fixed 陳述式 (C# 參考)
fixed 陳述式可讓記憶體回收行程避免重新配置可移動的變數。fixed 陳述式只允許在 unsafe 內容中。Fixed 也可用來建立固定大小緩衝區。
fixed 陳述式會設定一個指標至 Managed 變數，並在陳述式執行期間將此變數固定住。如果沒有 fixed，Managed 變數的指標也許沒有多大用處，因為記憶體回收可能會在非預期下重新配置變數。C# 編譯器只能讓您將指標指派到 fixed 陳述式中的 Managed 變數。
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;
}
}
您可以使用陣列、字串、固定大小的緩衝區或變數的位址初始化指標。下列範例說明變數位址、陣列和字串的用法。如需關於固定大小緩衝區的詳細資訊，請參閱固定大小緩衝區 (C# 程式設計手冊).
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.
}
}
}
只要都有相同的型別，您就可以初始化多個指標。
fixed (byte* ps = srcarray, pd = dstarray) {...}
若要初始化不同型別的指標，只需將 fixed 陳述式巢狀化，如下列範例所示。
fixed (int* p1 = &point.x)
{
fixed (double* p2 = &arr[5])
{
// Do something with p1 and p2.
}
}
執行陳述式中的程式碼後，任何已固定的變數將取消固定並且交付記憶體回收。因此請不要指向 fixed 陳述式之外的變數。
.gif "注意事項") 注意事項 |
|---|
不能修改在 fixed 陳述式中初始化的指標。 |
在不安全模式，您可以配置在堆疊上的記憶體；那裡並不需要交付給記憶體回收，因此不需要固定。如需詳細資訊，請參閱 stackalloc。
範例
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# 語言規格
如需詳細資訊，請參閱 C# 語言規格。語言規格是 C# 語法和用法的限定來源。