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LastIndexOf has improved handling of empty search strings

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String.LastIndexOf and related APIs now return correct values when searching for a zero-length (or zero-length equivalent) substring within a larger string.

Change description

In .NET Framework and .NET Core 1.0 - 3.1, String.LastIndexOf and related APIs might return an incorrect value when the caller searches for a zero-length substring.

Console.WriteLine("Hello".LastIndexOf("")); // prints '4' (incorrect)

ReadOnlySpan<char> span = "Hello";
Console.WriteLine(span.LastIndexOf("")); // prints '0' (incorrect)

Starting with .NET 5, these APIs return the correct value for LastIndexOf.

Console.WriteLine("Hello".LastIndexOf("")); // prints '5' (correct)

ReadOnlySpan<char> span = "Hello";
Console.WriteLine(span.LastIndexOf("")); // prints '5' (correct)

In these examples, 5 is the correct answer because "Hello".Substring(5) and "Hello".AsSpan().Slice(5) both produce an empty string, which is trivially equal to the empty substring that is sought.

Reason for change

This change was part of an overall bug fixing effort around string handling for .NET 5. It also helps unify our behavior between Windows and non-Windows platforms. For more information, see dotnet/runtime#13383 and dotnet/runtime##13382.

Version introduced

5.0

You don't need to take any action. The .NET 5 runtime provides the new behaviors automatically.

There is no compatibility switch to restore the old behavior.

Affected APIs