Troubleshooting Guide: Error 6274 – ALTER ASSEMBLY Serialization Format Mismatch

Why This Error Happens

This error occurs when a CLR user-defined type (UDT) or user-defined aggregate marked with Format.Native or a persisted layout is modified in a way that changes its binary serialization layout—such as reordering fields, changing data types, or altering the structure of the class. SQL Server stores persisted instances of these types (in tables, as column values) using their exact binary layout, so it blocks the ALTER ASSEMBLY operation to prevent silently corrupting existing data. This typically happens after a developer edits the .cs or .vb source of a UDT, recompiles the assembly, and attempts to deploy the update without accounting for already-persisted data in the database.

Step-by-Step Resolution Guide

Step 1: Identify Persisted Instances of the Type

• Run a query against sys.assembly_types to confirm which type triggered the error and check its is_binary_ordered and format properties.
• Identify all tables/columns using this UDT with a query like:

  SELECT t.name AS TypeName, c.name AS ColumnName, o.name AS TableName
  FROM sys.columns c
  JOIN sys.types t ON c.user_type_id = t.user_type_id
  JOIN sys.objects o ON c.object_id = o.object_id
  WHERE t.name = 'YourUDTName';

• Export or back up this data before proceeding, as the next steps require dropping and recreating the type.

Step 2: Script Out and Drop Dependent Objects

• Generate CREATE scripts for all tables, indexes, and constraints that reference the CLR type.
• Convert affected columns to a temporary intermediate format (e.g., VARBINARY(MAX) using CONVERT) to preserve the data outside the UDT structure.
• Drop the dependent columns/tables, then drop the assembly using:

  DROP ASSEMBLY YourAssemblyName;

Step 3: Rebuild the Assembly with Format Compatibility in Mind

• In Visual Studio or your build environment, ensure the updated class retains the original field order and types if you want to preserve Format.Native.
• If structural changes are unavoidable, switch the type’s serialization to Format.UserDefined and manually implement IBinarySerialize.Read and IBinarySerialize.Write to control versioning explicitly.
• Recompile the .dll and redeploy it using:

  CREATE ASSEMBLY YourAssemblyName
  FROM 'C:\Assemblies\YourAssembly.dll'
  WITH PERMISSION_SET = SAFE;

Step 4: Recreate Types, Tables, and Reload Data

• Run CREATE TYPE to re-register the UDT against the newly deployed assembly.
• Recreate the original tables/columns using the scripts saved in Step 2.
• Reload the exported data, converting it back from the intermediate format into the new UDT structure using your application logic or a migration script.

Alternative or Verification Steps

• Verify the fix: After redeployment, run ALTER ASSEMBLY again on a test instance with a minor, non-structural code change to confirm SQL Server no longer throws Error 6274.
• Alternative: If preserving the native format long-term is critical, avoid Format.Native entirely for UDTs expected to evolve—use Format.UserDefined with explicit versioning fields from the start to prevent this issue on future schema changes.