WPF

<window x:class="ComboBoxTest.Window1" xmlns:x="#unknown">
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300">
    <grid>
        <combobox margin="92,122,66,116">
                    ItemsSource="{Binding Path=Colours}"
                    DisplayMemberPath="Description"
                    SelectedItem="{Binding Path=SelectedColour, Mode=TwoWay}"
                    Grid.Column="1" 
                    Grid.Row="8" 
                    IsSynchronizedWithCurrentItem="True" 
                  
                  
                  />
    </combobox></grid>
</window>

using System.Windows;

namespace ComboBoxTest
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            this.DataContext = new ViewModel();
            InitializeComponent();
        }
    }
}

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

namespace ComboBoxTest
{

    public class Colour : IComparable
    {
        public string Description { get; set; }

        #region IComparable Members

        public int CompareTo(object obj)
        {
            return string.Compare(((Colour) obj).Description, this.Description);
        }

        #endregion
    }

    public class ViewModel : DependencyObject
    {
        public List<colour> Colours
        {
            get
            {
                List<colour> retval = new List<colour>();
                retval.Add(new Colour { Description = "Blue" });
                retval.Add(new Colour { Description = "Green" });
                retval.Add(new Colour { Description = "White" });
                retval.Add(new Colour { Description = "Yellow" });
                retval.Add(new Colour { Description = "Red" });
                retval.Add(new Colour { Description = "Silver" });
           

                return retval;
            }
        }

        private Colour _colour = new Colour { Description = "Yellow" };

        public Colour SelectedColour
        {
            get { return _colour; }
            set { _colour = value; }

        }
        public static readonly DependencyProperty SelectedColourProperty =
            DependencyProperty.Register("SelectedColour", typeof(Colour), typeof(ViewModel));



    }
}

public partial class AttenuationBox : UserControl
{ 
    private String justAtest;
    public String ATest { get { return justATest; } set { ...

    public String LowChRx
    {
        get { return tbLowChRX.Text; }
        set { tbLowChRX.Text = value; }
    }
...
}

<textbox name="tbLowChRX" text="{Binding Path=ATest}"></textbox>