C# ArcGIS桌面加载项程序无法获取AxMapControl对象

我有一个由ArcGIS engine SDK版本10.1支持的.NET程序类型，其目标是按圆圈选择层。IDE是Microsoft Visual Studio 2010。该程序可以编译，但运行时出错

一些代码如下所示：

using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
using ESRI.ArcGIS.ArcMapUI;
using ESRI.ArcGIS.Controls;
using ESRI.ArcGIS.Carto;
using ESRI.ArcGIS.Geometry;
using ESRI.ArcGIS.Display;
namespace SelectByCircle
{
    public class SelectByCircle : ESRI.ArcGIS.Desktop.AddIns.Tool
    {
        public SelectByCircle()
        {
        }
        protected override void OnUpdate()
        {
            Enabled = ArcMap.Application != null;
        }
        protected override void OnMouseDown(ESRI.ArcGIS.Desktop.AddIns.Tool.MouseEventArgs arg)
        {
            IHookHelper pHookHelper = new HookHelperClass();
            pHookHelper.Hook = this.Hook;
            IntPtr pHandle = new IntPtr(pHookHelper.ActiveView.ScreenDisplay.hWnd);      

            //axMapControl=null when debuging
            AxMapControl axMapControl = System.Windows.Forms.Form.FromHandle(pHandle) as AxMapControl;

            IMapControlDefault pMapControl = axMapControl.Object as IMapControlDefault;
            pMapControl.Map.ClearSelection();
            IGeometry pGeo;
            pGeo = pMapControl.TrackCircle();
            pMapControl.Map.SelectByShape(pGeo, null, false);
            pMapControl.Refresh(esriViewDrawPhase.esriViewGeoSelection, null, null);
        }
    }
}

通过调试，我发现程序运行时axMapControl为null。怎么了

我曾经使用ArcGIS引擎WinForm类型程序成功地从this.Hook中获得AxMapControl对象。那么有什么区别呢

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