Ref Structs vs Traditional Structs in .NET: The Performance Differences

In the world of .NET development, structs have long been a fundamental part of the language. However, with the introduction of C# 7.2, a new player entered the game: ref structs. This editions delves into the intricacies of ref structs, exploring how they differ from traditional structs and when you might want to use them. We’ll cover the basics, dive into the details, and provide code snippets to illustrate key concepts.

Before we dive into ref structs, let’s quickly recap what traditional structs are in .NET. Structs are value types that can contain data members and methods. They are typically used for small, immutable data structures and are stored on the stack rather than the heap.

Here’s a simple example of a traditional struct:

public struct Point
{
    public int X { get; set; }
    public int Y { get; set; }

    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }
}

Enter Ref Structs Ref structs, introduced in C# 7.2, are a special kind of struct designed to enable the creation of high-performance data structures. They are particularly useful when working with large amounts of data or when you need to avoid heap allocations.

Here’s a basic example of a ref struct:

public ref struct LargeBuffer
{
    public Span<byte> Buffer;

    public LargeBuffer(int size)
    {
        Buffer = new Span<byte>(new byte[size]);
    }
}

Key Differences

1. Storage Location: Traditional structs can be stored on both the stack and the heap, while ref structs are always stack-allocated.
2. Boxability: Unlike traditional structs, ref structs cannot be boxed. This means they can’t be converted to object or any interface type.
3. Usage Restrictions: Ref structs have several usage restrictions that don’t apply to traditional structs. For example, they can’t be used as field types in classes or traditional structs.
4. Ref Returns: Ref structs can be returned by reference, which is not possible with traditional structs.
5. Span and ReadOnlySpan: These are the most common examples of ref structs in the .NET framework.

Performance Implications

The primary advantage of ref structs is performance. By ensuring that these types are always stack-allocated and never boxed, they can significantly reduce the pressure on the garbage collector and improve overall application performance.

Use Cases

Ref structs are particularly useful in scenarios where you need to:

1. Work with large amounts of data efficiently.
2. Avoid heap allocations in performance-critical code paths.
3. Implement low-level algorithms or data structures.
4. Interop with unmanaged memory.

Limitations of Ref Structs

While ref structs offer performance benefits, they come with several limitations:

1. Cannot be used as generic type arguments.
2. Cannot be used in async methods or lambda expressions.
3. Cannot be used in iterator methods or async iterator methods.
4. Cannot be used as a field in a class or normal struct.
5. Cannot implement interfaces.

Code Examples

Let’s look at some code examples to illustrate the differences and use cases:

Traditional Struct Example

public struct Rectangle
{
    public int Width { get; set; }
    public int Height { get; set; }

    public Rectangle(int width, int height)
    {
        Width = width;
        Height = height;
    }

    public int Area() => Width * Height;
}

// Usage
Rectangle rect = new Rectangle(5, 10);
Console.WriteLine($"Area: {rect.Area()}");

Ref Struct Example

using System;

public ref struct MemoryBuffer
{
    private Span<byte> _buffer;

    public MemoryBuffer(int size)
    {
        _buffer = new Span<byte>(new byte[size]);
    }

    public void SetValue(int index, byte value)
    {
        if (index < 0 || index >= _buffer.Length)
            throw new ArgumentOutOfRangeException(nameof(index));

        _buffer[index] = value;
    }

    public byte GetValue(int index)
    {
        if (index < 0 || index >= _buffer.Length)
            throw new ArgumentOutOfRangeException(nameof(index));

        return _buffer[index];
    }

    public int Length => _buffer.Length;
}

// Usage
public static void Main()
{
    MemoryBuffer buffer = new MemoryBuffer(1024);
    buffer.SetValue(0, 42);
    Console.WriteLine($"Value at index 0: {buffer.GetValue(0)}");
}

In this example, MemoryBuffer is a ref struct that wraps a Span. It provides methods to set and get values in the buffer. Because it’s a ref struct, it’s always stack-allocated and can be used very efficiently in performance-critical scenarios.

Conclusion

Ref structs are a powerful feature in .NET that allow developers to create high-performance, stack-only types. While they come with certain limitations, they offer significant benefits in scenarios where performance is critical and heap allocations need to be minimized.

When deciding between traditional structs and ref structs, consider your specific use case. If you’re working on performance-critical code that deals with large amounts of data or requires low-level memory management, ref structs might be the right choice. However, for general-purpose small data structures, traditional structs remain a versatile and practical option.

As with any performance optimization, it’s important to measure and profile your code to ensure that using ref structs actually provides the expected benefits in your specific scenario. 💡