title: how to raise an event when a value in a cell of a wpf datagrid changes using MVVM?
body: <p>I need help with a wpf datagrid using the MVVM design pattern.</p>
  
  <p>I have a datagid that is bound to an observablecollection. The first column in the grid contains decimal values that cannot be edited. The second column contains a textbox into which a decimal value must be entered. The third column must display the difference between the value in the first column and the value in the second column AS IT IS ENTERED. I was hoping that handling the observablecollection's Collectionchanged event will allow met to determine when a field of one of the items in the collection has changed, but that does not seem to work.</p>
  
  <p>I've also tried handling the PropertyChanged event of the grid's selected item, but that's not working either.</p>
  
  <p>Can someone please indicate to me how to raise an event in the viewmodel whenever 'n value in a textbox, in a datagrid DataGridTemplateColumn, is changed? And then how do I set the calculated value in the third column's corresponding row?</p>