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LastIndexOf breaking change #21375

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Merged
merged 3 commits into from
Nov 9, 2020
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LastIndexOf breaking change #21375

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47ceb61
lastindexof breaking change
gewarren Nov 7, 2020
26a83f4
fix links
gewarren Nov 7, 2020
c267e55
fix file name
gewarren Nov 9, 2020
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5 changes: 5 additions & 0 deletions docs/core/compatibility/3.1-5.0.md
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Expand Up @@ -215,6 +215,7 @@ If you're migrating from version 3.1 of .NET Core, ASP.NET Core, or EF Core to v

## Core .NET libraries

- [LastIndexOf has improved handling of empty search strings](#lastindexof-has-improved-handling-of-empty-search-strings)
- [Global assembly cache APIs are obsolete](#global-assembly-cache-apis-are-obsolete)
- [Remoting APIs are obsolete](#remoting-apis-are-obsolete)
- [Most code access security APIs are obsolete](#most-code-access-security-apis-are-obsolete)
Expand All @@ -241,6 +242,10 @@ If you're migrating from version 3.1 of .NET Core, ASP.NET Core, or EF Core to v
- [CounterSet.CreateCounterSetInstance now throws InvalidOperationException if instance already exist](#countersetcreatecountersetinstance-now-throws-invalidoperationexception-if-instance-already-exists)
- [Microsoft.DotNet.PlatformAbstractions package removed](#microsoftdotnetplatformabstractions-package-removed)

[!INCLUDE [lastindexof-improved-handling-of-empty-values](../../../includes/core-changes/corefx/5.0/lastindexof-improved-handling-of-empty-values.md)]

***

[!INCLUDE [globalassemblycache-property-obsolete](../../../includes/core-changes/corefx/5.0/global-assembly-cache-apis-obsolete.md)]

***
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5 changes: 5 additions & 0 deletions docs/core/compatibility/corefx.md
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Expand Up @@ -11,6 +11,7 @@ The following breaking changes are documented on this page:

| Breaking change | Version introduced |
| - | :-: |
| [LastIndexOf has improved handling of empty search strings](#lastindexof-has-improved-handling-of-empty-search-strings) | 5.0 |
| [Global assembly cache APIs are obsolete](#global-assembly-cache-apis-are-obsolete) | 5.0 |
| [Remoting APIs are obsolete](#remoting-apis-are-obsolete) | 5.0 |
| [Most code access security APIs are obsolete](#most-code-access-security-apis-are-obsolete) | 5.0 |
Expand Down Expand Up @@ -57,6 +58,10 @@ The following breaking changes are documented on this page:

## .NET 5.0

[!INCLUDE [lastindexof-improved-handling-of-empty-values](../../../includes/core-changes/corefx/5.0/lastindexof-improved-handling-of-empty-values.md)]

***

[!INCLUDE [remoting-apis-obsolete](../../../includes/core-changes/corefx/5.0/remoting-apis-obsolete.md)]

***
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59 changes: 59 additions & 0 deletions includes/core-changes/corefx/5.0/lastindexof-improved-handling-of-empty-values.md
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@@ -0,0 +1,59 @@
### LastIndexOf has improved handling of empty search strings

<xref:System.String.LastIndexOf%2A?displayProperty=nameWithType> 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, <xref:System.String.LastIndexOf%2A?displayProperty=nameWithType> and related APIs might return an incorrect value when the caller searches for a zero-length substring.

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

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

Starting with .NET 5.0, these APIs return the correct value for `LastIndexOf`.

```csharp
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](https://github.com/dotnet/runtime/issues/13383) and [dotnet/runtime##13382](https://github.com/dotnet/runtime/issues/13382).

#### Version introduced

5.0

#### Recommended action

You don't need to take any action. The .NET 5.0 runtime provides the new behaviors automatically.

There is no compatibility switch to restore the old behavior.

#### Category

Core .NET libraries

#### Affected APIs

- <xref:System.String.LastIndexOf%2A?displayProperty=fullName>
- <xref:System.Globalization.CompareInfo.LastIndexOf%2A?displayProperty=fullName>
- <xref:System.MemoryExtensions.LastIndexOf%2A?displayProperty=fullName>

<!--

#### Affected APIs

- `Overload:System.String.LastIndexOf`
- `Overload:System.Globalization.CompareInfo.LastIndexOf`
- `Overload:System.MemoryExtensions.LastIndexOf`

-->