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What are binding declarations? (WPF .NET)

  • Artikkeli

Typically, developers declare the bindings directly in the XAML markup of the UI elements they want to bind data to. However, you can also declare bindings in code. This article describes how to declare bindings in both XAML and in code.

Prerequisites

Before reading this article, it's important that you're familiar with the concept and usage of markup extensions. For more information about markup extensions, see Markup Extensions and WPF XAML.

This article doesn't cover data binding concepts. For a discussion of data binding concepts, see Data binding overview.

Declare a binding in XAML

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. The clauses are Name=Value pairs, where Name is the name of the Binding property and Value is the value you're setting for the property.

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.

<TextBlock Text="{Binding Source={StaticResource myDataSource}, Path=Name}"/>

The previous example uses a simple data object type of Person. The following snippet is the code for that object:

class Person
{
    public string Name { get; set; }
    public DateTime Birthdate { get; set; }
}

Public Class Person

    Public Property Name As String
    Public Property Birthdate As DateTime
    
End Class

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.

For an example on creating a binding in XAML, see How to create a data binding.

Object element syntax

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:

<TextBlock>
    <TextBlock.Text>
        <Binding Source="{StaticResource myDataSource}" Path="Name"/>
    </TextBlock.Text>
</TextBlock>

For more information about the different terms, see XAML Syntax In Detail (.NET Framework).

MultiBinding and PriorityBinding

MultiBinding and PriorityBinding don't support the XAML extension syntax. That's why you must use the object element syntax if you're declaring a MultiBinding or a PriorityBinding in XAML.

Create a binding in code

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.

private void Window_Loaded(object sender, RoutedEventArgs e)
{
    // Make a new data source object
    var personDetails = new Person()
    {
        Name = "John",
        Birthdate = DateTime.Parse("2001-02-03")
    };

    // New binding object using the path of 'Name' for whatever source object is used
    var nameBindingObject = new Binding("Name");

    // Configure the binding
    nameBindingObject.Mode = BindingMode.OneWay;
    nameBindingObject.Source = personDetails;
    nameBindingObject.Converter = NameConverter.Instance;
    nameBindingObject.ConverterCulture = new CultureInfo("en-US");

    // Set the binding to a target object. The TextBlock.Name property on the NameBlock UI element
    BindingOperations.SetBinding(NameBlock, TextBlock.TextProperty, nameBindingObject);
}

Private Sub Window_Loaded(sender As Object, e As RoutedEventArgs)

    ' Make a new data source object
    Dim personDetails As New Person() With {
        .Name = "John",
        .Birthdate = Date.Parse("2001-02-03")
    }

    ' New binding object using the path of 'Name' for whatever source object is used
    Dim nameBindingObject As New Binding("Name")

    ' Configure the binding
    nameBindingObject.Mode = BindingMode.OneWay
    nameBindingObject.Source = personDetails
    nameBindingObject.Converter = NameConverter.Instance
    nameBindingObject.ConverterCulture = New CultureInfo("en-US")

    ' Set the binding to a target object. The TextBlock.Name property on the NameBlock UI element
    BindingOperations.SetBinding(NameBlock, TextBlock.TextProperty, nameBindingObject)

End Sub

The previous code set the following on the binding:

  • A path of the property on the data source object.
  • The mode of the binding.
  • The data source, in this case, a simple object instance representing a person.
  • An optional converter that processes the value coming in from the data source object before it's assigned to the target property.

When the object you're binding is a FrameworkElement or a FrameworkContentElement, you can call the SetBinding method on your object directly instead of using BindingOperations.SetBinding. For an example, see How to: Create a Binding in Code.

The previous example uses a simple data object type of Person. The following is the code for that object:

class Person
{
    public string Name { get; set; }
    public DateTime Birthdate { get; set; }
}

Public Class Person

    Public Property Name As String
    Public Property Birthdate As DateTime
    
End Class

Binding path syntax

Use the Path property to specify the source value you want to bind to:

  • In the simplest case, the Path property value is the name of the property of the source object to use for the binding, such as Path=PropertyName.

  • Subproperties of a property can be specified by a similar syntax as in C#. For instance, the clause Path=ShoppingCart.Order sets the binding to the subproperty Order of the object or property ShoppingCart.

  • To bind to an attached property, place parentheses around the attached property. For example, to bind to the attached property DockPanel.Dock, the syntax is Path=(DockPanel.Dock).

  • Indexers of a property can be specified within square brackets following the property name where the indexer is applied. For instance, the clause Path=ShoppingCart[0] sets the binding to the index that corresponds to how your property's internal indexing handles the literal string "0". Nested indexers are also supported.

  • Indexers and subproperties can be mixed in a Path clause; for example, Path=ShoppingCart.ShippingInfo[MailingAddress,Street].

  • Inside indexers. You can have multiple indexer parameters separated by commas (,). The type of each parameter can be specified with parentheses. For example, you can have Path="[(sys:Int32)42,(sys:Int32)24]", where sys is mapped to the System namespace.

  • When the source is a collection view, the current item can be specified with a slash (/). For example, the clause Path=/ sets the binding to the current item in the view. When the source is a collection, this syntax specifies the current item of the default collection view.

  • Property names and slashes can be combined to traverse properties that are collections. For example, Path=/Offices/ManagerName specifies the current item of the source collection, which contains an Offices property that is also a collection. Its current item is an object that contains a ManagerName property.

  • Optionally, a period (.) path can be used to bind to the current source. For example, Text="{Binding}" is equivalent to Text="{Binding Path=.}".

Escaping mechanism

  • Inside indexers ([ ]), the caret character (^) escapes the next character.

  • If you set Path in XAML, you also need to escape (using XML entities) certain characters that are special to the XML language definition:

    • Use &amp; to escape the character "&".

    • Use &gt; to escape the end tag ">".

  • Additionally, if you describe the entire binding in an attribute using the markup extension syntax, you need to escape (using backslash \) characters that are special to the WPF markup extension parser:

    • Backslash (\) is the escape character itself.

    • The equal sign (=) separates property name from property value.

    • Comma (,) separates properties.

    • The right curly brace (}) is the end of a markup extension.

Binding direction

Use the Binding.Mode property to specify the direction of the binding. The following modes are the available options for binding updates:

Binding mode Description
BindingMode.TwoWay Updates the target property or the property whenever either the target property or the source property changes.
BindingMode.OneWay Updates the target property only when the source property changes.
BindingMode.OneTime Updates the target property only when the application starts or when the DataContext undergoes a change.
BindingMode.OneWayToSource Updates the source property when the target property changes.
BindingMode.Default Causes the default Mode value of target property to be used.

For more information, see the BindingMode enumeration.

The following example shows how to set the Mode property:

<TextBlock Name="IncomeText" Text="{Binding Path=TotalIncome, Mode=OneTime}" />

To detect source changes (applicable to OneWay and TwoWay bindings), the source must implement a suitable property change notification mechanism such as INotifyPropertyChanged. For more information, see Providing change notifications.

For TwoWay or OneWayToSource bindings, you can control the timing of the source updates by setting the UpdateSourceTrigger property. For more information, see UpdateSourceTrigger.

Default behaviors

The default behavior is as follows if not specified in the declaration:

  • A default converter is created that tries to do a type conversion between the binding source value and the binding target value. If a conversion cannot be made, the default converter returns null.

  • If you don't set ConverterCulture, the binding engine uses the Language property of the binding target object. In XAML, this defaults to en-US or inherits the value from the root element (or any element) of the page, if one has been explicitly set.

  • As long as the binding already has a data context (for example, the inherited data context coming from a parent element), and whatever item or collection being returned by that context is appropriate for binding without requiring further path modification, a binding declaration can have no clauses at all: {Binding}. This is often the way a binding is specified for data styling, where the binding acts upon a collection. For more information, see Using Entire Objects as a Binding Source.

  • The default Mode varies between one-way and two-way depending on the dependency property that is being bound. You can always declare the binding mode explicitly to ensure that your binding has the desired behavior. In general, user-editable control properties, such as TextBox.Text and RangeBase.Value, default to two-way bindings, but most other properties default to one-way bindings.

  • The default UpdateSourceTrigger value varies between PropertyChanged and LostFocus depending on the bound dependency property as well. The default value for most dependency properties is PropertyChanged, while the TextBox.Text property has a default value of LostFocus.

See also