public sealed class SketchModifiedEventArgs : System.EventArgs 

// Occurs when a sketch is modified. SketchModified event is fired by 
//  - COTS construction tools (except annotation, dimension geometry types), 
//  - Edit Vertices, Reshape, Align Features
//  - 3rd party tools with FireSketchEvents = true
// Handles the SketchModified event, providing access to the current and previous sketch geometries when a sketch
// is modified.
// Register for the SketchModified event
SketchModifiedEvent.Subscribe(sketchModifiedEventArgs
  =>
{
  // if not an undo operation
  if (!sketchModifiedEventArgs.IsUndo)
  {
    // what was the sketch before the change?
    var prevSketch = sketchModifiedEventArgs.PreviousSketch;
    // what is the current sketch?
    var currentSketch = sketchModifiedEventArgs.CurrentSketch;
    if (currentSketch is Polyline polyline)
    {
      // Examine the current (last) vertex in the line sketch
      var lastSketchPoint = polyline.Points.Last();
      // do something with the last point
    }
  }
});