Document breaking change: Disallow loading non-Explicit types with explicit field offsets

docs/core/compatibility/10.0.md

@@ -89,6 +89,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 
 | Title | Type of change    | Introduced version |
 |-------|-------------------|--------------------|
+| [Disallow loading non-Explicit types with explicit field offsets](interop/10.0/explicit-field-offset-validation.md) | Binary incompatible/behavioral change | Preview 4 |
 | [Single-file apps no longer look for native libraries in executable directory](interop/10.0/native-library-search.md) | Behavioral change | Preview 6 |
 | [Specifying DllImportSearchPath.AssemblyDirectory only searches the assembly directory](interop/10.0/search-assembly-directory.md) | Behavioral change | Preview 5 |
 

docs/core/compatibility/interop/10.0/explicit-field-offset-validation.md

@@ -0,0 +1,123 @@
+---
+title: "Breaking change: Disallow loading non-Explicit types with explicit field offsets"
+description: "Learn about the breaking change in .NET 10 where the runtime enforces stricter validation for type layouts and throws TypeLoadException if non-explicit layout types specify explicit field offsets."
+ms.date: 10/13/2025
+ai-usage: ai-assisted
+---
+
+# Disallow loading non-Explicit types with explicit field offsets
+
+Starting in .NET 10 Preview 4, the .NET runtime enforces stricter validation for type layouts. Specifically, the runtime now throws a <xref:System.TypeLoadException> if a type with a non-explicit layout (for example, `Auto` or `Sequential`) specifies explicit field offsets using the <xref:System.Runtime.InteropServices.FieldOffsetAttribute>. This change aligns the runtime's behavior with the ECMA-335 specification, which only permits explicit field offsets for types with an `Explicit` layout.
+
+## Version introduced
+
+.NET 10 Preview 4
+
+## Previous behavior
+
+In earlier versions of .NET, the runtime ignored explicit field offsets on types with `Auto` or `Sequential` layouts. For example:
+
+```csharp
+using System;
+using System.Runtime.InteropServices;
+
+[StructLayout(LayoutKind.Sequential)]
+public struct MyStruct
+{
+    [FieldOffset(0)] // Ignored in .NET versions prior to 10 Preview 4
+    public int Field1;
+
+    [FieldOffset(4)] // Ignored in .NET versions prior to 10 Preview 4
+    public int Field2;
+}
+
+class Program
+{
+    static void Main()
+    {
+        Console.WriteLine("Struct loaded successfully.");
+    }
+}
+```
+
+The above code would execute without error, and the `[FieldOffset]` attributes would be ignored.
+
+## New behavior
+
+Starting in .NET 10 Preview 4, the runtime enforces the ECMA-335 specification and throws a <xref:System.TypeLoadException> if a type with `Auto` or `Sequential` layout specifies explicit field offsets. The following code now throws an exception:
+
+```csharp
+using System;
+using System.Runtime.InteropServices;
+
+[StructLayout(LayoutKind.Sequential)]
+public struct MyStruct
+{
+    [FieldOffset(0)] // Causes a TypeLoadException in .NET 10 Preview 4 and later
+    public int Field1;
+
+    [FieldOffset(4)] // Causes a TypeLoadException in .NET 10 Preview 4 and later
+    public int Field2;
+}
+
+class Program
+{
+    static void Main()
+    {
+        Console.WriteLine("Struct loaded successfully.");
+    }
+}
+```
+
+**Exception thrown:**
+
+```output
+System.TypeLoadException: Explicit field offsets are not allowed on types with Sequential or Auto layout.
+```
+
+## Type of breaking change
+
+This change can affect [binary compatibility](../../categories.md#binary-compatibility) and is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+This change was introduced to align the .NET runtime with the ECMA-335 specification, which explicitly disallows explicit field offsets on types with `Auto` or `Sequential` layouts. The previous behavior of ignoring these offsets was non-compliant and could lead to unexpected behavior or incorrect assumptions about memory layout.
+
+## Recommended action
+
+To resolve this issue, update your code to use `LayoutKind.Explicit` for types that specify explicit field offsets. For example:
+
+```csharp
+using System;
+using System.Runtime.InteropServices;
+
+[StructLayout(LayoutKind.Explicit)] // Use Explicit layout
+public struct MyStruct
+{
+    [FieldOffset(0)]
+    public int Field1;
+
+    [FieldOffset(4)]
+    public int Field2;
+}
+
+class Program
+{
+    static void Main()
+    {
+        Console.WriteLine("Struct loaded successfully.");
+    }
+}
+```
+
+Alternatively, if explicit field offsets aren't required, remove the <xref:System.Runtime.InteropServices.FieldOffsetAttribute> attributes and rely on the default behavior of `Sequential` or `Auto` layout.
+
+## Affected APIs
+
+- <xref:System.Runtime.InteropServices.StructLayoutAttribute?displayProperty=fullName>
+- <xref:System.Runtime.InteropServices.FieldOffsetAttribute?displayProperty=fullName>
+
+## See also
+
+- [ECMA-335 specification](https://www.ecma-international.org/publications-and-standards/standards/ecma-335/)
+- [Customize structure marshalling](../../../../standard/native-interop/customize-struct-marshalling.md)

docs/core/compatibility/toc.yml

@@ -98,6 +98,8 @@ items:
                 href: globalization/10.0/version-override.md
           - name: Interop
             items:
+              - name: Disallow loading non-Explicit types with explicit field offsets
+                href: interop/10.0/explicit-field-offset-validation.md
               - name: Single-file apps no longer look for native libraries in executable directory
                 href: interop/10.0/native-library-search.md
               - name: Specifying DllImportSearchPath.AssemblyDirectory only searches the assembly directory