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：
代码
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。
