The Remarks in this topic have some doc bugs, which will be fixed in an upcoming release.

One of the Remarks states that Silverlight always processes Tag as a string. This is not entirely correct.

1) If you are using the Silverlight JavaScript API, you can only use a string, but that's probably not relevant to mention here, because this is the documentation for the managed version.

2) If you want to set the Tag value to an object in the initial XAML, that is possible. However, whatever object you use has to support a XAML usage in some fashion. One possibility is to use attribute syntax along with either a binding or a resource reference, for instance:

<Button Tag="{Binding OrderBusinessObj}" .../>

or

<Button Tag="{StaticResource OrderBusinessObj}" .../>

Another possibility is to use property element syntax along with an element usage for a custom object, along with a "my" mapping to the object-defining assembly+namespace. For instance:

<Button>

  <Button.Tag>

    <my:OrderBusinessObj .../>

  </Button.Tag>

</Button>

3) If you want to set Tag in code at run-time there are no particular restrictions on which object you use. However, bear in mind that if you want to use properties of the object you set as Tag for UI purposes, using Tag might not be the best design. It might be better to associate a given object to a FrameworkElement through a Binding relationship instead, perhaps by setting the object into a single FrameworkElement's DataContext. With a Binding relationship, you can then bind particular properties of your object to properties of the FrameworkElement, using converters on the binding and/or DataTemplate techniques in order to get the right type-match and presentation for your object data in the UI.