add docs for MSBuild breaking change around serialization of certain types

docs/core/compatibility/7.0.md

@@ -47,11 +47,12 @@ If you're migrating an app to .NET 7, the breaking changes listed here might aff
 | [AllowRenegotiation default is false](networking/7.0/allowrenegotiation-default.md) | ❌ | ❌ | Preview 3 |
 | [Custom ping payloads on Linux](networking/7.0/ping-custom-payload-linux.md) | ❌ | ✔️ | Preview 2 |
 
-## SDK
+## SDK and MSBuild
 
 | Title | Binary compatible | Source compatible | Introduced |
 | - | :-: | :-: | - |
 | [Version requirements for .NET 7 SDK](sdk/7.0/vs-msbuild-version.md) | ✔️ | ✔️ | 7.0.100 |
+| [MSBuild serialization of custom types in .NET 7](sdk/7.0/custom-serialization.md) | ❌ | ❌ | 7.0.100 |
 
 ## Serialization
 

docs/core/compatibility/sdk/7.0/custom-serialization.md

@@ -0,0 +1,32 @@
+---
+title: "Breaking change: Serialization of custom types in .NET 7"
+description: Learn about a breaking change in MSBuild for .NET 7 where serialization using BinaryFormatter of certain user-defined types is removed.
+author: baronfel
+ms.date: 05/06/2022
+---
+
+# BinaryFormatter serialization of custom BuildEventArgs and ITaskItems removed for .NET 7
+
+MSBuild in .NET 7 doesn't support serialization of custom `BuildEventArgs`-derived and `ITaskItem`-derived types via the `BinaryFormatter` serializer.
+
+## Version introduced
+
+MSBuild 17.4 (.NET SDK 7.0.100)
+
+## Old behavior
+
+MSBuild used BinaryFormatter to preserve custom types that derived from BuildEventArgs and ITaskItem across certain boundaries, most notably when running in a multi-process environment.
+
+## New behavior
+
+MSBuild will no longer support this mechanism, so code that used custom types derived from BuildEventArgs and ITaskItem may fail.
+
+## Reason for change
+
+BinaryFormatter was [made obsolete in .NET 5](https://github.com/dotnet/designs/blob/main/accepted/2020/better-obsoletion/binaryformatter-obsoletion.md). Per this plan, all first-party code in the dotnet GitHub organization must migrate away from its use by .NET 7. This change impacts user-exposed functionality of MSBuild.
+
+## Recommended action
+
+* Engage with the MSBuild team on [this GitHub discussion](https://github.com/dotnet/msbuild/discussions/7582) about your specific use cases and how you can migrate away from the `TranslateDotNet` mechanism.
+
+* Avoid returning custom derived types from tasks or when logging.

docs/core/compatibility/toc.yml

@@ -61,10 +61,12 @@ items:
           href: networking/7.0/allowrenegotiation-default.md
         - name: Custom ping payloads on Linux
           href: networking/7.0/ping-custom-payload-linux.md
-      - name: SDK
+      - name: SDK and MSBuild
         items:
         - name: Version requirements for .NET 7 SDK
           href: sdk/7.0/vs-msbuild-version.md
+        - name: Serialization of custom types in .NET 7
+          href: sdk/7.0/custom-serialization.md
       - name: Serialization
         items:
         - name: Deserialize Version type with leading or trailing whitespace
@@ -965,6 +967,8 @@ items:
         items:
         - name: Version requirements for .NET 7 SDK
           href: sdk/7.0/vs-msbuild-version.md
+        - name: Serialization of custom types in .NET 7
+          href: sdk/7.0/custom-serialization.md
       - name: .NET 6
         items:
         - name: -p option for `dotnet run` is deprecated