle. Filtering preserves the selected item if that selection remains in view after the filtering.
Otherwise, the current item pointer is set to the first item of the filtered collection view. The notion of a current item is useful not only for navigation of items in a collection, but also for the master-detail binding scenario. Consider the app UI in the What is data binding section again. In that app, the selection within the ListBox determines the content shown in the ContentControl. To put it in another way, when a ListBox item is selected, the ContentControl shows the details of the selected item.
You can implement the master-detail scenario simply by having two or more controls bound to the same view. The following example from the Data binding demo shows the markup of the ListBox and the ContentControl you see on the app UI in the What is data binding section.
Notice that both of the controls are bound to the same source, the listingDataView static resource see the definition of this resource in the How to create a view section. This binding works because when a singleton object the ContentControl in this case is bound to a collection view, it automatically binds to the CurrentItem of the view. The CollectionViewSource objects automatically synchronize currency and selection. If your list control isn't bound to a CollectionViewSource object as in this example, then you would need to set its IsSynchronizedWithCurrentItem property to true for this to work.
For other examples, see Bind to a collection and display information based on selection. NET Framework and Use the master-detail pattern with hierarchical data. You may have noticed that the above example uses a template. In fact, the data would not be displayed the way we wish without the use of templates the one explicitly used by the ContentControl and the one implicitly used by the ListBox.
We now turn to data templating in the next section. Without the use of data templates, our app UI in the What is data binding section would look like the following. As shown in the example in the previous section, both the ListBox control and the ContentControl are bound to the entire collection object or more specifically, the view over the collection object of AuctionItem s.
Without specific instructions of how to display the data collection, the ListBox displays the string representation of each object in the underlying collection, and the ContentControl displays the string representation of the object it's bound to. To solve that problem, the app defines DataTemplates.
As shown in the example in the previous section, the ContentControl explicitly uses the detailsProductListingTemplate data template. Contents Exit focus mode. Please rate your experience Yes No. Any additional feedback? Submit and view feedback for This product This page.
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