public int ActiveFieldIndex {get;}

var tv = TableView.Active;
if (tv == null)
  return;

// get active field, active field name
var activeFieldIdx = tv.ActiveFieldIndex;
var fldDesc = tv.GetField(activeFieldIdx);
var fldName = fldDesc.Name;

// set active field by name
tv.SetActiveField("STATE_NAME");

// or set active field by index
tv.SetActiveField(3);