Styles, DataTemplates, and Implicit Keys
Earlier, it was stated that all items in a ResourceDictionary must have a key. However, that does not mean that all resources must have an explicit x:Key. Several object types support an implicit key when defined as a resource, where the key value is tied to the value of another property. This is known as an implicit key, whereas an x:Key attribute is an explicit key. You can overwrite any implicit key by specifying an explicit key.
One very important scenario for resources is when you define a Style. In fact, a Style is almost always defined as an entry in a resource dictionary, because styles are inherently intended for reuse. For more information about styles, see Styling and Templating.
Styles for controls can be both created with and referenced with an implicit key. The theme styles that define the default appearance of a control rely on this implicit key. The implicit key from the standpoint of requesting it is the Type of the control itself. The implicit key from the standpoint of defining the resource is the TargetType of the style. Therefore, if you are creating themes for custom controls, creating styles that interact with existing theme styles, you do not need to specify an x:Key Attribute for that Style. And if you want to use the themed styles, you do not need to specify any style at all. For instance, the following style definition works, even though the Style resource does not appear to have a key:
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<Style TargetType="Button">
<Setter Property="Background">
<Setter.Value>
<LinearGradientBrush>
<GradientStop Offset="0.0" Color="AliceBlue"/>
<GradientStop Offset="1.0" Color="Salmon"/>
</LinearGradientBrush>
</Setter.Value>
</Setter>
<Setter Property="FontSize" Value="18"/>
</Style>
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That style really does have a key: the implicit key typeof(Button). In markup, you can specify a TargetType directly as the type name (or you can optionally use {x:Type...} to return a Type.
Through the default theme style mechanisms used by WPF, that style is applied as the runtime style of a Button on the page, even though the Button itself does not attempt to specify its Style property or a specific resource reference to the style. Your style defined in the page is found earlier in the lookup sequence earlier than the theme dictionary style, using the same key that the theme dictionary style has. You could just specify <Button>Hello</Button> anywhere in the page, and the style you defined with TargetType of Button would apply to that button. If you want, you can still explicitly key the style with the same type value as TargetType, for clarity in your markup, but that is optional.
Implicit keys for styles do not apply on a control if OverridesDefaultStyle is true (also note that OverridesDefaultStyle might be set as part of native behavior for the control class, rather than explicitly on an instance of the control). Also, in order to support implicit keys for derived class scenarios, the control must override DefaultStyleKey (all existing controls provided as part of WPF do this). For more information about styles, themes, and control design, see Guidelines for Designing Stylable Controls.
DataTemplate also has an implicit key. The implicit key for a DataTemplate is the DataType property value. DataType can also be specified as the name of the type rather than explicitly using {x:Type...}. For details, see Data Templating Overview.