mef中出现意外结果

我是MEF的初学者。我写了这段代码，但我不明白为什么程序会显示这样的结果。

namespace ConsoleApplication1
{
    public class MEFTest
    {
         [Import]
        public string Text { get; set; }
        [Import]
        public IExtension Ext { get; set; }
        public void ShowText()
        {
            AssemblyCatalog asscatalog = new AssemblyCatalog(typeof(Extension2).Assembly);
            CompositionContainer container = new CompositionContainer(asscatalog);
            CompositionBatch batch = new CompositionBatch();
            batch.AddPart(this);
            container.Compose(batch);
            Console.WriteLine(Text);
            Console.WriteLine(Ext.Text);
        }
    }
    class Program
    {
        static void Main( string[] args )
        {
            MEFTest mef = new MEFTest();
            mef.ShowText();
            Console.ReadKey();
        }
    }
    public interface IExtension
    {
        string Text { get; set; }
    }
    [Export]
    public class Extension1 : IExtension
    {
        [Export]
        public string Text { get; set; }
        public Extension1()
        {
            Text = "From Extension1.";
        }
    }
    [Export(typeof(IExtension))]
    public class Extension2 : IExtension
    {
       // [Export(typeof(IExtension).GetProperties()[0].GetType())]
        public string Text { get; set; }
        public Extension2()
        {
            Text = "From Extension2.";
        }
    }
} 

结果：

来自扩展1。来自Extension2。