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RuntimeHelpers.GetSubArray returns different type

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The type of array instance returned by RuntimeHelpers.GetSubArray<T>(T[], Range) has changed to match the source array. RuntimeHelpers.GetSubArray is used by C# compiler to implement range operator for arrays.

This behavior change can be observed only by code that uses covariant array conversions.

Previous behavior

Previously, RuntimeHelpers.GetSubArray<T>(T[] array, Range range) returned an array instance of type T[].

For example, the type of array instance returned by RuntimeHelpers.GetSubArray<object>(new string[1], ...) was object[].

New behavior

Starting in .NET 9, RuntimeHelpers.GetSubArray<T>(T[] array, Range range) returns an array instance of the same type as the array parameter.

For example, the type of array instance returned by RuntimeHelpers.GetSubArray<object>(new string[1], ...) is string[].

Version introduced

.NET 9 Preview 1

Type of breaking change

This change is a behavioral change.

Reason for change

The design of C# pattern-matching features assumes that the type of array instance returned by RuntimeHelpers.GetSubArray<T>(T[], Range) matches the source array. The previous behavior led to unexpected behavior of certain complex pattern expressions that used slicing of covariant arrays. For more information, see dotnet/roslyn#69053.

The recommended action is to remove dependency of the affected code on array covariance.

For example, change:

object[] arr = new string[1];
M(arr[1..2]);

to:

string[] arr = new string[1];
M(arr[1..2]);

Affected APIs