use select columns for calculations

Question

  var result = from assc in context.A
 join b in B on .........    
       select new classXYZ {
  	 total = 
     	spent = ..............
         Left = total - spent
   };

I have a linq query which returns an object classXYZ , the column Left, of which is a calculation on other two columns.

It doesnt allow that calculation.

Any thoughts on why wont it pick up.

Second thing is i am trying to use the result object to update that value like

result.Left = result.Total - result.Spent.

Why wont this also work?

Thanks