Binding to ancestors from within a ResourceDictionary

Question

How can I bind to a UserControl's property from within its ResourceDictionary? I want an object I declare in my resources to have the same DataContext as the UserControl it is contained in:

<UserControl
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:Some.Namespace"
    DataContext="{Binding Path=ViewModel, RelativeSource={RelativeSource Self}}">
    <UserControl.Resources>
        <local:SomeClass
            x:Key="SomeClass"
            DataContext="{Binding Path=DataContext, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}}" />
    </UserControl.Resources>
</UserControl>

At runtime I get the error:

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='UserControl', AncestorLevel='1''. BindingExpression:Path=DataContext; DataItem=null; target element is 'SomeClass' (Name=''); target property is 'DataContext' (type 'Object')

Answer 1

My workaround was to set the DataContext of the resource in the code-behind.

.xaml

<local:SomeType x:Key="SomeKey" SomeProperty="{Binding ... }" />

.xaml.cs

public SomeControl()
{
    InitializeComponent();
    ((SomeType)this.Resources["SomeKey"]).DataContext = this;
}