fixed 语句(C# 参考)
fixed 语句禁止垃圾回收器重定位可移动的变量。fixed 语句只在不安全的上下文中是允许的。Fixed 还可用于创建固定大小缓冲区。
fixed 语句设置指向托管变量的指针,并在执行该语句期间“固定”此变量。如果没有 fixed 语句,则指向可移动托管变量的指针的作用很小,因为垃圾回收可能不可预知地重定位变量。C# 编译器只允许在 fixed 语句中分配指向托管变量的指针。
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;
}
}
可通过使用数组、字符串、大小固定的缓冲区或变量地址初始化指针。以下示例演示可变地址、数组和字符串的用法。关于固定大小缓冲器的更多信息,请参见 固定大小的缓冲区(C# 编程指南)。
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.
}
}
}
只要指针的类型相同,就可以初始化多个指针。
fixed (byte* ps = srcarray, pd = dstarray) {...}
要初始化不同类型的指针,只需嵌套 fixed 语句,如下面的示例所示。
fixed (int* p1 = &point.x)
{
fixed (double* p2 = &arr[5])
{
// Do something with p1 and p2.
}
}
执行完语句中的代码后,任何固定变量都被解除固定并受垃圾回收的制约。因此,不要指向 fixed 语句之外的那些变量。
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无法修改在 fixed 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。堆栈不受垃圾回收的制约,因此不需要被锁定。有关更多信息,请参见 stackalloc。
示例
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
*/
C# 语言规范
有关更多信息,请参见 C# 语言规范。该语言规范是 C# 语法和用法的权威资料。