What is Memory Pinning and the fixed statement, and how do they affect the Garbage Collector?

Short answer

Pinning prevents the GC from moving an object at a specific RAM address while passing it to unmanaged (C/C++) code; it can hinder memory compaction and cause severe Heap fragmentation.

Explanation

The GC normally compacts memory and moves objects to create contiguous free space. If calling a native C function through P/Invoke that expects a fixed address, fixed pins the object and prevents the GC from moving it while the block runs. Excessive pinning of Gen 0/1 objects interferes with GC compaction and creates memory fragmentation.

Code example

unsafe {
    byte[] buffer = new byte[100];
    fixed (byte* p = buffer) {
        // The pointer p is fixed here and cannot be moved by the GC
        NativeMethods.FillBuffer(p, 100);
    }
}

Common mistakes

• Pinning objects for very long periods, preventing Heap compaction and causing Memory Fragmentation.

Follow-up questions

• How does the .NET Garbage Collector work, and what are the three generations (Gen 0, Gen 1, Gen 2)?
• What is the Large Object Heap (LOH) in .NET, and why is it prone to Memory Fragmentation?

Sources

• Microsoft Learn — fixed statement (C# reference)