title: How to validate all items in WPF DataGrid column?
body: <p>Hello everyone I have a problem:
  In WPF DataGrid I have an editable column and this colomn is measuring in percent (%), and sum of value in this column for all row should be not more than 100%. 
  For example in grid we have 3 row, and in this column we have 20% 20% 60% values. 
  in sum will be 100%.And user can change 60% into 75% the sum will be 
  115%, the question is how to validate that sum of all items when user changes values?
  Standard ValidationRule works only for one cell it doesn't help me.</p>
  
  <p>Help me please with suggestion or solution of this problem.</p>
  
  <p>Thanks in advance.</p>