Fixed的说明
- Fixed 语句可防止垃圾回收器重新定位可移动的变量。
- Fixed 语句仅允许存在于不安全的上下文中。
- Fixed 可用于创建固定大小的缓冲区。
unsafe static void TestMethod() { // Assume that the following class exists. //class Point //{ // public int x; // public int y; //} // Variable pt is a managed variable, subject to garbage collection. Point pt = new Point(); // Using fixed allows the address of pt members to be taken, // and "pins" pt so that it is not relocated. fixed (int* p = &pt.x) { *p = 1; } }
可以通过使用一个数组、字符串、固定大小的缓冲区或变量的地址来初始化指针。 下面的示例说明了变量地址、数组和字符串的使用。 有关固定大小的缓冲区的详细信息,请参阅固定大小的缓冲区。
static unsafe void Test2() { Point point = new Point(); double[] arr = { 0, 1.5, 2.3, 3.4, 4.0, 5.9 }; string str = "Hello World"; // The following two assignments are equivalent. Each assigns the address // of the first element in array arr to pointer p. // You can initialize a pointer by using an array. fixed (double* p = arr) { /*...*/ } // You can initialize a pointer by using the address of a variable. fixed (double* p = &arr[0]) { /*...*/ } // The following assignment initializes p by using a string. fixed (char* p = str) { /*...*/ } // The following assignment is not valid, because str[0] is a char, // which is a value, not a variable. //fixed (char* p = &str[0]) { /*...*/ } // You can initialize a pointer by using the address of a variable, such // as point.x or arr[5]. fixed (int* p1 = &point.x) { fixed (double* p2 = &arr[5]) { // Do something with p1 and p2. } } }
可以初始化多个指针,只要它们的类型相同。
fixed (byte* ps = srcarray, pd = dstarray) {...}
若要初始化不同类型的指针,只需嵌套 fixed 语句,如下面的示例中所示。
fixed (int* p1 = &point.x) { fixed (double* p2 = &arr[5]) { // Do something with p1 and p2. } }
执行该语句中的代码之后,任何固定的变量都将被解锁并受垃圾回收的约束。 因此,请勿指向 fixed 语句之外的那些变量。
注:不能修改在固定语句中初始化的指针。
在不安全模式中,可以在堆栈上分配内存,在这种情况下,内存不受垃圾回收的约束,因此不需要固定。 有关详细信息,请参阅 stackalloc。
class Point { public int x, y; } class FixedTest2 { // Unsafe method: takes a pointer to an int. unsafe static void SquarePtrParam (int* p) { *p *= *p; } unsafe static void Main() { Point pt = new Point(); pt.x = 5; pt.y = 6; // Pin pt in place: fixed (int* p = &pt.x) { SquarePtrParam (p); } // pt now unpinned. Console.WriteLine ("{0} {1}", pt.x, pt.y); } } /* Output: 25 6 */参考地址: https://docs.microsoft.com/zh-cn/dotnet/csharp/language-reference/keywords/fixed-statement