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GetGeneration breaking change #35228

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May 4, 2023
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GetGeneration breaking change #35228

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1 change: 1 addition & 0 deletions docs/core/compatibility/8.0.md
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Expand Up @@ -23,6 +23,7 @@ If you're migrating an app to .NET 8, the breaking changes listed here might aff
| [AnonymousPipeServerStream.Dispose behavior](core-libraries/8.0/anonymouspipeserverstream-dispose.md) | Behavioral change | Preview 1 |
| [Backslash mapping in Unix file paths](core-libraries/8.0/file-path-backslash.md) | Behavioral change | Preview 1 |
| [FileStream writes when pipe is closed](core-libraries/8.0/filestream-disposed-pipe.md) | Behavioral change | Preview 1 |
| [GC.GetGeneration might return Int32.MaxValue](core-libraries/8.0/getgeneration-return-value.md) | Behavioral change | Preview 4 |
| [GetFolderPath behavior on Unix](core-libraries/8.0/getfolderpath-unix.md) | Behavioral change | Preview 1 |
| [ITypeDescriptorContext nullable annotations](core-libraries/8.0/itypedescriptorcontext-props.md) | Source incompatible | Preview 1 |
| [Legacy Console.ReadKey removed](core-libraries/8.0/console-readkey-legacy.md) | Behavioral change | Preview 1 |
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41 changes: 41 additions & 0 deletions docs/core/compatibility/core-libraries/8.0/getgeneration-return-value.md
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---
title: ".NET 8 breaking change: GC.GetGeneration might return Int32.MaxValue"
description: Learn about the .NET 8 breaking change in core .NET libraries where GC.GetGeneration might return Int32.MaxValue for certain object types.
ms.date: 05/02/2023
---
# GC.GetGeneration might return Int32.MaxValue

Starting in .NET 8, <xref:System.GC.GetGeneration%2A?displayProperty=nameWithType> might return <xref:System.Int32.MaxValue?displayProperty=nameWithType> for objects allocated on non-GC heaps (also referred as "frozen" heaps), where previously it returned 2. When and how the runtime allocates objects on non-GC heaps is an internal implementation detail. String literals, for example, are allocated on a non-GC heap, and the following method call might return <xref:System.Int32.MaxValue?displayProperty=nameWithType>.

```csharp
int gen = int GetGeneration("string");
```

## Previous behavior

Previously, <xref:System.GC.GetGeneration%2A?displayProperty=nameWithType> returned integer values in the range of 0-2.

## New behavior

Starting in .NET 8, <xref:System.GC.GetGeneration%2A?displayProperty=nameWithType> can return a value of 0, 1, 2, or <xref:System.Int32.MaxValue?displayProperty=nameWithType>.

## Version introduced

.NET 8 Preview 4

## Type of breaking change

This change is a [behavioral change](../../categories.md#behavioral-change).

## Reason for change

.NET introduced a new, non-GC kind of heap that's slightly different from the existing heaps, which are large object heap (LOH), small object heap (SOH), and pinned object heap (POH).

## Recommended action

Make sure you're not using the return value from `GC.GetGeneration()` as an array indexer or for anything else where <xref:System.Int32.MaxValue?displayProperty=nameWithType> is unexpected.

## Affected APIs

- <xref:System.GC.GetGeneration(System.Object)?displayProperty=fullName>
- <xref:System.GC.GetGeneration(System.WeakReference)?displayProperty=fullName>
4 changes: 4 additions & 0 deletions docs/core/compatibility/toc.yml
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Expand Up @@ -18,6 +18,8 @@ items:
href: core-libraries/8.0/file-path-backslash.md
- name: FileStream writes when pipe is closed
href: core-libraries/8.0/filestream-disposed-pipe.md
- name: GC.GetGeneration might return Int32.MaxValue
href: core-libraries/8.0/getgeneration-return-value.md
- name: GetFolderPath behavior on Unix
href: core-libraries/8.0/getfolderpath-unix.md
- name: ITypeDescriptorContext nullable annotations
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href: core-libraries/8.0/file-path-backslash.md
- name: FileStream writes when pipe is closed
href: core-libraries/8.0/filestream-disposed-pipe.md
- name: GC.GetGeneration might return Int32.MaxValue
href: core-libraries/8.0/getgeneration-return-value.md
- name: GetFolderPath behavior on Unix
href: core-libraries/8.0/getfolderpath-unix.md
- name: ITypeDescriptorContext nullable annotations
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