Ref Structs, Stackalloc, and Low Level Memory Tricks in High Performance C#

In modern high performance .NET applications, memory management and data locality significantly influence application performance and efficiency. Managed memory in .NET offers convenience but often comes at the cost of garbage collection (GC) overhead, which can introduce latency, especially in high throughput, real time, or latency sensitive applications. Reducing this GC pressure is essential for systems requiring deterministic performance, such as financial trading systems, real time telemetry, or high frequency data processing. We rely on low level memory management strategies to ensure predictable, high speed execution, minimising interruptions caused by GC cycles.

Ref structs (ref struct) provide an effective solution for scenarios where explicit control over memory allocation is critical. Unlike regular classes or structs, ref structs are exclusively stack allocated, meaning they never contribute to heap allocations. This guarantees that they do not trigger garbage collection processes, significantly enhancing runtime predictability and efficiency. Additionally, ref structs cannot be captured by asynchronous methods or lambdas, nor can they be boxed. These restrictions ensure that ref structs remain fully optimised for stack based execution, maintaining a clear separation from heap managed objects and preventing unintended allocations. Another essential tool for memory optimisation is stack allocation using the stackalloc keyword. Stack allocation allows us to explicitly allocate temporary memory directly on the stack, avoiding heap allocations entirely. This is particularly beneficial for performance critical methods, especially those involving short lived buffers or frequently accessed temporary data. By using stackalloc, applications can significantly reduce heap allocations, resulting in fewer garbage collections, lower latency, and higher overall throughput. This stack based memory allocation method is highly suitable for scenarios where memory lifetime is short, predictable, and strictly scoped within a single method execution.

Understanding Ref Structs

Ref structs (ref struct) are types allocated only on the stack. They cannot be boxed or captured by lambdas and asynchronous methods, making them excellent for ensuring high performance stack allocations without GC involvement.

Here's an example of defining a ref struct:

ref struct SpanBasedBuffer
{
    private Span<byte> _buffer;

    public SpanBasedBuffer(Span<byte> buffer)
    {
        _buffer = buffer;
    }

    public void Fill(byte value)
    {
        for (int i = 0; i < _buffer.Length; i++)
        {
            _buffer[i] = value;
        }
    }
}

Ref structs cannot escape their defining context, ensuring safe stack usage.

Stackalloc: High Speed Stack Memory

The stackalloc keyword lets you allocate memory directly on the stack. It's particularly useful in high frequency methods.

public void ProcessData()
{
    Span<int> stackAllocatedSpan = stackalloc int[256];
    for (int i = 0; i < stackAllocatedSpan.Length; i++)
    {
        stackAllocatedSpan[i] = i * i;
    }

    // Use stackAllocatedSpan efficiently without heap allocations.
}

Since stack memory allocation is very fast and avoids GC overhead, stackalloc is ideal for short lived, performance critical data.

Combining Ref Structs with Stackalloc

You can combine ref structs and stack allocations to leverage efficient, GC free memory operations:

ref struct FixedSizeBuffer
{
    private Span<byte> _buffer;

    public FixedSizeBuffer(int size)
    {
        _buffer = stackalloc byte[size];
    }

    public Span<byte> Buffer => _buffer;
}

public void UseFixedSizeBuffer()
{
    var buffer = new FixedSizeBuffer(128);
    buffer.Buffer.Fill(0xFF);
    // Perform operations without heap allocation
}

Low Level Memory Tricks with Unsafe Operations

Using unsafe blocks provides direct memory access and pointer arithmetic, enabling fine grained performance optimisations.

public unsafe void MemoryCopy(byte[] source, byte[] destination, int length)
{
    fixed (byte* srcPtr = source, destPtr = destination)
    {
        Buffer.MemoryCopy(srcPtr, destPtr, destination.Length, length);
    }
}

Parsing Binary Protocols

Efficiently parse binary protocols:

public void ParseBinaryMessage(ReadOnlySpan<byte> data)
{
    ref readonly byte header = ref data[0];
    int messageLength = BitConverter.ToInt32(data.Slice(1, 4));

    // process remaining data
    ReadOnlySpan<byte> messageBody = data.Slice(5, messageLength);
}

Using Span and Memory

Optimise data processing with Span<T> and Memory<T>:

public void CopyData(Memory<byte> source, Memory<byte> destination)
{
    source.Span.CopyTo(destination.Span);
}

Unsafe Code for Complex Structures

Directly manipulate struct memory layout:

public unsafe struct Vector3
{
    public float X;
    public float Y;
    public float Z;
}

public unsafe void ZeroVector(ref Vector3 vector)
{
    fixed (float* ptr = &vector.X)
    {
        for (int i = 0; i < 3; i++)
        {
            ptr[i] = 0;
        }
    }
}

Performance Considerations

While these techniques significantly improve performance, they come with limitations and risks:

• Ref structs cannot be heap allocated.

• Stack memory is limited and unsuitable for large allocations.

• Unsafe operations bypass type safety, potentially introducing errors.

Ref structs, stackalloc, and low level memory manipulation gives us the ability to achieve performance in high frequency, latency sensitive scenarios. Mastering these tools allows us to build applications that scale efficiently, dramatically reducing heap allocations and garbage collection overhead. Using these techniques strategically in critical parts of an application can provide substantial performance gains, particularly in scenarios where traditional managed memory models create bottlenecks. They offer us granular control over memory management, significantly enhancing application responsiveness and reliability. We must remember though, it is crucial to maintain a disciplined approach when employing low level memory manipulation, thoroughly testing and profiling the application to avoid unintended side effects.

By integrating ref structs, stackalloc, and unsafe memory operations, you can optimise applications for environments where predictable, low latency execution is essential. Adopting these practices ensures applications remain performant and competitive, pushing the boundaries of what's achievable with managed code.