C#

namespace Tabcontrol 
{    
    public partial class PresetTab : UserControl   //3rd Tabitem ,preset.xaml.cs 
    {         
        public PresetTab() 
        { 
            InitializeComponent();             
        } 
        public  void AddPresetmenu(string pMenu)     
        { 
           menubox.Items.Add(pMenu);    //menubox is listbox 
        }  
    }     
} 

namespace Tabcontrol 
{   
    public partial class ToolBar : UserControl 
    { 
        PresetTab tab = new PresetTab();      
        public ToolBar() 
        {             
            InitializeComponent();  
        } 
         public void Click(object sender, MouseButtonEventArgs e) 
         {          
          tab.AddPresetmenu(menu);      //Im calling from tabcontrol,toolbar.xaml.cs                
            } 
     } 
}