用一个小程序重新认识“&”与“&&”

上午写了个提ＣＡＤ　ＰＬ拐点的程序，很简单，但是遇到提示选择时用ｅｓｃ取消ＣＡＤ就会报错。程序如下：

using System;

using System.Text;

using Autodesk.AutoCAD.ApplicationServices;

using Autodesk.AutoCAD.Runtime;

using Autodesk.AutoCAD.DatabaseServices;

using Autodesk.AutoCAD.EditorInput;

using Autodesk.AutoCAD.Geometry;

using System.IO;

[assembly:CommandClass(typeof(提拐点坐标.Class1))]

/*Copyright © 519 2008

 

AutoCAD version:AutoCAD 2008

Description:

 

To use 提拐点坐标.dll:

 

1. Start AutoCAD and open a new drawing.

2. Type netload and select 提拐点坐标.dll.

3. Execute the Test command.

 

Autodesk references added to this project are acdbmgd.dll,acmgd.dll.*/

 

namespace 提拐点坐标

{

    /// <summary>

    /// Summary for Class1.

    /// </summary>

    public class Class1:IExtensionApplication

    {

        public void Initialize()

        {

            Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument.Editor.WriteMessage("\n调试程序命令GD");//初始化操作

        }

        public void Terminate()

        {

            //清除操作

        }

        Editor ed = Application.DocumentManager.MdiActiveDocument.Editor;

        Database db = Autodesk.AutoCAD.DatabaseServices.HostApplicationServices.WorkingDatabase;

        Autodesk.AutoCAD.DatabaseServices.TransactionManager tm = Autodesk.AutoCAD.DatabaseServices.HostApplicationServices.WorkingDatabase.TransactionManager;

        [CommandMethod("GD")]

        public void Test()

        {

            try

            {

                Transaction trans = tm.StartTransaction();

                using (trans)

                {

                    BlockTable bt = (BlockTable)trans.GetObject(db.BlockTableId, OpenMode.ForRead);

                    BlockTableRecord btr = (BlockTableRecord)trans.GetObject(db.CurrentSpaceId, OpenMode.ForWrite);

                    PromptSelectionOptions ps = new PromptSelectionOptions();

                    ps.MessageForAdding = "选择要提取的多段线";

                    TypedValue[] filter ={ new TypedValue(0, "LWPOLYLINE") };

                     PromptSelectionResult sr = ed.GetSelection(ps, new SelectionFilter(filter));

                    if (sr.Status == PromptStatus.OK && sr.Value.Count != 0)

                    {

                        StreamWriter sw = new StreamWriter(@"c://点坐标.txt");

                        ObjectId[] ss = sr.Value.GetObjectIds();

                        foreach (ObjectId obj in ss)

                        {

                            Polyline pl = (Polyline)trans.GetObject(obj, OpenMode.ForRead);

                            int vn = pl.NumberOfVertices;

                            for (int i=0; i < vn; i++)

                            {

                                Point3d pt3d = pl.GetPoint3dAt(i);

                                sw.WriteLine(pt3d.ToString());

                            }

                        }

                        sw.Close();

                        System.Diagnostics.Process.Start("notepad.exe", @"c://点坐标.txt");

                    }                  

                }

            }

            finally

            { }

        }

    }

}

 

主程序没有问题，程序中能报错的地方只有if (sr.Status == PromptStatus.OK & sr.Value.Count != 0)这句，如果选择被取消sr.Status应该为PromptStatus.Cancel，sr.Status == PromptStatus.OK 这句没有问题，如果有问题就可能出现再sr.Value.Count != 0这句。即如果程序判断出sr.Status == PromptStatus.OK为false仍然要执行
sr.Value.Count != 0这句的话CAD就会报错。

查看MSDN上关于“&”的解释为：& 运算符计算两个运算符，与第一个操作数的值无关。

再查看关于“&&”的解释：

x && y

对应于操作

x & y

不同的是，如果 x 为 false，则不计算 y（因为不论 y 为何值，“与”操作的结果都为 false）。这被称作为“短路”计算。

不能重载条件“与”运算符，但常规逻辑运算符和运算符 true 与 false 的重载，在某些限制条件下也被视为条件逻辑运算符的重载。