How to fix Error 6227 on Windows Server 2025 / Windows 11

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Why This Error Happens

Error 6227 occurs when SQL Server’s CLR integration attempts to serialize a user-defined type (UDT) or table type that has been explicitly marked with Format.Native for serialization, but the underlying .NET type violates the strict requirements for native serialization (such as containing reference types, non-sequential layout, or fields that aren’t blittable). This typically surfaces after deploying a CLR assembly compiled under a newer .NET runtime (as used in SQL Server 2025) where stricter type-layout validation is enforced during assembly registration or execution. It can also occur if the assembly was compiled with mismatched StructLayout attributes or if the assembly was altered/corrupted after being loaded into the database.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Type and Assembly

  • Run the following query to locate the assembly and type referenced in the error message: SELECT * FROM sys.assembly_types WHERE name = 'YourTypeName';
  • Cross-reference with sys.assemblies to find the .dll source: SELECT * FROM sys.assemblies WHERE assembly_id = (SELECT assembly_id FROM sys.assembly_types WHERE name = 'YourTypeName');
  • Note the exact namespace and class name shown in the error (e.g., MyNamespace.MyType).

Step 2: Correct the StructLayout Attribute in Source Code

  • Open the original C# project used to build the CLR assembly.
  • Ensure the type is decorated with [StructLayout(LayoutKind.Sequential)] and [Serializable] directly above the class or struct definition.
  • Remove any reference-type fields (like string without [MarshalAs], arrays, or nested classes) that are not blittable — replace with fixed-size value types or marshaled equivalents.
  • Rebuild the .dll targeting the .NET runtime version compatible with SQL Server 2025 CLR hosting (check via SELECT * FROM sys.dm_clr_properties;).

Step 3: Redeploy the Assembly to SQL Server

  • Drop the existing assembly and dependent types: DROP TYPE dbo.YourTypeName; DROP ASSEMBLY YourAssemblyName;
  • Re-register the corrected assembly: CREATE ASSEMBLY YourAssemblyName FROM 'C:\Path\To\YourAssembly.dll' WITH PERMISSION_SET = SAFE;
  • Recreate the user-defined type: CREATE TYPE dbo.YourTypeName EXTERNAL NAME YourAssemblyName.[MyNamespace.MyType];

Step 4: Validate CLR Strict Security Settings

  • On SQL Server 2025, confirm clr strict security is enabled (default) using: EXEC sp_configure 'clr strict security';
  • If set to 0 for legacy compatibility, consider re-enabling it (1) and signing the assembly with a certificate or asymmetric key instead, since disabling it is not a long-term fix and may mask future serialization errors.

Alternative or Verification Steps

  • Verify the fix by executing a sample INSERT/SELECT statement against the recreated type to confirm no serialization errors appear in the SQL Server Error Log or Event Viewer (Event ID 6227 should no longer trigger).
  • If rebuilding the assembly is not feasible, as a workaround, change the serialization format from Format.Native to Format.UserDefined in the type definition and implement IBinarySerialize manually to control serialization logic explicitly.