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fixed Statement (C# Reference)

The fixed statement prevents the garbage collector from relocating a movable variable. The fixed statement is only permitted in an unsafe context. Fixed can also be used to create fixed size buffers.

The fixed statement sets a pointer to a managed variable and "pins" that variable during the execution of the statement. Without fixed, pointers to movable managed variables would be of little use since garbage collection could relocate the variables unpredictably. The C# compiler only lets you assign a pointer to a managed variable in a fixed statement.

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;
    }        

}

You can initialize a pointer by using an array, a string, a fixed-size buffer, or the address of a variable. The following example illustrates the use of variable addresses, arrays, and strings. For more information about fixed-size buffers, see Fixed Size Buffers (C# Programming Guide).

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.
        }
    }
}

You can initialize multiple pointers, as long as they are all of the same type.

fixed (byte* ps = srcarray, pd = dstarray) {...}

To initialize pointers of different types, simply nest fixed statements, as shown in the following example.

fixed (int* p1 = &point.x)
{
    fixed (double* p2 = &arr[5])
    {
        // Do something with p1 and p2.
    }
}

After the code in the statement is executed, any pinned variables are unpinned and subject to garbage collection. Therefore, do not point to those variables outside the fixed statement.

NoteNote

Pointers initialized in fixed statements cannot be modified.

In unsafe mode, you can allocate memory on the stack, where it is not subject to garbage collection and therefore does not need to be pinned. For more information, see 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
     */

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

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