Core libraries breaking changes for Preview 6

docs/core/compatibility/9.0.md

@@ -28,9 +28,11 @@ If you're migrating an app to .NET 9, the breaking changes listed here might aff
 | [Adding a ZipArchiveEntry with CompressionLevel sets ZIP central directory header general-purpose bit flags](core-libraries/9.0/compressionlevel-bits.md) | Behavioral change | Preview 5 |
 | [API obsoletions with custom diagnostic IDs](core-libraries/9.0/obsolete-apis-with-custom-diagnostics.md) | Source incompatible | Preview 1  |
 | [Creating type of array of System.Void not allowed](core-libraries/9.0/type-instance.md) | Behavioral change   | Preview 1          |
+| [Default `Equals()` and `GetHashCode()` throw for types marked with `InlineArrayAttribute`](core-libraries/9.0/inlinearrayattribute.md) | Behavioral change   | Preview 6          |
 | [Inline array struct size limit is enforced](core-libraries/9.0/inlinearray-size.md) | Behavioral change   | Preview 1          |
 | [InMemoryDirectoryInfo prepends rootDir to files](core-libraries/9.0/inmemorydirinfo-prepends-rootdir.md) | Behavioral change   | Preview 1          |
 | [RuntimeHelpers.GetSubArray returns different type](core-libraries/9.0/getsubarray-return.md) | Behavioral change   | Preview 1          |
+| [Support for empty environment variables](core-libraries/9.0/empty-env-variable.md) | Behavioral change   | Preview 6          |
 
 ## Networking
 

docs/core/compatibility/core-libraries/9.0/empty-env-variable.md

@@ -0,0 +1,46 @@
+---
+title: "Breaking change: Support for empty environment variables"
+description: Learn about the .NET 9 breaking change in core .NET libraries where passing `string.Empty` to `Environment.SetEnvironmentVariable` no longer deletes the environment variable.
+ms.date: 07/10/2024
+---
+# Support for empty environment variables
+
+Support was added to be able to set an environment variable to the empty string using <xref:System.Environment.SetEnvironmentVariable(System.String,System.String)?displayProperty=nameWithType>. As part of this work, the behavior of setting the <xref:System.Diagnostics.ProcessStartInfo.Environment?displayProperty=nameWithType> and <xref:System.Diagnostics.ProcessStartInfo.EnvironmentVariables?displayProperty=nameWithType> properties was changed to be consistent with that of <xref:System.Environment.SetEnvironmentVariable(System.String,System.String)?displayProperty=nameWithType>.
+
+## Previous behavior
+
+Previously:
+
+- Both `Environment.SetEnvironmentVariable("TEST", string.Empty)` and `Environment.SetEnvironmentVariable("TEST", null)` deleted the environment variable.
+- Both `ProcessStartInfo.Environment["TEST"] = string.Empty` and `ProcessStartInfo.Environment["TEST"] = null` set the environment variable in the child process to an empty value.
+
+## New behavior
+
+Starting in .NET 9:
+
+- `Environment.SetEnvironmentVariable("TEST", string.Empty)` sets the environment variable value to an empty value. `Environment.SetEnvironmentVariable("TEST", null)` behavior is unchanged, that is, it still deletes the environment variable.
+- `ProcessStartInfo.Environment["TEST"] = null` deletes the environment variable. `ProcessStartInfo.Environment["TEST"] = string.Empty` behavior is unchanged, that is, it still sets the environment variable to an empty value.
+
+## Version introduced
+
+.NET 9 Preview 6
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+Before this change, it wasn't possible to use <xref:System.Environment.SetEnvironmentVariable(System.String,System.String)?displayProperty=nameWithType> to set an environment variable to an empty value, which is a valid environment variable value on all supported platforms.
+
+## Recommended action
+
+To delete an environment variable using <xref:System.Environment.SetEnvironmentVariable(System.String,System.String)?displayProperty=nameWithType>, change your code to pass `null` instead of `string.Empty` as the value argument.
+
+To set the environment variable to an empty value using <xref:System.Diagnostics.ProcessStartInfo.Environment?displayProperty=nameWithType> or <xref:System.Diagnostics.ProcessStartInfo.EnvironmentVariables?displayProperty=nameWithType>, change your code to set these properties to `string.Empty` instead of to `null`.
+
+## Affected APIs
+
+- <xref:System.Environment.SetEnvironmentVariable(System.String,System.String)?displayProperty=fullName>
+- <xref:System.Diagnostics.ProcessStartInfo.Environment?displayProperty=fullName>
+- <xref:System.Diagnostics.ProcessStartInfo.EnvironmentVariables?displayProperty=fullName>

docs/core/compatibility/core-libraries/9.0/inlinearrayattribute.md

@@ -0,0 +1,37 @@
+---
+title: "Breaking change: Default `Equals()` and `GetHashCode()` throw for types marked with `InlineArrayAttribute`"
+description: Learn about the .NET 9 breaking change in core .NET libraries where the default implementations of `Equals()` and `GetHashCode()` throw an exception for types marked with `InlineArrayAttribute`.
+ms.date: 07/10/2024
+---
+# Default `Equals()` and `GetHashCode()` throw for types marked with `InlineArrayAttribute`
+
+The default behavior for <xref:System.ValueType.Equals(System.Object)> and <xref:System.ValueType.GetHashCode> on types marked with <xref:System.Runtime.CompilerServices.InlineArrayAttribute> is now to throw a <xref:System.NotSupportedException>. Library authors should override these two methods if they're expected to not throw.
+
+## Previous behavior
+
+Previously, the default implementations only used the placeholder `ref` field when computing equality or the hash code.
+
+## New behavior
+
+Starting in .NET 9, a <xref:System.NotSupportedException> is always thrown from the default implementations for <xref:System.ValueType.Equals(System.Object)> and <xref:System.ValueType.GetHashCode> when <xref:System.Runtime.CompilerServices.InlineArrayAttribute> is applied to a type.
+
+## Version introduced
+
+.NET 9 Preview 6
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+The current behavior is incorrect for both determining equality and computing the hash code, and users are led into a false sense of correctness when calling these functions.
+
+## Recommended action
+
+Library authors should implement both <xref:System.ValueType.Equals(System.Object)> and <xref:System.ValueType.GetHashCode> on all types marked with <xref:System.Runtime.CompilerServices.InlineArrayAttribute>.
+
+## Affected APIs
+
+- <xref:System.ValueType.Equals(System.Object)?displayProperty=fullName>
+- <xref:System.ValueType.GetHashCode?displayProperty=fullName>

docs/core/compatibility/toc.yml

@@ -20,12 +20,16 @@ items:
         href: core-libraries/9.0/compressionlevel-bits.md
       - name: Creating type of array of System.Void not allowed
         href: core-libraries/9.0/type-instance.md
+      - name: "`Equals`/`GetHashCode` throw for `InlineArrayAttribute` types"
+        href: core-libraries/9.0/inlinearrayattribute.md
       - name: Inline array struct size limit is enforced
         href: core-libraries/9.0/inlinearray-size.md
       - name: InMemoryDirectoryInfo prepends rootDir to files
         href: core-libraries/9.0/inmemorydirinfo-prepends-rootdir.md
       - name: RuntimeHelpers.GetSubArray returns different type
         href: core-libraries/9.0/getsubarray-return.md
+      - name: Support for empty environment variables
+        href: core-libraries/9.0/empty-env-variable.md
     - name: Networking
       items:
       - name: HttpListenerRequest.UserAgent is nullable
@@ -1180,12 +1184,16 @@ items:
         href: core-libraries/9.0/compressionlevel-bits.md
       - name: Creating type of array of System.Void not allowed
         href: core-libraries/9.0/type-instance.md
+      - name: "`Equals`/`GetHashCode` throw for `InlineArrayAttribute` types"
+        href: core-libraries/9.0/inlinearrayattribute.md
       - name: Inline array struct size limit is enforced
         href: core-libraries/9.0/inlinearray-size.md
       - name: InMemoryDirectoryInfo prepends rootDir to files
         href: core-libraries/9.0/inmemorydirinfo-prepends-rootdir.md
       - name: RuntimeHelpers.GetSubArray returns different type
         href: core-libraries/9.0/getsubarray-return.md
+      - name: Support for empty environment variables
+        href: core-libraries/9.0/empty-env-variable.md
     - name: .NET 8
       items:
       - name: Activity operation name when null