Generic type parameters could not accept ref struct types like Span<T> or ReadOnlySpan<T>. This meant high-performance code using spans could not participate in generic abstractions such as IEnumerable<T> or IReadOnlyList<T>.

C# 13 introduces the allows ref struct anti-constraint, enabling generic type parameters to accept ref struct types while the compiler enforces the necessary lifetime restrictions.

Code

Now

static T Sum<T>(params ReadOnlySpan<T> values) where T : INumber<T>, allows ref struct
{
    T result = T.Zero;
    foreach (var value in values)
        result += value;
    return result;
}

// Works with Span<T> for zero-allocation scenarios
var total = Sum(1, 2, 3, 4, 5);

Before

// Could not pass Span<T> to generic methods
// Had to write non-generic overloads or use arrays instead
static T Sum<T>(params T[] values) where T : INumber<T>
{
    T result = T.Zero;
    foreach (var value in values)
        result += value;
    return result;
}

Notes

• allows ref struct is an anti-constraint that relaxes the default restriction rather than adding one
• The compiler ensures ref struct values are not used in ways that would violate their lifetime rules (e.g. boxing, storing in fields of non-ref struct types)
• Enables the BCL to offer Span<T>-based overloads in generic APIs

More information

• What's new in C# 13.0
• Specification