title: Compilation error when assign value to struct in dictionary
body: <p>folks, 
   I've get compilation error('Can not modify the return value of dictionary because it's not a variable') in next code  :</p>
  
  <pre><code>public class BaseForm : Form
  {
  
      protected void StoreGridViewPosition(DataGridView grid)
      {
  
          if (grids.ContainsKey(grid))
          {
              grids[grid].RowIndex = grid.CurrentCell.RowIndex;
              grids[grid].ColumnIndex = grid.CurrentCell.ColumnIndex;
          }
  
          Cell s = new Cell();
          s.RowIndex = 213;
      }
  
      protected void LoadGridViewPosition(DataGridView grid)
      {
      }
  
      private Dictionary&lt;DataGridView, Cell&gt; grids = new Dictionary&lt;DataGridView, Cell&gt;();
  
      private struct Cell
      {
          public int RowIndex;
          public int ColumnIndex;
      }
  }
  </code></pre>
  
  <p>But if i'll replace struct(Cell) with class, then it works fine. 
  Why it happens ?</p>