ResourceDictionary and XAML resource references

You can define the UI or resources for your app using XAML. Resources are typically definitions of some object that you expect to use more than once. To refer to a XAML resource later, you specify a key for a resource that acts like its name. You can reference a resource throughout an app or from any XAML page within it. You can define your resources using a ResourceDictionary element from the Windows Runtime XAML. Then, you can reference your resources by using a StaticResource markup extension or ThemeResource markup extension.

The XAML elements you might want to declare most often as XAML resources include Style, ControlTemplate, animation components, and Brush subclasses. Here, we explain how to define a ResourceDictionary and keyed resources, and how XAML resources relate to other resources that you define as part of your app or app package. We also explain resource dictionary advanced features such as MergedDictionaries and ThemeDictionaries.

Prerequisites

We assume that you understand XAML markup and have read the XAML overview.

Define and use XAML resources

XAML resources are objects that are referenced from markup more than once. Resources are defined in a ResourceDictionary, typically in a separate file or at the top of the markup page, like this.

<Page
    x:Class="MSDNSample.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Page.Resources>
        <x:String x:Key="greeting">Hello world</x:String>
        <x:String x:Key="goodbye">Goodbye world</x:String>
    </Page.Resources>

    <TextBlock Text="{StaticResource greeting}" Foreground="Gray" VerticalAlignment="Center"/>
</Page>

In this example:

  • <Page.Resources>…</Page.Resources> - Defines the resource dictionary.
  • <x:String> - Defines the resource with the key "greeting".
  • {StaticResource greeting} - Looks up the resource with the key "greeting", which is assigned to the Text property of the TextBlock.

Note  Don't confuse the concepts related to ResourceDictionary with the Resource build action, resource (.resw) files, or other "resources" that are discussed in the context of structuring the code project that produces your app package.

Resources don't have to be strings; they can be any shareable object, such as styles, templates, brushes, and colors. However, controls, shapes, and other FrameworkElements are not shareable, so they can't be declared as reusable resources. For more info about sharing, see the XAML resources must be shareable section later in this topic.

Here, both a brush and a string are declared as resources and used by controls in a page.

<Page
    x:Class="SpiderMSDN.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Page.Resources>
        <SolidColorBrush x:Key="myFavoriteColor" Color="green"/>
        <x:String x:Key="greeting">Hello world</x:String>
    </Page.Resources>

    <TextBlock Foreground="{StaticResource myFavoriteColor}" Text="{StaticResource greeting}" VerticalAlignment="Top"/>
    <Button Foreground="{StaticResource myFavoriteColor}" Content="{StaticResource greeting}" VerticalAlignment="Center"/>
</Page>

All resources need to have a key. Usually that key is a string defined with x:Key=”myString”. However, there are a few other ways to specify a key:

  • Style and ControlTemplate require a TargetType, and will use the TargetType as the key if x:Key is not specified. In this case, the key is the actual Type object, not a string. (See examples below)
  • DataTemplate resources that have a TargetType will use the TargetType as the key if x:Key is not specified. In this case, the key is the actual Type object, not a string.
  • x:Name can be used instead of x:Key. However, x:Name also generates a code behind field for the resource. As a result, x:Name is less efficient than x:Key because that field needs to be initialized when the page is loaded.

The StaticResource markup extension can retrieve resources only with a string name (x:Key or x:Name). However, the XAML framework also looks for implicit style resources (those which use TargetType rather than x:Key or x:Name) when it decides which style & template to use for a control that hasn't set the Style and ContentTemplate or ItemTemplate properties.

Here, the Style has an implicit key of typeof(Button), and since the Button at the bottom of the page doesn't specify a Style property, it looks for a style with key of typeof(Button):

<Page
    x:Class="MSDNSample.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Page.Resources>
        <Style TargetType="Button">
              <Setter Property="Background" Value="red"/>
        </Style>
    </Page.Resources> 
       <!-- This button will have a red background. -->
       <Button Content="Button" Height="100" VerticalAlignment="Center" Width="100"/>
</Page>

For more info about implicit styles and how they work, see Styling controls and Control templates.

Look up resources in code

You access members of the resource dictionary like any other dictionary.

Caution  When you perform a resource lookup in code, only the resources in the Page.Resources dictionary are looked at. Unlike the StaticResource markup extension, the code doesn't fall back to the Application.Resources dictionary if the resources aren’t found in the first dictionary.

This example shows how to retrieve the redButtonStyle resource out of a page’s resource dictionary:

<Page
    x:Class="MSDNSample.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Page.Resources>
        <Style TargetType="Button" x:Key="redButtonStyle">
            <Setter Property="Background" Value="red"/>
        </Style>
    </Page.Resources>
</Page>
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
            Style redButtonStyle = (Style)this.Resources["redButtonStyle"];
        }
    }

To look up app-wide resources from code, use Application.Current.Resources to get the app's resource dictionary, as shown here.

<Application
    x:Class="MSDNSample.App"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:SpiderMSDN">
    <Application.Resources>
        <Style TargetType="Button" x:Key="appButtonStyle">
            <Setter Property="Background" Value="red"/>
        </Style>
    </Application.Resources>

</Application>
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
            Style appButtonStyle = (Style)Application.Current.Resources["appButtonStyle"];
        }
    }

You can also add an application resource in code.

There are two things to keep in mind when doing this.

  • First, you need to add the resources before any page tries to use the resource.
  • Second, you can’t add resources in the App’s constructor.

You can avoid both problems if you add the resource in the Application.OnLaunched method, like this.

// App.xaml.cs

sealed partial class App : Application
{
    protected override void OnLaunched(LaunchActivatedEventArgs e)
    {
        Frame rootFrame = Window.Current.Content as Frame;
        if (rootFrame == null)
        {
            SolidColorBrush brush = new SolidColorBrush(Windows.UI.Color.FromArgb(255, 0, 255, 0)); // green
            this.Resources["brush"] = brush;
            // … Other code that VS generates for you …
        }
    }
}

Every FrameworkElement can have a ResourceDictionary

FrameworkElement is a base class that controls inherit from, and it has a Resources property. So, you can add a local resource dictionary to any FrameworkElement.

Here, a resource dictionary is added to a page element.

<Page
    x:Class="MSDNSample.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Page.Resources>
        <x:String x:Key="greeting">Hello world</x:String>
    </Page.Resources>

    <Border>
        <Border.Resources>
            <x:String x:Key="greeting">Hola mundo</x:String>
        </Border.Resources>
        <TextBlock Text="{StaticResource greeting}" Foreground="Gray" VerticalAlignment="Center"/>
    </Border>
</Page>

Here, both the Page and the Border have resource dictionaries, and they both have a resource called "greeting". The TextBlock is inside the Border, so its resource lookup looks first to the Border’s resources, then the Page’s resources, and then the Application resources. The TextBlock will read "Hola mundo".

To access that element’s resources from code, use that element’s Resources property. Accessing a FrameworkElement’s resources in code, rather than XAML, will look only in that dictionary, not in parent element’s dictionaries.

<Page
    x:Class="MSDNSample.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <Page.Resources>
        <x:String x:Key="greeting">Hello world</x:String>
    </Page.Resources>

    <Border x:Name="border">
        <Border.Resources>
            <x:String x:Key="greeting">Hola mundo</x:String>
        </Border.Resources>
    </Border>
</Page>
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
            string str = (string)border.Resources["greeting"];
        }
    }

Merged resource dictionaries

A merged resource dictionary combines one resource dictionary into another, usually in another file.

Tip  You can create a resource dictionary file in Microsoft Visual Studio by using the Add > New Item… > Resource Dictionary option from the Project menu.

Here, you define a resource dictionary in a separate XAML file called Dictionary1.xaml.

<!-- Dictionary1.xaml -->
<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MSDNSample">

    <SolidColorBrush x:Key="brush" Color="Red"/>

</ResourceDictionary>

To use that dictionary, you merge it with your page’s dictionary:

<Page
    x:Class="MSDNSample.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Page.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Dictionary1.xaml"/>
            </ResourceDictionary.MergedDictionaries>

            <x:String x:Key="greeting">Hello world</x:String>

        </ResourceDictionary>
    </Page.Resources>

    <TextBlock Foreground="{StaticResource brush}" Text="{StaticResource greeting}" VerticalAlignment="Center"/>
</Page>

Here's what happens in this example. In <Page.Resources>, you declare <ResourceDictionary>. The XAML framework implicitly creates a resource dictionary for you when you add resources to <Page.Resources>; however, in this case, you don’t want just any resource dictionary, you want one that contains merged dictionaries.

So you declare <ResourceDictionary>, then add things to its <ResourceDictionary.MergedDictionaries> collection. Each of those entries takes the form <ResourceDictionary Source="Dictionary1.xaml"/>. To add more than one dictionary, just add a <ResourceDictionary Source="Dictionary2.xaml"/> entry after the first entry.

After <ResourceDictionary.MergedDictionaries>…</ResourceDictionary.MergedDictionaries>, you can optionally put additional resources in your main dictionary. You use resources from a merged to dictionary just like a regular dictionary. In the example above, {StaticResource brush} finds the resource in the child/merged dictionary (Dictionary1.xaml), while {StaticResource greeting} finds its resource in the main page dictionary.

In the resource-lookup sequence, a MergedDictionaries dictionary is checked only after a check of all the other keyed resources of that ResourceDictionary. After searching that level, the lookup reaches the merged dictionaries, and each item in MergedDictionaries is checked. If multiple merged dictionaries exist, these dictionaries are checked in the inverse of the order in which they are declared in the MergedDictionaries property. In the following example, if both Dictionary2.xaml and Dictionary1.xaml declared the same key, the key from Dictionary2.xaml is used first because it's last in the MergedDictionaries set.

<Page
    x:Class="MSDNSample.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Page.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Dictionary1.xaml"/>
                <ResourceDictionary Source="Dictionary2.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Page.Resources>

    <TextBlock Foreground="{StaticResource brush}" Text="greetings!" VerticalAlignment="Center"/>
</Page>

Within the scope of any one ResourceDictionary, the dictionary is checked for key uniqueness. However, that scope does not extend across different items in different MergedDictionaries files.

You can use the combination of the lookup sequence and lack of unique key enforcement across merged-dictionary scopes to create a fallback value sequence of ResourceDictionary resources. For example, you might store user preferences for a particular brush color in the last merged resource dictionary in the sequence, using a resource dictionary that synchronizes to your app's state and user preference data. However, if no user preferences exist yet, you can define that same key string for a ResourceDictionary resource in the initial MergedDictionaries file, and it can serve as the fallback value. Remember that any value you provide in a primary resource dictionary is always checked before the merged dictionaries are checked, so if you want to use the fallback technique, don't define that resource in a primary resource dictionary.

Theme resources and theme dictionaries

A ThemeResource is similar to a StaticResource, but the resource lookup is reevaluated when the theme changes.

In this example, you set the foreground of a TextBlock to a value from the current theme.

<TextBlock Text="hello world" Foreground="{ThemeResource FocusVisualWhiteStrokeThemeBrush}" VerticalAlignment="Center"/>

A theme dictionary is a special type of merged dictionary that holds the resources that vary with the theme a user is currently using on his or her device. For example, the "light" theme might use a white color brush whereas the "dark" theme might use a dark color brush. The brush changes the resource that it resolves to, but otherwise the composition of a control that uses the brush as a resource could be the same. To reproduce the theme-switching behavior in your own templates and styles, instead of using MergedDictionaries as the property to merge items into the main dictionaries, use the ThemeDictionaries property.

Each ResourceDictionary element within ThemeDictionaries must have an x:Key value. The value is a string that names the relevant theme—for example, "Default", "Dark", "Light", or "HighContrast". Typically, Dictionary1 and Dictionary2 will define resources that have the same names but different values.

Here, you use red text for the light theme and blue text for the dark theme.

<!-- Dictionary1.xaml -->
<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MSDNSample">

    <SolidColorBrush x:Key="brush" Color="Red"/>

</ResourceDictionary>

<!—Dictionary2.xaml -->
<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:MSDNSample">

    <SolidColorBrush x:Key="brush" Color="blue"/>

</ResourceDictionary>

In this example, you set the foreground of a TextBlock to a value from the current theme.

<Page
    x:Class="MSDNSample.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <Page.Resources>
        <ResourceDictionary>
            <ResourceDictionary.ThemeDictionaries>
                <ResourceDictionary Source="Dictionary1.xaml" x:Key="Light"/>
                <ResourceDictionary Source="Dictionary2.xaml" x:Key="Dark"/>
            </ResourceDictionary.ThemeDictionaries>
        </ResourceDictionary>
    </Page.Resources>
    <TextBlock Foreground="{StaticResource brush}" Text="hello world" VerticalAlignment="Center"/>
</Page>

For theme dictionaries, the active dictionary to be used for resource lookup changes dynamically, whenever ThemeResource markup extension is used to make the reference and the system detects a theme change. The lookup behavior that is done by the system is based on mapping the active theme to the x:Key of a specific theme dictionary.

It can be useful to examine the way that the theme dictionaries are structured in the default XAML design resources, which parallel the templates that the Windows Runtime uses by default for its controls. Open the XAML files in \(Program Files)\Windows Kits\10\DesignTime\CommonConfiguration\Neutral\UAP\<SDK version>\Generic using a text editor or your IDE. Note how the theme dictionaries are defined first in generic.xaml, and how each theme dictionary defines the same keys. Each such key is then referenced by elements of composition in the various keyed elements that are outside the theme dictionaries and defined later in the XAML. There's also a separate themeresources.xaml file for design that contains only the theme resources and extra templates, not the default control templates. The theme areas are duplicates of what you'd see in generic.xaml.

When you use XAML design tools to edit copies of styles and templates, the design tools extract sections from the XAML design resource dictionaries and place them as local copies of XAML dictionary elements that are part of your app and project.

For more info and for a list of the theme-specific and system resources that are available to your app, see XAML theme resources.

Lookup behavior for XAML resource references

Lookup behavior is the term that describes how the XAML resources system tries to find a XAML resource. The lookup occurs when a key is referenced as a XAML resource reference from somewhere in the app's XAML. First, the resources system has predictable behavior for where it will check for the existence of a resource based on scope. If a resource isn't found in the initial scope, the scope expands. The lookup behavior continues on throughout the locations and scopes that a XAML resource could possibly be defined by an app or by the system. If all possible resource lookup attempts fail, an error often results. It's usually possible to eliminate these errors during the development process.

The lookup behavior for XAML resource references starts with the object where the actual usage is applied and its own Resources property. If a ResourceDictionary exists there, that ResourceDictionary is checked for an item that has the requested key. This first level of lookup is rarely relevant because you usually do not define and then reference a resource on the same object. In fact, a Resources property often doesn't exist here. You can make XAML resource references from nearly anywhere in XAML; you aren't limited to properties of FrameworkElement subclasses.

The lookup sequence then checks the next parent object in the runtime object tree of the app. If a FrameworkElement.Resources exists and holds a ResourceDictionary, the dictionary item with the specified key string is requested. If the resource is found, the lookup sequence stops and the object is provided to the location where the reference was made. Otherwise, the lookup behavior advances to the next parent level towards the object tree root. The search continues recursively upwards until the root element of the XAML is reached, exhausting the search of all possible immediate resource locations.

Note  It is a common practice to define all the immediate resources at the root level of a page, both to take advantage of this resource-lookup behavior and also as a convention of XAML markup style.

If the requested resource is not found in the immediate resources, the next lookup step is to check the Application.Resources property. Application.Resources is the best place to put any app-specific resources that are referenced by multiple pages in your app's navigation structure.

Control templates have another possible location in the reference lookup: theme dictionaries. A theme dictionary is a single XAML file that has a ResourceDictionary element as its root. The theme dictionary might be a merged dictionary from Application.Resources. The theme dictionary might also be the control-specific theme dictionary for a templated custom control.

Finally, there is a resource lookup against platform resources. Platform resources include the control templates that are defined for each of the system UI themes, and which define the default appearance of all the controls that you use for UI in a Windows Runtime app. Platform resources also include a set of named resources that relate to system-wide appearance and themes. These resources are technically a MergedDictionaries item, and thus are available for lookup from XAML or code once the app has loaded. For example, the system theme resources include a resource named "SystemColorWindowTextColor" that provides a Color definition to match app text color to a system window's text color that comes from the operating system and user preferences. Other XAML styles for your app can refer to this style, or your code can get a resource lookup value (and cast it to Color in the example case).

For more info and for a list of the theme-specific and system resources that are available to a UWP app that uses XAML, see XAML theme resources.

If the requested key is still not found in any of these locations, a XAML parsing error/exception occurs. In certain circumstances, the XAML parse exception may be a run-time exception that is not detected either by a XAML markup compile action, or by a XAML design environment.

Because of the tiered lookup behavior for resource dictionaries, you can deliberately define multiple resource items that each have the same string value as the key, as long as each resource is defined at a different level. In other words, although keys must be unique within any given ResourceDictionary, the uniqueness requirement does not extend to the lookup behavior sequence as a whole. During lookup, only the first such object that's successfully retrieved is used for the XAML resource reference, and then the lookup stops. You could use this behavior to request the same XAML resource by key at various positions within your app's XAML but get different resources back, depending on the scope from which the XAML resource reference was made and how that particular lookup behaves.

Forward references within a ResourceDictionary

XAML resource references within a particular resource dictionary must reference a resource that has already been defined with a key, and that resource must appear lexically before the resource reference. Forward references cannot be resolved by a XAML resource reference. For this reason, if you use XAML resource references from within another resource, you must design your resource dictionary structure so that the resources that are used by other resources are defined first in a resource dictionary.

Resources defined at the app level cannot make references to immediate resources. This is equivalent to attempting a forward reference, because the app resources are actually processed first (when the app first starts, and before any navigation-page content is loaded). However, any immediate resource can make a reference to an app resource, and this can be a useful technique for avoiding forward-reference situations.

XAML resources must be shareable

For an object to exist in a ResourceDictionary, that object must be shareable.

Being shareable is required because, when the object tree of an app is constructed and used at run time, objects cannot exist at multiple locations in the tree. Internally, the resource system creates copies of resource values to use in the object graph of your app when each XAML resource is requested.

A ResourceDictionary and Windows Runtime XAML in general supports these objects for shareable usage:

You can also use custom types as a shareable resource if you follow the necessary implementation patterns. You define such classes in your backing code (or in runtime components that you include) and then instantiate those classes in XAML as a resource. Examples are object data sources and IValueConverter implementations for data binding.

Custom types must have a default constructor, because that's what a XAML parser uses to instantiate a class. Custom types used as resources can't have the UIElement class in their inheritance, because a UIElement can never be shareable (it's always intended to represent exactly one UI element that exists at one position in the object graph of your runtime app).

UserControl usage scope

A UserControl element has a special situation for resource-lookup behavior because it has the inherent concepts of a definition scope and a usage scope. A UserControl that makes a XAML resource reference from its definition scope must be able to support the lookup of that resource within its own definition-scope lookup sequence—that is, it cannot access app resources. From a UserControl usage scope, a resource reference is treated as being within the lookup sequence towards its usage page root (just like any other resource reference made from an object in a loaded object tree) and can access app resources.

ResourceDictionary and XamlReader.Load

You can use a ResourceDictionary as either the root or a part of the XAML input for the XamlReader.Load method. You can also include XAML resource references in that XAML if all such references are completely self-contained in the XAML submitted for loading. XamlReader.Load parses the XAML in a context that is not aware of any other ResourceDictionary objects, not even Application.Resources. Also, don't use {ThemeResource} from within XAML submitted to XamlReader.Load.

Using a ResourceDictionary from code

Most of the scenarios for a ResourceDictionary are handled exclusively in XAML. You declare the ResourceDictionary container and the resources within as a XAML file or set of XAML nodes in a UI definition file. And then you use XAML resource references to request those resources from other parts of XAML. Still, there are certain scenarios where your app might want to adjust the contents of a ResourceDictionary using code that executes while the app is running, or at least to query the contents of a ResourceDictionary to see if a resource is already defined. These code calls are made on a ResourceDictionary instance, so you must first retrieve one—either an immediate ResourceDictionary somewhere in the object tree by getting FrameworkElement.Resources, or Application.Current.Resources.

In C# or Microsoft Visual Basic code, you can reference a resource in a given ResourceDictionary by using the indexer (Item). A ResourceDictionary is a string-keyed dictionary, so the indexer uses the string key instead of an integer index. In Visual C++ component extensions (C++/CX) code, use Lookup.

When using code to examine or change a ResourceDictionary, the behavior for APIs like Lookup or Item does not traverse from immediate resources to app resources; that's a XAML parser behavior that only happens as XAML pages are loaded. At run time, scope for keys is self-contained to the ResourceDictionary instance that you are using at the time. However, that scope does extend into MergedDictionaries.

Also, if you request a key that does not exist in the ResourceDictionary, there may not be an error; the return value may simply be provided as null. You may still get an error, though, if you try to use the returned null as a value. The error would come from the property's setter, not your ResourceDictionary call. The only way you'd avoid an error is if the property accepted null as a valid value. Note how this behavior contrasts with XAML lookup behavior at XAML parse time; a failure to resolve the provided key from XAML at parse time results in a XAML parse error, even in cases where the property could have accepted null.

Merged resource dictionaries are included into the index scope of the primary resource dictionary that references the merged dictionary at run time. In other words, you can use Item or Lookup of the primary dictionary to find any objects that were actually defined in the merged dictionary. In this case, the lookup behavior does resemble the parse-time XAML lookup behavior: if there are multiple objects in merged dictionaries that each have the same key, the object from the last-added dictionary is returned.

You are permitted to add items to an existing ResourceDictionary by calling Add (C# or Visual Basic) or Insert (C++/CX). You could add the items to either immediate resources or app resources. Either of these API calls requires a key, which satisfies the requirement that each item in a ResourceDictionary must have a key. However, items that you add to a ResourceDictionary at run time are not relevant to XAML resource references. The necessary lookup for XAML resource references happens when that XAML is first parsed as the app is loaded (or a theme change is detected). Resources added to collections at run time weren't available then, and altering the ResourceDictionary doesn't invalidate an already retrieved resource from it even if you change the value of that resource.

You also can remove items from a ResourceDictionary at run time, make copies of some or all items, or other operations. The members listing for ResourceDictionary indicates which APIs are available. Note that because ResourceDictionary has a projected API to support its underlying collection interfaces, your API options differ depending on whether you are using C# or Visual Basic versus C++/CX.

ResourceDictionary and localization

A XAML ResourceDictionary might initially contain strings that are to be localized. If so, store these strings as project resources instead of in a ResourceDictionary. Take the strings out of the XAML, and instead give the owning element an x:Uid directive value. Then, define a resource in a resources file. Provide a resource name in the form XUIDValue.PropertyName and a resource value of the string that should be localized.

Custom resource lookup

For advanced scenarios, you can implement a class that can have different behavior than the XAML resource reference lookup behavior described in this topic. To do this, you implement the class CustomXamlResourceLoader, and then you can access that behavior by using the CustomResource markup extension for resource references rather than using StaticResource or ThemeResource. Most apps won't have scenarios that require this. For more info, see CustomXamlResourceLoader.