Bruce Johnson, ObjectSharp Consulting
bjohnson@objectsharp.com

Data Bondage

The Basic Functionality
.Title
.Text Data Book Class
TitleTextBox
Binding
Target Source Source Data
Target Element
Property Property Object

DataContext Matters
• A DataContext in WPF is similar to a DataSource in
WinForms/ASP.NET
• Provides the basis for data binding
Book b = new Book() { Title=\"The Firm\",
Author=\"John Grisham\" };
this.DataContext = b;

Binding Expressions
• We saw a simple binding expression in our
walkthrough
Text=\"{Binding Path=Title}\"
• Binding expressions can be more complex
Text=\"{Binding Path=DateCreated.Month,
Mode=OneWay}\"

Data bindings can have different
modes
• One way
• Two way
• One time
• One way to source
• Default
– The characteristics of the target property
determines the mode

Formatting strings during binding
• StringFormat property of Binding class
• Formats strings during transfer to target
property
• Uses standard formatting strings, previously
defined in .NET
<TextBlock Text=\"{Binding Path=Price,
StringFormat=Only {0:c} with purchase!}\" />

Bindings have silent failure
• If a binding fails, no exception is generated
• The program’s output will note the binding
failure

Collections as the DataContext
 It’s common to use a list of data items instead
of a single one
 For example, a collection of books instead of a
single book:
Private Books As New List(Of Book)
Private Sub Window1_Loaded(ByVal sender As Object, …
Books.Add( _
New Book(\"Ender's Game\", \"Orson Scott Card\"))
Books.Add( _
New Book(\"Old Man's War\", \"John Scalzi\"))
Me.DataContext = Books
End Sub

Navigating with a ListBox
• Add an attribute to the XAML definition of the
Listbox
<ListBox ItemsSource=\"{Binding}\"
DisplayMemberPath=\"Title\"
IsSynchronizedWithCurrentItem=\"True\" … >
• Listbox now navigates the Books collection
If your ListBox fails to navigate
the list, always check the
IsSynchronizedWithCurrentItem
property

Lots of possible data sources…
 XML data (XPath attribute instead of Path)
 ADO.NET Datasets, DataTables
 Resource lists defined in XAML
You can bind to any control property that is a
dependency property
IF you can match data types
But what if you can’t?

Value converters
• WPF has some built-in converters
– For example, a string name of a color will be
converted to a brush
• You can write your own converters
– Convert one type to another
– Reformat strings (if StringFormat not sufficient)
• Implement the IValueConverter interface
– Convert method – data source property  WPF
element property
– ConvertBack method – WPF element property  data
source property

Interesting applications
• Bind data field to visibility of element
– Built-in BooleanToVisibilityConverter
• Bind month number to month name
• Bind graphic to data field
– Have image vary based on value of data field

MultiBinding
• A MultiBinding instance binds several data
fields in the source to one target property
• Collection of Binding instances gets the data
fields
• Converter assembles and parses the fields

Validation during binding
• Binding can have a collection of validation rules
• Built-in rules handle exceptions and IDataErrorInfo
– IDataErrorInfo support is new in FX3.5 SP1
– Not available in Silverlight (even 3)
• You can write your own validation rules
– Inherit from ValidationRule class
– Override the Validate method
– Return a ValidationResult object
• ValidationResult has a static ValidResult when there's no error

Displaying validation errors
• By default, all you get is a red rectangle
– Yes, I hate it too. It’s ugly.
– It can be overridden with a control template
– You’ll see this in the Control Templates section
• Validation class has a static GetErrors method to
return errors on an element
• Validation class has an Errors attached property
for an element
– You can use some fancy binding on this property to
display error messages in, e.g., tooltips
– However, binding information must be present in
every binding that is being validated

Another option – Error event
• Validation class has an attached event named Error
– Set this event on some container that holds all of your
fields
• Event args tells you if error is being added or
removed
• Event args includes an Error object with
ErrorContent property

Binding element-to-element
• Use ElementName property of binding instead
of setting a DataContext
• Allows you to tie user interface elements
together
• Example – zooming with Slider and TextBox

No-code zooming
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height=\"*\" />
<RowDefinition Height=\"8*\" />
</Grid.RowDefinitions>
<StackPanel Name=\"StackPanel1\" Orientation=\"Horizontal\">
<Slider Height=\"22\" Name=\"ZoomSlider\" Width=\"100\"
Minimum=\"10\" Maximum=\"100\" Value=\"50\" />
<TextBox Height=\"23\" Name=\"TextBox1\"
Text=\"{Binding Path=Value, ElementName=ZoomSlider}\"
Width=\"120\" Margin=\"5\" />
</StackPanel>
<Ellipse Width=\"{Binding Path=Value, ElementName=ZoomSlider}\"
Height=\"{Binding Path=Value, ElementName=ZoomSlider}\"
Grid.Row=\"1\" Name=\"Ellipse1\" Stroke=\"Black\" Fill=\"Red\">
</Grid>

RelativeSource binding
• Allows properties of an element to be bound to
other properties of that element
Height=
\"{Binding RelativeSource={RelativeSource
Self}, Path=Width}\"
• Allows properties of an element to be bound to
properties further up in the tree of elements

Questions?
• My contact information
– EMail: bjohnson@objectsharp.com
– Twitter: LACanuck
– Blog: http://www.objectsharp.com/blogs/bruce
– MSN: lacanadians@hotmail.com