提示说，windows不在System 的namespace 里

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using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;

我用VS2008创建一个silverlight 的project
编译后出错，提示我 Error 1 The type or namespace name 'Windows' does not exist in the namespace 'System' (are you missing an assembly reference?) C:\Documents and Settings\c\My Documents\Visual Studio 2008\Projects\SilverlightProject1\SilverlightProject1\Page.xaml.cs 2 14 SilverlightProject1

请问这是怎么回事