WPF Show data from multiple DataContexts in ToolTip of ItemsControl

Question

I am trying to display a tooltip for an item generated by an ItemsControl that needs to pull data from conceptually unrelated sources. For example, say I have a

Answer 1

After an hour of hair pulling I have come to the conviction that you can't reference another DataContext inside a DataTemplate for a ToolTip. For other Bindings it is perfectly possible as other posters have proven. That's why you can't use the RelativeSource trick either. What you can do is implement a static property on your Item class and reference that:

    
        
            
                
                     
                        
                            
                                
                                    
                                        
                                        
                                    
                                    
                                    
                                
                            
                        
                    
                
            
        
    


using System.Collections.Generic;
using System.Windows;

namespace ToolTipSpike
{
    public partial class Window1 : Window
    {

        public List Items { get; private set; }
        public Window1()
        {
            InitializeComponent();
            var itemList = new List
                  {
                      new Item { ItemName = "First Item", ItemDescription = "This is the first item." },
                      new Item { ItemName = "Second Item", ItemDescription = "This is the second item." }
                  };
            this.Items = itemList;
            this.DataContext = this;
       }
    }

     public class Item
     {
         static Item()
         {
             GlobalText = "Additional Text";
         }
         public static string GlobalText { get; set; }
         public string ItemName{ get; set;}
         public string ItemDescription{ get; set;}
     }
}