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Resources and Code

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This overview concentrates on how Windows Presentation Foundation (WPF) resources can be accessed or created using code rather than Extensible Application Markup Language (XAML) syntax. For more information on general resource usage and resources from a XAML syntax perspective, see XAML Resources.

Accessing Resources from Code

The keys that identify resources if they are defined through XAML are also used to retrieve specific resources if you request the resource in code. The simplest way to retrieve a resource from code is to call either the FindResource or the TryFindResource method from framework-level objects in your application. The behavioral difference between these methods is what happens if the requested key is not found. FindResource raises an exception; TryFindResource will not raise an exception but returns null. Each method takes the resource key as an input parameter, and returns a loosely typed object. Typically, a resource key is a string, but there are occasional nonstring usages; see the Using Objects as Keys section for details. Typically you would cast the returned object to the type required by the property that you are setting when requesting the resource. The lookup logic for code resource resolution is the same as the dynamic resource reference XAML case. The search for resources starts from the calling element, then continues to successive parent elements in the logical tree. The lookup continues onwards into application resources, themes, and system resources if necessary. A code request for a resource will properly account for runtime changes in resource dictionaries that might have been made subsequent to that resource dictionary being loaded from XAML, and also for realtime system resource changes.

The following is a brief code example that finds a resource by key and uses the returned value to set a property, implemented as a Click event handler.

void SetBGByResource(object sender, RoutedEventArgs e)
{
  Button b = sender as Button;
  b.Background = (Brush)this.FindResource("RainbowBrush");
}

Private Sub SetBGByResource(ByVal sender As Object, ByVal e As RoutedEventArgs)
  Dim b As Button = TryCast(sender, Button)
  b.Background = CType(Me.FindResource("RainbowBrush"), Brush)
End Sub

An alternative method for assigning a resource reference is SetResourceReference. This method takes two parameters: the key of the resource, and the identifier for a particular dependency property that is present on the element instance to which the resource value should be assigned. Functionally, this method is the same and has the advantage of not requiring any casting of return values.

Still another way to access resources programmatically is to access the contents of the Resources property as a dictionary. Accessing the dictionary contained by this property is also how you can add new resources to existing collections, check to see if a given key name is already taken in the collection, and other dictionary/collection operations. If you are writing a WPF application entirely in code, you can also create the entire collection in code, assign keys to it, and then assign the finished collection to the Resources property of an established element. This will be described in the next section.

You can index within any given Resources collection, using a specific key as the index, but you should be aware that accessing the resource in this way does not follow the normal runtime rules of resource resolution. You are only accessing that particular collection. Resource lookup will not be traversing the scope to the root or the application if no valid object was found at the requested key. However, this approach may have performance advantages in some cases precisely because the scope of the search for the key is more constrained. See the ResourceDictionary class for more details on how to work with the resource dictionary directly.

Creating Resources with Code

If you want to create an entire WPF application in code, you might also want to create any resources in that application in code. To achieve this, create a new ResourceDictionary instance, and then add all the resources to the dictionary using successive calls to ResourceDictionary.Add. Then, use the ResourceDictionary thus created to set the Resources property on an element that is present in a page scope, or the Application.Resources. You could also maintain the ResourceDictionary as a standalone object without adding it to an element. However, if you do this, you must access the resources within it by item key, as if it were a generic dictionary. A ResourceDictionary that is not attached to an element Resources property would not exist as part of the element tree and has no scope in a lookup sequence that can be used by FindResource and related methods.

Using Objects as Keys

Most resource usages will set the key of the resource to be a string. However, various WPF features deliberately do not use a string type to specify keys, instead this parameter is an object. The capability of having the resource be keyed by an object is used by the WPF style and theming support. The styles in themes which become the default style for an otherwise non-styled control are each keyed by the Type of the control that they should apply to. Being keyed by type provides a reliable lookup mechanism that works on default instances of each control type, and type can be detected by reflection and used for styling derived classes even though the derived type otherwise has no default style. You can specify a Type key for a resource defined in XAML by using the x:Type Markup Extension. Similar extensions exist for other nonstring key usages that support WPF features, such as ComponentResourceKey Markup Extension.

See also