Cannot apply indexing with [] to an expression of type 'ISet<string>'
<p>I am getting Error <strong>'Cannot apply indexing with [] to an expression of type 'ISet''</strong></p>

<p>for this code snippet</p>

<pre><code>foreach (GridViewRow grv in customTableDataList.UniGrid.GridView.Rows)
{
  if (grv != null)
  {
    if (null != grv.FindControl(ItemCheckBoxID) &amp;&amp; ((CheckBox)grv.FindControl(ItemCheckBoxID)).Checked)
    {
      //At this line I am getting error.
      itemIds += customTableDataList.UniGrid.ActionsID[rowCounter] + ", ";  
    }
    rowCounter++;
  }
}
</code></pre>

<p>Can anyone give some more details to resolve this.</p>