Fixed Keyword in C#

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In C#, the fixed keyword is used in conjunction with pointers to declare a pointer variable and "pin" it to a memory location, preventing the garbage collector from moving the object it points to. The fixed keyword is typically used in unsafe code blocks and is useful when working with unmanaged memory, interoperating with native code, or when direct memory manipulation is required for performance reasons.

Usage of the fixed keyword: 

unsafe
{
    int[] numbers = { 1, 2, 3, 4, 5 };

    fixed (int* ptr = numbers)
    {
        // 'ptr' is now pinned to the memory location of 'numbers'.
        // You can work with 'ptr' and perform direct memory manipulation here.
    }
}

In this example, we use the fixed keyword to declare a pointer variable ptr and "pin" it to the memory location of the numbers array. This ensures that the memory location of the array remains fixed during the block of code enclosed by the unsafe context. It is important to use the unsafe keyword because working with pointers is inherently unsafe and requires special permissions.

Points to note about the fixed keyword:

  1. The fixed keyword can only be used in unsafe code blocks.

  2. It is essential to be extremely cautious when using the fixed keyword and pointers, as incorrect usage can lead to memory leaks, undefined behavior, or program crashes.

  3. The fixed keyword is mainly used when working with unmanaged code or when interacting with native APIs that require pointers to fixed memory locations.

  4. In managed code, you generally don't need to use the fixed keyword, as the .NET garbage collector handles memory management automatically for most situations.

Because the fixed keyword involves low-level memory management, it is typically used in advanced scenarios or when optimizing performance in very specific situations. In most cases, managed C# code with safe and high-level abstractions is preferred for better safety and maintainability.

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