Use NotNullWhen(true) instead of MaybeNullWhen(false) for concrete-type Try-methods - #655
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…te-type Try-methods PR #642 annotated all Try* out-parameters with [MaybeNullWhen(false)] on a non-nullable T, which is the correct pattern for unconstrained generic type parameters (as in Dictionary<TKey,TValue>.TryGetValue) but not for concrete reference types. For concrete types, the BCL convention (e.g. Uri.TryCreate) is [NotNullWhen(true)] out T? - this also gives callers a compiler warning if they dereference the out value without checking the return value first, which MaybeNullWhen(false) on non-nullable T silently allows. Updates IHelperResolver, IObjectDescriptorProvider (and implementers), IFormatterProvider (and implementers), TypeExtensions.IsAssignableToGenericType, and BlockAccumulatorContext.IsDetachedClosingElement. Generic-TValue Try-methods (LookupSlim, DictionarySlim, FixedSizeDictionary, CascadeIndex, ObservableIndex) are left as-is since they correctly mirror the BCL generic pattern. Fixes #654
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Summary
PR #642 annotated all
Try*out-parameters with[MaybeNullWhen(false)] out T value(non-nullableT), regardless of whetherTwas an unconstrained generic type parameter or a concrete reference type. That pattern is only correct for the generic case (mirroringDictionary<TKey,TValue>.TryGetValue, which can't writeTValue?without aclassconstraint). For concrete reference types, the BCL's own convention (e.g.Uri.TryCreate) is[NotNullWhen(true)] out T?- which also gives a stronger compiler guarantee: dereferencing the out value without checking the return value first now correctly warns, whereasMaybeNullWhen(false)on a non-nullableTsilently allowed it.This is a compile-time-only change (nullable annotations erase to metadata, not IL), so it's not binary breaking, and normal
if (TryX(..., out var x))call sites are unaffected.IHelperResolver.TryResolveHelper/TryResolveBlockHelperIObjectDescriptorProvider.TryGetDescriptorand all implementers (ObjectDescriptorProvider,ObjectDescriptorFactory,LayoutViewModel.DescriptorProvider, the dictionary/enumerable/dynamic descriptor providers)ObjectDescriptor.TryCreate(both overloads)IFormatterProvider.TryCreateFormatterand all implementersTypeExtensions.IsAssignableToGenericTypeBlockAccumulatorContext.IsDetachedClosingElementLeft unchanged: the generic-
TValuecollection Try-methods (LookupSlim,DictionarySlim,FixedSizeDictionary,CascadeIndex,ObservableIndex) - these correctly mirror the BCL's own generic pattern already.Fixes #654
Test plan
dotnet buildacross all target frameworks (netstandard1.3/2.0/2.1, net6.0, net8.0) - 0 errors, no new warnings introduced (verified via clean-rebuild diff against master)dotnet test- 1867/1867 passing