silverlight在TextBlock中动态修改不能显示中文?

< TextBlock  Width ="29"  Height ="16"  Canvas.Left ="4"  Canvas.Top ="1"  Foreground ="#FFC5C5C5"  Text ="中文"  TextWrapping ="Wrap"  FontFamily ="Tahoma"  FontSize ="11" />

正常用没有问题

  xamlString = xamlString + "   < TextBlock  x:Name ='MtileTarget$0'  Canvas.Top ='9'  Canvas.Left ='$6'
         FontFamily ='Arial'  FontSize ='34'  FontWeight ='bold'  Text ='$1'  /> ";
    xamlString = xamlString + "   < TextBlock  Canvas.Top ='36'  Canvas.Left ='37.5'  FontFamily ='Arial'
   FontSize ='12'  FontWeight ='bold'  Text ='$5'  /> ";
   

上面这段是从FebCTPSamples\TileText中拿到的，但这个例子输入中文显示为方框，日文也是。
下面这么改也无法显示，英文则可以。

  var  wpfeControl  =  document.getElementById( " wpfeControl1 " );
     var  root  =  wpfeControl.findName( " MtileTarget1 " );
   root[ " Text " ] = " ぁ " ;