If you have updated from the Q32009 version of Telerik RadSilverlight controls to the latest Q1 2010 release of the RadSilverlight controls and you were using MultipleSelect property in the RadGrid then you will get this error. Its very simple to resolve this error this property on the RadGrid has been declared obsolete and has been replaced by a new property called “SelectionMode” If you were using MultipleSelect = False, then now you can use SelectionMode = Single , this is the default value and it says only one item can be selected at a time. If you were using MultipleSelect = True , then now you can use SelectionMode = Extended. this means that items are added to the collection by combining the mouse clicks with Ctrl or Shift keys There is one more value to this property and this is like SelectionMode = Multiple this means that items are added to collection on the click of the item and are removed from the collection when you click again on the item. So above mentioned is the solution to your problem if you have recently upgraded from the older version of RadSilverlight controls to the new version of the controls. Happy Programming!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!