C# 13 `allows ref struct` Anti-Constraint

1Concept

Historically, generic types could not accept `ref struct` types like `ReadOnlySpan<T>`. C# 13 introduces `where T : allows ref struct`, enabling high-performance generic libraries to operate over Spans.

2Architecture Diagram

Legacy Generics:
T cannot be Span<T> or ReadOnlySpan<T> (Compile Error)

C# 13 Generics:
where T : allows ref struct ──> T can now be Span<T> with zero stack escape violations!

3Code Example

using System;

public class RefStructGenericDemo
{
    // C# 13 allows ref struct constraint
    public static void ProcessBuffer<T>(T buffer) where T : allows ref struct
    {
        Console.WriteLine($"Processing buffer of type: {typeof(T).Name}");
    }

    public static void Main()
    {
        ReadOnlySpan<char> span = "HighPerformanceDotNet9".AsSpan();
        ProcessBuffer(span);
    }
}

4Expected Output

Processing buffer of type: ReadOnlySpan`1

5Key Takeaways

• ✓`where T : allows ref struct` enables generic abstractions over `Span<T>` and `Memory<T>`.
• ✓The compiler enforces that `T` cannot be boxed or escape the thread stack.
• ✓Major architectural feature in .NET 9 for zero-allocation libraries.