Difference between Style and ControlTemplate

Could you tell me what is the main differences between Style and ControlTemplate ? When or why to use one or the other ?

To my eyes, they are exactly the very same. As I am beginner I think that I am wrong, thus my question.

In a style you set properties of a control.

<Style x:Key="MyButtonStyle" TargetType="Button">
    <Setter Property="Background" Value="Red"/>
</Style>

<Button Style="{StaticResource MyButtonStyle}"/>

All buttons that use this style will have their Backgrounds set to Red.

In a template you define the UI (structure) of the control.

<ControlTemplate x:Key="MyButtonTemplate" TargetType="Button">
    <Grid>
        <Rectangle Fill="Green"/>
        <ContentPresenter/>
    </Grid>
</ControlTemplate>

<Button Template="{StaticResource MyButtonTemplate}"/>

All buttons that use this template will have a green background that cannot be changed.

Values set in a template can only be replaced by replacing the entire template. Values in a style can be replaced by setting the value explicitly when using the control. That is why is better to use the properties of the control by using TemplateBinding instead of coding values.

<ControlTemplate x:Key="MyButtonTemplate" TargetType="Button">
    <Grid>
        <Rectangle Fill="{TemplateBinding Background}"/>
        <ContentPresenter/>
    </Grid>
</ControlTemplate>

Now the template uses the value of the Background property of the button it is applied to, so it can be customized:

<Button Template="{StaticResource MyButtonTemplate}" Background="Yellow"/>

Another useful feature is that controls can pick up a default style without having a specific style being assigned to them. You can't do that with a template.

Just remove the x:Key attribute of the style (again: you can't do this with templates). All buttons in the visual tree below the style will have this style applied.

Combining Templates and Styles is extra powerful: you can set the Template property in the style:

<Style TargetType="Button">
    <Setter Property="Background" Value="Red"/>
    <Setter Property="Template">
        <Setter.Value>
             <ControlTemplate TargetType="Button">
                 <Grid>
                     <Rectangle Fill="{TemplateBinding Background}"/>
                     <ContentPresenter/>
                 </Grid>
             </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

No indeed you are quite wrong. Styles set properties on controls. ControlTemplate is a property shared by most controls that specify how they are rendered.

To elaborate, you can use a style to group settings for a bunch of properties so you can re-use that to standardize your controls. Styles can be set explicitly on controls or applied too all of a certain type.

Control Templates can be set by a style or set explicitly on a control to change the way it appears. All controls have default templates (and styles for that matter) that are embedded in the .net wpf assemblies. It is quite enlightening to see these and understand how the wpf developers implemented the normal versions of all controls. If you have Expression blend installed, look in its "SystemThemes" folder.

UPDATE:

To understand how Styles and ControlTemplates can "add controls". In some way or another, the ControlTemplate is the only way to define the controls a control is made up of. But, some default .net controls allow you to use controls in place of text.

For example:

<GroupBox>
  <GroupBox.Header>
    <CheckBox/>
  </GroupBox.Header>
</GroupBox>

This "adds" a checkbox to the groupbox without changing the ControlTemplate, but this is because the default ControlTemplate for GroupBox allows anything as the Header. This is done by using special controls such as ContentPresenter.

However, sometimes the default ControlTemplate for a control doesn't allow you to change something that you want to change via properties. Then you must change the ControlTemplate.

Whether you set the Properties of a control (Content, Header, ControlTemplate, IsEnabled, etc.) directly or via a style does not matter, Styles are only a convenience.

Hopefully this answers your question more clearly.

You can think of a Style as a convenient way to apply a set of property values to more than one element. You can change the default appearance by setting properties, such as FontSize and FontFamily, on each TextBlock element directly. However, if you want your TextBlock elements to share some properties, you can create a Style in the Resources section of your XAML file.

On the other hand, a ControlTemplate specifies the visual structure and visual behavior of a control. You can customize the appearance of a control by giving it a new ControlTemplate. When you create a ControlTemplate, you replace the appearance of an existing control without changing its functionality. For example, you can make the buttons in your application round instead of the default square shape, but the button will still raise the Click event.

Ref: http://msdn.microsoft.com/en-us/library/ms745683.aspx

I found some interesting differences in The difference between styles and templates (msdn)

Style: You can set only pre-existing properties in the style. For example, you cannot set a default value for a property that belongs to a new part that you added to the template.

Template: When you modify a template, you have access to more parts of a control than when you modify a style. For example, you can change the way the pop-up list appears in a combo box, or you change the look of the button that triggers the pop-up list in the combo box by modifying the items template.

---

Style: You can use styles to specify the default behavior of a control. For example, in a style for a button, you can specify a trigger so that when users move their mouse pointer over the button, the background color will change. These property changes are instantaneous (they cannot be animated gradually).

Template: You can specify the behavior of any new and existing parts in a template by using triggers. For example, you can specify a trigger so that when users move their mouse pointer over a button, the color of one of the parts will change. These property changes can be instantaneous or animated gradually to produce a smooth transition.

OK, I had the exact same question and the answers I found in this thread pointed me in the right direction so I'm sharing, if only so I can understand it better myself.

A Style is more flexible than a ControlTemplate.

From Windows Presentation Foundation Unleashed, Adam Nathan and gang (writers) state this:

• "Besides the convenience of combining a template [with a style using the Style's ControlTemplate setter] with arbitrary property settings, there are important advantages of doing this [setting the ControlTemplate setter on a style]:

1. It gives you the effect of default templates. For example, when a typed Style gets applied to elements by default, and that Style contains a custom control template, the control template gets applied without any explicitly markings on those elements.
2. It enables you to provide default yet overridable property valus that control the look of the template. In other words, it enables you to respect the templated parent's properties but still provide your own default values."

In other words, creating a style allows the user of the Style's Template setter to override the values set, even if they did not use a TemplateBinding ({TemplateBinding Width} for example). If you hardcoded the Width in your style, the user of the Style could still override it, but if you hardcoded that Width property in a Template, the user is stuck with it.

Also, (and this is kind of confusing) when using a ContentTemplate with a TemplateBinding the onus is on the user to set that property otherwise it will use the default property for the TargetType. If you use a style, you can override the default property of the TargetType by using a setter for the property and then applying a TemplateBinding referencing back to that setter. The book explains it better, page 338 (Mixing Templates with Styles)