class="htmledit_views">

以c#为例。

一，打开unsafe编程开关

   工程中默认的配置是不支持代码的非安全性的，所以第一次使用你得：

      解决方案管理器→→Properties→→生成选项卡→→允许不安全代码。

 

二，何时要用unsafe

1.一个原则

   unsafe  关键字表示不安全上下文，该上下文是任何涉及指针的操作所必需的。

2.什么时候用指针

    msdn里说 “在 C# 中很少需要使用指针，但仍有一些需要使用的情况。例如，在下列情况中使用允许采用指针的不安全上下文是正确的： 处理磁盘上的现有结构， 涉及内部包含指针的结构的高级COM或平台调用方案， 性能关键代码”

• Dealing with existing class="tags" href="/tags/STRUCT.html" title=struct>structures on disk
• Advanced COM or Platform Invoke scenarios that involve class="tags" href="/tags/STRUCT.html" title=struct>structures with class="tags" href="/tags/POINTERS.html" title=pointers>pointers in them
• Performance-critical code

 

   总之，一条判断标准：当你使用C/C++能给带给你的利大于弊时都可以使用unsafe编程。

三，unsafe的使用方法（几个例子）

1.unsafe一个方法体

class</a>="c-sharp:nogutter:collapse"> 1. <a class="tags" href="/tags/CLASS.html" title=class>class</a> UnsafeTest1 2. { 3. unsafe static void AddParam(int *a) 4. { 5. *a = *a 10000; 6. } . 7. unsafe public static void Main() 8. { 9. int i = 10000; 10. AddParam (&i); 11. Console.WriteLine(i); 12. } 13. } 

 

2.unsafe一段代码

 

class</a>="c-sharp:collapse"> private void button1_Click(object sender, EventArgs e) { unsafe { int *pInt; } }

 

3.网上找的一个较大unsafe例子

class</a>="c-sharp:nogutter:collapse">using System; using System.Collections.Generic; using System.Text; namespace UnsafeCode { #region Unsafe types and members (just for testing purposes) // This entire <a <a class="tags" href="/tags/CLASS.html" title=class>class</a>="tags" href="/tags/STRUCT.html" title=struct>struct</a>ure is 'unsafe' and can // be used only in an unsafe context. public unsafe <a <a class="tags" href="/tags/CLASS.html" title=class>class</a>="tags" href="/tags/STRUCT.html" title=struct>struct</a> Node { public int Value; public Node* Left; public Node* Right; } // This <a <a class="tags" href="/tags/CLASS.html" title=class>class</a>="tags" href="/tags/STRUCT.html" title=struct>struct</a> is safe, but the Node* members // are not. Technically, you may <a <a class="tags" href="/tags/CLASS.html" title=class>class</a>="tags" href="/tags/ACCESS.html" title=access>access</a> 'Value' from // outside an unsafe context, but not 'Left' and 'Right'. public <a <a class="tags" href="/tags/CLASS.html" title=class>class</a>="tags" href="/tags/STRUCT.html" title=struct>struct</a> Node2 { public int Value; // These can be <a <a class="tags" href="/tags/CLASS.html" title=class>class</a>="tags" href="/tags/ACCESS.html" title=access>access</a>ed only in an unsafe context! public unsafe Node2* Left; public unsafe Node2* Right; } #endregion #region Simple Point / PointRef <a <a class="tags" href="/tags/CLASS.html" title=class>class</a>="tags" href="/tags/STRUCT.html" title=struct>struct</a> Point { public int x; public int y; public override <a class="tags" href="/tags/STRING.html" title=string>string</a> ToString() { return <a class="tags" href="/tags/STRING.html" title=string>string</a>.Format("({0}, {1})", x, y); } } <a class="tags" href="/tags/CLASS.html" title=class>class</a> PointRef { public int x; public int y; public override <a class="tags" href="/tags/STRING.html" title=string>string</a> ToString() { return <a class="tags" href="/tags/STRING.html" title=string>string</a>.Format("({0}, {1})", x, y); } } #endregion <a class="tags" href="/tags/CLASS.html" title=class>class</a> Program { static void Main(<a class="tags" href="/tags/STRING.html" title=string>string</a>[] args) { Console.WriteLine("***** Fun with Pointers *****"); #region Swap 2 ints safely and unsafely // Values for swap. int i = 10, j = 20; // Swap values 'safely'. Console.WriteLine("/n***** Safe swap *****"); Console.WriteLine("Values before safe swap: i = {0}, j = {1}", i, j); SafeSwap(ref i, ref j); Console.WriteLine("Values after safe swap: i = {0}, j = {1}", i, j); // Swap values 'unsafely'. Console.WriteLine("/n***** Unsafe swap *****"); Console.WriteLine("Values before unsafe swap: i = {0}, j = {1}", i, j); unsafe { UnsafeSwap(&i, &j); } Console.WriteLine("Values after unsafe swap: i = {0}, j = {1}", i, j); Console.WriteLine(); #endregion UsePointerToPoint(); Console.WriteLine(); UseSizeOfOperator(); Console.ReadLine(); } public static void SafeSwap(ref int i, ref int j) { int temp = i; i = j; j = temp; } #region Various unsafe methods unsafe static void PrintValueAndAddress() { int myInt; // Define an int pointer, and // assign it the address of myInt. int* ptrToMyInt = &myInt; // Assign value of myInt using pointer indirection. *ptrToMyInt = 123; // Print some stats. Console.WriteLine("Value of myInt {0}", myInt); Console.WriteLine("Address of myInt {0:X}", (int)&ptrToMyInt); } unsafe static void SquareIntPointer(int* myIntPointer) { // Work with pointer types here! // Square the value just for a test. *myIntPointer *= *myIntPointer; } unsafe public static void UnsafeSwap(int* i, int* j) { int temp = *i; *i = *j; *j = temp; } unsafe static void UsePointerToPoint() { // Access members via pointer. Point point; Point* p = &point; p->x = 100; p->y = 200; Console.WriteLine(p->ToString()); // Access members via pointer indirection. Point point2; Point* p2 = &point2; (*p2).x = 100; (*p2).y = 200; Console.WriteLine((*p2).ToString()); } unsafe static void UnsafeStackAlloc() { char* p = stackalloc char[256]; for (int k = 0; k < 256; k++) p[k] = (char)k; } unsafe public static void UseAndPinPoint() { PointRef pt = new PointRef(); pt.x = 5; pt.y = 6; // pin pt in place so it will not // be moved or GC-ed. fixed (int* p = &pt.x) { // Use int* variable here! } // pt is now unpinned, and ready to be GC-ed. Console.WriteLine("Point is: {0}", pt); } unsafe static void UseSizeOfOperator() { Console.WriteLine("The size of short is {0}.", sizeof(short)); Console.WriteLine("The size of int is {0}.", sizeof(int)); Console.WriteLine("The size of long is {0}.", sizeof(long)); Console.WriteLine("The size of Point is {0}.", sizeof(Point)); } #endregion } }

四，使用unsafe时可能用到的fixed

   1.为什么要用fixed

       Unsafe的代码托管代码 (managed code)和非托管代码（Unmanaged Code）之间，它也是在CLR的环境中执行，但是可以用来直接操作内存。但由于代码是在CLR下托管执行，为了减少内存碎片C#的自动垃圾回收机制会允许已经分配的内存在运行时进行位置调整，所以如果我们多次调用的话就可能 导致指针指向其他的变量。比如*pInt为指向一个变量的地址为1001，CLR在重新内存整理分配后该变量就存储在地址为5001的地方。而原来1001的地方可能会被分配其他变量，要解决这个问题我们就需要使用Fixed关键字。

   2.使用fixed的例子

      fixed 语句禁止垃圾回收器重定位可移动的变量。fixed 语句只能出现在不安全的上下文中。Fixed 还可用于创建固定大小的缓冲区。

class</a>="c-sharp:collapse">using System; <a class="tags" href="/tags/CLASS.html" title=class>class</a> CaryData { public int data; } <a class="tags" href="/tags/CLASS.html" title=class>class</a> CProgram { unsafe static void ChangeValue(int* pInt) { *pInt = 23; } public unsafe static void Main() { CaryData cd = new CaryData(); Console.WriteLine("改变前: {0}", cd.data); fixed (int* p = &cd.data) { ChangeValue(p); } Console.WriteLine("改变后: {0}", cd.data); } }

五，参考

1.http://msdn.microsoft.com/zh-cn/library/chfa2zb8.aspx

2.http://msdn.microsoft.com/zh-cn/library/f58wzh21.aspx

3.http://msdn.microsoft.com/en-us/library/t2yzs44b%28v=VS.80%29.aspx

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