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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 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.

// assume class Point { public int x, y; }
// 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 it isn't relocated.
fixed ( int* p = &pt.x )
{
    *p = 1; 
}

You can initialize a pointer with the address of an array or a string:


        fixed (int* p = arr) ...  // equivalent to p = &arr[0]
fixed (char* p = str) ... // equivalent to p = &str[0]

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 type, simply nest fixed statements:


        fixed (int* p1 = &p.x)
{
    fixed (double* p2 = &array[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.

// statements_fixed.cs
// compile with: /unsafe
using System;

class Point
{ 
    public int x, y; 
}

class FixedTest 
{
    // 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 following sections in the C# Language Specification:

  • 18.3 Fixed and moveable variables

  • 18.6 The fixed statement

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