Remove some allocation from NotifyCollectionChangedEventArgs #54899
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When initialized with a single object rather than a list of objects (single object is very common), the args is allocating an object[] and then wrapping that object[] in a ReadOnlyList. We can instead just allocate a simple read-only IList wrapper for the object directly; that also makes accessing it faster. Additionally, when constructing an instance using `public NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction action, IList? changedItems, int index, int oldIndex)`, even though the same collection instance is used for both the new and old items, it wraps it twice. We can wrap it once. Along the way I also simplified the code, enabling the fields to become readonly, removing duplicate assignments of fields, etc.
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Tagging subscribers to this area: @eiriktsarpalis Issue DetailsWhen initialized with a single object rather than a list of objects (single object is very common), the args is allocating an object[] and then wrapping that object[] in a ReadOnlyList. We can instead just allocate a simple read-only IList wrapper for the object directly; that also makes accessing it faster. Additionally, when constructing an instance using Along the way I also simplified the code, enabling the fields to become readonly, removing duplicate assignments of fields, etc.
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src/libraries/System.ObjectModel/src/System/Collections/CollectionHelpers.cs
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When initialized with a single object rather than a list of objects (single object is very common), the args is allocating an object[] and then wrapping that object[] in a ReadOnlyList. We can instead just allocate a simple read-only IList wrapper for the object directly; that also makes accessing it faster.
Additionally, when constructing an instance using
public NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction action, IList? changedItems, int index, int oldIndex), even though the same collection instance is used for both the new and old items, it wraps it twice. We can wrap it once.Along the way I also simplified the code, enabling the fields to become readonly, removing duplicate assignments of fields, etc.