[PartCreationPolicy(CreationPolicy.Shared)]

using System;
using System.ComponentModel.Composition;

namespace SlMefTest
{
   [Export]
   public class WebServiceData
   {
       public int Result { set; get; }
      
       public WebServiceData()
       {
           var rnd = new Random();
           Result = rnd.Next();
       }
   }

}

<UserControl x:Class="SlMefTest.MainPage"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
   xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
   xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
   mc:Ignorable="d" d:DesignHeight="300" d:DesignWidth="400">
   <StackPanel>
       <Button Content="MainPageButton" Height="23"
               HorizontalAlignment="Left"
               Margin="10,10,0,0" Name="button1"
               VerticalAlignment="Top" Width="98" Click="button1_Click" />
       <StackPanel Name="panel1" Margin="5"/>
   </StackPanel>
</UserControl>

using System.ComponentModel.Composition;
using System.Windows;

namespace SlMefTest
{
   public partial class MainPage
   {
       [Import]
       public WebServiceData Data { set; get; }

       public MainPage()
       {
           InitializeComponent();
           this.Loaded += mainPageLoaded;
       }

       void mainPageLoaded(object sender, RoutedEventArgs e)
       {
           CompositionInitializer.SatisfyImports(this);
           panel1.Children.Add(new SilverlightControl1());
       }

       private void button1_Click(object sender, RoutedEventArgs e)
       {
           MessageBox.Show(Data.Result.ToString());
       }
   }
}

<UserControl x:Class="SlMefTest.SilverlightControl1"
   xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
   xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
   xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
   mc:Ignorable="d"
   d:DesignHeight="300" d:DesignWidth="400">
  
   <Grid x:Name="LayoutRoot" Background="White">
       <Button Content="UserControlButton"
               Height="23"
               HorizontalAlignment="Left"
               Margin="10,10,0,0"
               Name="button1"
               VerticalAlignment="Top"
               Width="125"
               Click="button1_Click" />
   </Grid>
</UserControl>

using System.ComponentModel.Composition;
using System.Windows;

namespace SlMefTest
{
   public partial class SilverlightControl1
   {
       [Import]
       public WebServiceData Data { set; get; }

       public SilverlightControl1()
       {
           InitializeComponent();
           this.Loaded += silverlightControl1Loaded;
       }

       void silverlightControl1Loaded(object sender, RoutedEventArgs e)
       {
           CompositionInitializer.SatisfyImports(this);
       }

       private void button1_Click(object sender, RoutedEventArgs e)
       {
           MessageBox.Show(Data.Result.ToString());
       }
   }
}