Custom selection extension
SelectByLineTool.cs
// Copyright 2010 ESRI
// 
// All rights reserved under the copyright laws of the United States
// and applicable international laws, treaties, and conventions.
// 
// You may freely redistribute and use this sample code, with or
// without modification, provided you include the original copyright
// notice and use restrictions.
// 
// See the use restrictions.
// 

using System;
using System.Collections.Generic;
using System.Text;
using ESRI.ArcGIS.ArcMapUI;
using ESRI.ArcGIS.Carto;
using ESRI.ArcGIS.Geometry;
using ESRI.ArcGIS.esriSystem;

namespace SelectionSample
{
  public class SelectByLineTool : ESRI.ArcGIS.Desktop.AddIns.Tool
  {
    private bool m_isMouseDown = false;
    private ESRI.ArcGIS.Display.INewLineFeedback m_lineFeedback;
    private IActiveView m_focusMap;

    public SelectByLineTool()
    {

    }

    protected override void OnMouseDown(MouseEventArgs arg)
    {
      IMxDocument mxDoc = ArcMap.Document;
      m_focusMap = mxDoc.FocusMap as IActiveView;
      IPoint point = m_focusMap.ScreenDisplay.DisplayTransformation.ToMapPoint(arg.X, arg.Y) as IPoint;

      if (m_lineFeedback == null)
      {
        m_lineFeedback = new ESRI.ArcGIS.Display.NewLineFeedback();
        m_lineFeedback.Display = m_focusMap.ScreenDisplay;
        m_lineFeedback.Start(point);
      }
      else
      {
        m_lineFeedback.AddPoint(point);
      }

      m_isMouseDown = true;
    }

    protected override void OnDoubleClick()
    {
      m_focusMap.PartialRefresh(esriViewDrawPhase.esriViewGeoSelection, null, null);

      IPolyline polyline;

      if (m_lineFeedback != null)
      {
        polyline = m_lineFeedback.Stop();
        if (polyline != null)
          ArcMap.Document.FocusMap.SelectByShape(polyline, null, false);
      }


      m_focusMap.PartialRefresh(esriViewDrawPhase.esriViewGeoSelection, null, null);

      m_lineFeedback = null;
      m_isMouseDown = false;
    }

    protected override void OnMouseMove(MouseEventArgs arg)
    {
      if (!m_isMouseDown) return;

      IPoint point = m_focusMap.ScreenDisplay.DisplayTransformation.ToMapPoint(arg.X, arg.Y) as IPoint;
      m_lineFeedback.MoveTo(point);
    }

    protected override void OnUpdate()
    {
      if (!SelectionExtension.IsExtensionEnabled())
      {
        this.Enabled = false;
        return;
      }

      this.Enabled = SelectionExtension.HasSelectableLayer();
    }
  }
}