Add LDM notes for April 24, 2019
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Andy Gocke
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meetings/2019/LDM-2019-04-24.md
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meetings/2019/LDM-2019-04-24.md
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# C# LDM Notes for April 24, 2019
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## Agenda
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1. MaybeNull and related attributes
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## Discussion
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There are two proposals on the table:
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- https://github.com/cston/csharplang/blob/MaybeNull/proposals/MaybeNull.md
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- https://gist.github.com/MadsTorgersen/e94bc6802b96ce9cc65bc3dd39b7f6a2
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The proposals are primarily focused on providing nullable information for the caller of methods
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where certain nullable contracts are outside the ability of our current annotation syntax to
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express.
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We've also gathered the following information on nullable contracts as they currently
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exist in CoreFX:
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> We’ve annotated most of corelib at this point. I just quickly went through looking at some of the ~600 TODO-NULLABLE follow-up comments we have for examples of public API signatures that are currently less-than-correct and that would benefit from further ability to annotate/attribute. I did not include things that just impact locals or fields or other non-API stuff.
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> Generic methods or method on generic type that might return default(T), e.g.
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> - Array.Find<T>(T[] array, Predicate<T> match). Returns default(T) if there isn’t a match.
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> - Lazy<T>.ValueForDebugDisplay. Returns default(T) if the Lazy<T> hasn’t yet been initialized.
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> - Marshal.PtrToStructure<T>(IntPtr). If you pass in IntPtr.Zero, you’ll get back default(T).
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> Generic methods accepting an unconstrained T with argument that should not be null
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> - Marshal.GetFunctionPointerForDelegate
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> Try- methods on generic type that outs default(T), e.g.
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> - ConcurrentQueue<T>.TryDequeue(out T)
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> - IProducerConsumerCollection<T>.TryTake(out T)
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> - WeakReference<T>.TryGetTarget(out T target)
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> Try- methods with non-generics, e.g.
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> - Version.TryParse
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> - Semaphore.TryOpenExisting
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> Try- methods that return a struct wrapper around a reference type that’ll be non-null if returning true
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> - MemoryMarshal.TryGetArray(…, out ArraySegment<T>)
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> Interfaces where T should be nullable on some methods but non-nullable on others, e.g.
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> - IEqualityComparer<T>: Equals(T x, T y) should have nullable arguments, but GetHashCode(T obj) should have a non-nullable argument
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> Fields/properties of type T that start life as default(T), and maybe become default(T) again
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> - AsyncLocal<T>.Value
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> - ThreadLocal<T>.Value
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> - StrongBox<T>.Value
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> Array slots that need to be settable to default(T)
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> - Pretty much every collection we have, nulling out a slot when an element is removed
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> Methods where whether the return value can be null is impacted by whether an argument is null
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> - RegistryKey.GetValue
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> - Path.GetFullName
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> - Path.ChangeExtension
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> - Path.GetExtension
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> - Path.GetFileNameWithoutExtension
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> - Convert.ToString(string?)
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> - Delegate.Combine
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> Methods where whether the return value can be null is impacted by whether an argument is true/false
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> - Type.GetType(…, bool throwOnError)
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> Methods where one argument will be non-null if another is non-null
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> - Volatile.Write
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> - Interlocked.Exchange
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> - Interlocked.CompareExchange (this one’s more complicated still)
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> Methods that accept an delegate<object?> and object?, but where the argument to Action<object?> will be non-null iff the object? is non-null
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> - Task.Factory.StartNew
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> - ExecutionContext.Run
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> - SynchronizationContext.Post
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> - CancellationToken.Register
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> - ThreadPool.QueueUserWorkItem
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> - Task ctor
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> - IValueTaskSource.OnCompleted
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> Ref arguments that will be non-null upon return
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> - Array.Resize(ref T[]);
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> - LazyInitializer.EnsureInitialized(ref T) (this one’s complicated if T is a nullable value type)
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The first question is whether or not the annotation of the method
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changes, e.g.
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```C#
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public static class Enumerable
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{
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[return: MaybeNull]
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public static T FirstOrDefault<T>(this IEnumerable<T> src) {...}
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public static T Identity<T>(T t) => t;
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}
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```
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Here `FirstOrDefault<string>` does not produce a `string?`, but the flow state of values returned
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from calls is a string with a MaybeNull state. Following through, for
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`Identity(FirstOrDefault<string>(...))`, the inferred type of `T` for `Identity<T>` would be
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`string?` because the flow state is used to construct the annotation of the substituted type
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argument.
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As written, if you were to provide a `FirstOrDefault<string>` override for an analogous
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`FirstOrDefault<T>` instance method, you could not write `string?` for the return type, despite
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the `MaybeNull` attribute.
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The proposal does not attempt to address modifying what the methods accept, only
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what they produce, meaning that providing `MaybeNull` on a parameter does not
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indicate that the input can be nullable.
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However, we think `MaybeNull` may be common or impactful enough to consider syntax, like allowing
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`T?` on an unconstrained type parameter or `T : object?`, to express `MaybeNull`. Importantly,
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this would not address all the patterns we've found while annotating CoreFX, like
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`Interlocked.CompareExchange` or `Debug.Assert`.
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The other problem is that the list of attributes which are needed to represent all
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patterns is unbounded for all the code, but even the set of attributes needed for
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just what we know is common is quite large.
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**Conclusion**
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We think the best approach right now is to try to address ~80% of the common cases
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using a mixture of attributes and special casing for extremely complicated contracts
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like `Interlocked.CompareExchange`.
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For `MaybeNull` and `T?` specifically, we are conflicted between the potential value
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that `T?` can provide, especially in annotating nested types, and the additional
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complexity in both language and implementation. One thing we're particularly worried
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about is the design work in adopting `T?` and revisiting some previous decisions.
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We're going to examine that work and if it's low in comparison to the `MaybeNull`
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attribute work, there's substantial support for implementing the feature. If we do,
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implement `T?`, we do not want to include the `MaybeNull`, specifically.
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@ -29,9 +29,10 @@ Overview of meetings and agendas for 2019
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## Apr 24, 2019
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### Nullable Reference Types
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[C# Language Design Notes for Apr 24, 2019](LDM-2019-04-24.md)
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MaybeNull and related nullable reference type attributes
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- MaybeNullAttribute: see https://github.com/cston/csharplang/blob/MaybeNull/proposals/MaybeNull.md
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## Apr 22, 2019
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