Collectible assembly breaking change

docs/core/compatibility/7.0.md

@@ -29,6 +29,7 @@ If you're migrating an app to .NET 7, the breaking changes listed here might aff
 | - | :-: | :-: | - |
 | [API obsoletions with non-default diagnostic IDs](core-libraries/7.0/obsolete-apis-with-custom-diagnostics.md) | ✔️ | ❌ | Preview 1 |
 | [C++/CLI projects in Visual Studio](core-libraries/7.0/cpluspluscli-compiler-version.md)
+| [Collectible Assembly in non-collectible AssemblyLoadContext](core-libraries/7.0/collectible-assemblies.md) | ❌ | ✔️ | Preview 5 |
 | [FullPath and OldFullPath return fully qualified path](core-libraries/7.0/filesystemeventargs-fullpath.md) | ✔️ | ❌ | Preview 1 |
 | [Generic type constraint on PatternContext\<T>](core-libraries/7.0/patterncontext-generic-constraint.md) | ❌ | ❌ | Preview 3 |
 | [SerializationFormat.Binary is obsolete](core-libraries/7.0/serializationformat-binary.md) | ❌ | ❌ | Preview 2 |

docs/core/compatibility/core-libraries/7.0/collectible-assemblies.md

@@ -0,0 +1,41 @@
+---
+title: ".NET 7 breaking change: Collectible Assembly in non-collectible AssemblyLoadContext"
+description: Learn about the .NET 7 breaking change in core .NET libraries where resolving a collectible Assembly in a non-collectible AssemblyLoadContext results in a FileLoadException.
+ms.date: 05/10/2022
+---
+# Collectible Assembly in non-collectible AssemblyLoadContext
+
+.NET incorrectly allowed garbage-collectible assemblies to resolve into a non-collectible <xref:System.Runtime.Loader.AssemblyLoadContext>. In some cases, this lead to runtime crashes or unexpected <xref:System.NullReferenceException> exceptions. This change prevents the incorrect behavior by throwing an exception when the <xref:System.Runtime.Loader.AssemblyLoadContext.Load(System.Reflection.AssemblyName)?displayProperty=nameWithType> or <xref:System.Runtime.Loader.AssemblyLoadContext.Resolving?displayProperty=nameWithType> event returns a collectible <xref:System.Type.Assembly> and the <xref:System.Runtime.Loader.AssemblyLoadContext> is non-collectible.
+
+## Previous behavior
+
+Returning a collectible <xref:System.Type.Assembly> in the <xref:System.Runtime.Loader.AssemblyLoadContext.Load(System.Reflection.AssemblyName)?displayProperty=nameWithType> override or the <xref:System.Runtime.Loader.AssemblyLoadContext.Resolving?displayProperty=nameWithType> event of a non-collectible <xref:System.Runtime.Loader.AssemblyLoadContext> doesn't cause any exceptions to be thrown.
+
+## New behavior
+
+Returning a collectible <xref:System.Type.Assembly> in the <xref:System.Runtime.Loader.AssemblyLoadContext.Load(System.Reflection.AssemblyName)?displayProperty=nameWithType> override or the <xref:System.Runtime.Loader.AssemblyLoadContext.Resolving?displayProperty=nameWithType> event of a non-collectible <xref:System.Runtime.Loader.AssemblyLoadContext> throws a <xref:System.IO.FileLoadException?displayProperty=fullName> with a <xref:System.NotSupportedException> as the inner exception.
+
+## Version introduced
+
+.NET 7 Preview 5
+
+## Type of breaking change
+
+This change can affect [binary compatibility](../../categories.md#binary-compatibility).
+
+## Reason for change
+
+This change fixes a bug. The collectible <xref:System.Type.Assembly> would be garbage-collected while the <xref:System.Runtime.Loader.AssemblyLoadContext> that has a reference to it is alive for the rest of the process lifetime. If the code running in that context references anything from that `Assembly` after it's collected, it would crash the runtime or result in a <xref:System.NullReferenceException>, <xref:System.AccessViolationException>, or other kinds of bad behavior.
+
+## Recommended action
+
+Don't return collectible assemblies in <xref:System.Runtime.Loader.AssemblyLoadContext.Load(System.Reflection.AssemblyName)?displayProperty=nameWithType> or the <xref:System.Runtime.Loader.AssemblyLoadContext.Resolving?displayProperty=nameWithType> event of a non-collectible <xref:System.Runtime.Loader.AssemblyLoadContext>. A possible workaround is to change the `AssemblyLoadContext` to be collectible by passing `true` for the `isCollectible` parameter in its constructor, and then keep a reference to that `AssemblyLoadContext` forever to make sure it's never collected.
+
+## Affected APIs
+
+- <xref:System.Runtime.Loader.AssemblyLoadContext.Load(System.Reflection.AssemblyName)?displayProperty=fullName>
+- <xref:System.Runtime.Loader.AssemblyLoadContext.Resolving?displayProperty=fullName> event
+
+## See also
+
+- [Use collectible AssemblyLoadContext](../../../../standard/assembly/unloadability.md#use-collectible-assemblyloadcontext)

docs/core/compatibility/toc.yml

@@ -43,6 +43,8 @@ items:
           href: core-libraries/7.0/obsolete-apis-with-custom-diagnostics.md
         - name: C++/CLI projects in Visual Studio
           href: core-libraries/7.0/cpluspluscli-compiler-version.md
+        - name: Collectible Assembly in non-collectible AssemblyLoadContext
+          href: core-libraries/7.0/collectible-assemblies.md
         - name: FullPath and OldFullPath return fully qualified path
           href: core-libraries/7.0/filesystemeventargs-fullpath.md
         - name: Generic type constraint on PatternContext<T>
@@ -729,6 +731,8 @@ items:
           href: core-libraries/7.0/obsolete-apis-with-custom-diagnostics.md
         - name: C++/CLI projects in Visual Studio
           href: core-libraries/7.0/cpluspluscli-compiler-version.md
+        - name: Collectible Assembly in non-collectible AssemblyLoadContext
+          href: core-libraries/7.0/collectible-assemblies.md
         - name: FullPath and OldFullPath return fully qualified path
           href: core-libraries/7.0/filesystemeventargs-fullpath.md
         - name: Generic type constraint on PatternContext<T>