How to fix Error 6229 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6229 – Native Serialization Layout Error

Why This Error Happens

This error occurs when a SQL CLR (Common Language Runtime) assembly contains a managed type marked for native serialization (typically via [Serializable] or used as a User-Defined Type/Aggregate) but lacks the required [StructLayout(LayoutKind.Sequential)] attribute. SQL Server 2025’s CLR integration requires explicit, deterministic memory layout for any type that crosses the native/managed boundary, since native serialization relies on predictable field ordering rather than the CLR’s default “Auto” layout. This typically happens after compiling a custom User-Defined Type (UDT), User-Defined Aggregate (UDA), or CLR Table-Valued Function assembly without properly decorating the struct or class, often following a migration from an older .NET Framework project to .NET 8/9 targeting SQL Server 2025.

Step-by-Step Resolution Guide

Step 1: Locate the Offending Assembly and Type

  • Identify the exact assembly name from the error message (the %.*ls.%.*ls placeholders reference Namespace.TypeName).
  • Run the following query to find the assembly in SQL Server:
  • SELECT a.name, a.clr_name, m.assembly_class FROM sys.assemblies a JOIN sys.assembly_modules m ON a.assembly_id = m.assembly_id;
  • Open the corresponding .cs or .vb source file in Visual Studio 2025 where the type is defined.

Step 2: Apply the Correct StructLayout Attribute

  • Open the source file and locate the class or struct declaration that is marked with [Serializable] or implements IBinarySerialize.
  • Add the [StructLayout(LayoutKind.Sequential)] attribute directly above the type declaration:
  • using System.Runtime.InteropServices;
  • [Serializable]
  • [StructLayout(LayoutKind.Sequential)]
  • [SqlUserDefinedType(Format.Native)]
  • public struct MyUdt : INullable { ... }
  • If the type contains reference types (e.g., string, arrays) that cannot support Sequential layout natively, switch the Format to Format.UserDefined instead and implement IBinarySerialize manually.

Step 3: Rebuild and Redeploy the Assembly

  • Recompile the project in Visual Studio 2025 targeting the .NET Framework 4.8.1 or the SQL CLR-compatible runtime profile (do not target .NET 8+ directly for Format.Native types).
  • Drop and re-register the assembly in SQL Server:
  • ALTER ASSEMBLY MyAssembly FROM 'C:\Deploy\MyAssembly.dll' WITH PERMISSION_SET = SAFE;
  • Re-create any dependent UDTs or aggregates that referenced the old binary layout:
  • DROP TYPE dbo.MyUdt; CREATE TYPE dbo.MyUdt EXTERNAL NAME MyAssembly.[Namespace.MyUdt];

Step 4: Validate Permission Set and Trust Configuration

  • Confirm the assembly’s permission set (SAFE, EXTERNAL_ACCESS, or UNSAFE) matches your clr strict security configuration in sys.configurations.
  • For SQL Server 2025 on Windows Server 2025, ensure the assembly is signed and added to the trusted assemblies list if clr strict security is enabled (default = 1):
  • EXEC sp_add_trusted_assembly @hash = <SHA-512 hash>, @description = N'MyAssembly';

Alternative or Verification Steps

  • Verify the fix: Run a test query invoking the UDT or UDA (e.g., SELECT dbo.MyUdt::Parse('test')) and confirm no Error 6229 is raised; check SQL Server error logs (%ProgramFiles%\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG) for confirmation.
  • Alternative approach: If modifying source code isn’t feasible (third-party assembly), switch the SQL registration to Format.UserDefined where supported, or request an updated binary from the vendor compiled with proper LayoutKind.Sequential attributes.