<Window x:Class="Binding.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:MyCLR="clr-namespace:Binding"
    Title="Window1" Height="300" Width="300">
    <Window.Resources>
        <MyCLR:TextToObject x:Key="MyConv"/>
    </Window.Resources>
    <Grid>
        <TextBox Margin="48,95,110,0" Name="textBox1" Height="22" VerticalAlignment="Top" 
         Text="{Binding ElementName=slider1, Path=Value, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
        <ListBox Margin="62,123,62,39" Name="listBox1" Background="LightSalmon">
            <ListBox.ContextMenu>
                <ContextMenu x:Name="MyMenu" DataContext="{Binding PlacementTarget, RelativeSource={RelativeSource Self}}"> 
                    <MenuItem x:Name="MyItem" Header="{Binding ElementName=slider1, Path=Value, Converter={StaticResource MyConv}}"/>
                </ContextMenu>
            </ListBox.ContextMenu>
        </ListBox>
        <Slider Height="22" Margin="85,36,93,0" Name="slider1" VerticalAlignment="Top" SmallChange="1" />
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Data;
using System.Globalization;

namespace Binding
{
    [ValueConversion (typeof(double), typeof(object))]
    class TextToObject:IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            object val = (object)value;
            return val;
        }
        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            double val = (double)value;
            return val;
        }
    }
}