C# 13 will allow interfaces on ref struct. Until now, without this possibility ref struct missed out on abstraction. For example, while Span<T> acts like a sequential list, it cannot be used with methods that accept IReadOnlyList<T> or a IEnumerable<T>. This requires separate methods for Span<T> with nearly identical implementations. Allowing ref struct to implement interfaces will enable consistent abstraction and operation reuse across different types.
A Word on ref struct
C# 7.2 introduced ref struct to get the benefit of pointers in a safe context and to prevent memory allocation in many common situations.
A ref struct is a structure that can only exist on the thread stack. To enforce this, the C# compiler imposes several constraints, such as preventing the boxing of a ref struct instance and ensuring that a field of a non ref struct type cannot be typed with a ref struct.
ref structs are designed to enhance performance. The most well-known ref struct is Span<T>. Internally, Span<T> holds a managed pointer that can only exist on the thread stack. A managed pointer is a special .NET runtime feature that has been available since .NET 1.0. Before the introduction of Span<T>, managed pointers were only used for parameter passing by reference through the ref and out keywords.
Managed pointers can point to various types of memory, such as a character within a string, an item in an array, a field within an object or even unmanaged memory. They are tracked by the Garbage Collector and are faster than regular object references due to their stack-only nature. These advantages motivated Microsoft to create Span<T> and ref struct to leverage the power of managed pointers.
More explanations in details in this article: Managed pointers, Span, ref struct, C#11 ref fields and the scoped keyword
Wait, interface means boxing?
Usually, interface on structure leads to boxing. This program…
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MyStruct val = new(); IDisposable d = val; struct MyStruct : IDisposable { public void Dispose() { } } |
…compiles to this IL with the box instruction:
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IL_0000: ldloca.s 0 IL_0002: initobj MyStruct IL_0008: ldloc.0 IL_0009: box MyStruct IL_000e: stloc.1 IL_000f: ret |
Boxing a value on the heap is incompatible with a ref struct that must reside on the thread stack. However, there is an exception where using an interface on a structure does not result in boxing: within a generic parameter context. The following program avoids any boxing:
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MyStruct val = new(); DoDispose(val); static void DoDispose<T>(T obj) where T : IDisposable { obj.Dispose(); } struct MyStruct : IDisposable { public void Dispose() { } } |
The anti-constraint ‘allows ref struct’ on generic type
Until C# 13, it was not possible to use a ref struct as a generic type parameter:
This is why C# 13 introduces the anti-constraint allows ref struct on generic type. In C# 13 this program compiles:
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MyStruct val = new(); Method(val); static void Method<T>(T obj) where T: allows ref struct { // use obj } ref struct MyStruct { } |
allows ref struct is the first C# anti-constraint on a generic type: instead of imposing a restriction, it allows a broader range of types to be used as generic parameters.
Putting it all together
In the current .NET 9 preview 6 version, Span<T> and ReadOnlySpan<T> do not implement any interface. This is likely due to potential breaking changes with their stack-only nature being incompatible with the enumerator pattern. Indeed, this pattern requires an IEnumerator<T> object on the heap at some point. However, to implement an abstract algorithm that can work with both ReadOnlySpan<T> and char[], we can still create a wrapper ref struct that implements IReadOnlyList<T> without an enumerator. Here is the code:
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using System.Collections; const string str = "Hello .NET 9 and C# 13"; ReadOnlySpan<char> span = str; // GetCustomHash() can be invoked on various types that implement IReadOnlyList<char> // including a ref struct with no heap allocation required int h1 = GetCustomHash(new ReadOnlySpanWrapper<char>(span)); char[] array = str.ToCharArray(); int h2 = GetCustomHash(array); Assert.IsTrue(h1 == h2); // Algorithm that can work with an abstracted span static int GetCustomHash<T>(T list) where T : IReadOnlyList<char>, allows ref struct { int hash = 0; int count = list.Count; for(int i = 0; i < count; i++) { // http://www.cse.yorku.ca/~oz/hash.html (hash * 33) + c // the magic of number 33 (why it works better than many other constants, prime or not) // has never been adequately explained. hash += (hash << 5) + list[i]; } return hash; } // ref struct wrapper ref struct ReadOnlySpanWrapper<T> : IReadOnlyList<T> { readonly ReadOnlySpan<T> _span; internal ReadOnlySpanWrapper(ReadOnlySpan<T> span) { _span = span; } public T this[int index] => _span[index]; public int Count => _span.Length; // No way to box a ref struct into an interface public IEnumerator<T> GetEnumerator() { throw new NotImplementedException();} IEnumerator IEnumerable.GetEnumerator() { throw new NotImplementedException(); } } static class Assert { public static void IsTrue(bool b) { System.Diagnostics.Debug.Assert(b); } } |
Conclusion
With ref struct interfaces, Microsoft provides a new tool to enhance the performance of C# programs with no design sacrifice thanks to abstraction. Since 2017, there has been ongoing discussion about the possibility of ref struct implementing interfaces. There, some developers exposed real-world examples where this language feature could shine.
In our last example, we had to use a wrapper to use a span as an IReadOnlyList<char>. I wonder if anything will be done to prevent this glitch.