Technique using a binding converter to return an object representing viewmodel state for displaying an adorner.

Adorners.cs

using System;
using System.Globalization;
using System.Windows.Data;

namespace WpfApplication1
{
  /*
  
    see: http://cuppster.com/2014/07/10/using-the-state-pattern-for-adorning-in-xaml/
   
    <!-- in resources ... -->
    <local:FooStateConverter x:Key="FooStateConverter" />
   
    <!-- data template for a specific state -->
    <DataTemplate DataType="{x:Type local:FooStateDefaultAdorner}">
        <Canvas>
          <Image Source="/Images/DefaultOverlay.png" Canvas.Left="5" Canvas.Top="5"/>
        </Canvas>
    </DataTemplate>

    ...

    <!-- inside the data template for the view model for Foo -->
    <Grid>
	    <Image Source="/Images/FooBaseImage.png"/>
	    <AdornerDecorator>
	      <ContentControl Content="{Binding Converter={StaticResource FooStateConverter}}"/>
	    </AdornerDecorator>
  
  */

  public class FooState
  {
    // a base class is convient so that the generic state class
    // can be more type-safe and return a known base class instead of 'object'
  }

  public class FooState<T> : FooState
  {
    // each generic class declared will recieve its own _instance static member
    // ... that's the trick
    private static FooState _instance;

    public FooState()
    {
      _instance = this;
    }

    public new static Type GetType()
    {
      // returns the type of the generic parameter NOT of this class
      return typeof(T);
    }

    public static FooState Instance
    {
      get
      {
        if (null == _instance)
        {
          Type t = GetType();
          Activator.CreateInstance(t);
        }
        return _instance;
      }
    }
  }

  public class FooStateDefaultAdorner : FooState<FooStateDefaultAdorner>
  {
    // use a XAML DataTemplate to present this object
  }

  public class FooStateInvalidAdorner : FooState<FooStateInvalidAdorner>
  {
    // use a XAML DataTemplate to present this object
  }

  public class FooStateConverter : IValueConverter
  {

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
      // Inspect viewmodel or model for state
      // and create the appropriate FooState class

      /*
      
      1. cast 'value' to your view model class
      2. query the state of the view model
      3. return a specific FooState instance based on that state
      
      e.g.
      
      var myViewModel = object as MyViewModel;
      if (myViewModel.IsDefault)
        return FooStateDefaultAdorner.Instance;
      else if (myViewModel.IsInvalid)
        return FooStateInvalidAdorner.Instance;
      */

      // fall-through...
      return null;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
      return null;
    }
  }
}