How to fix Error 6245 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Error 6245 – Invalid Serialization Format

Why This Error Happens

Error 6245 occurs when SQL Server 2025 encounters a Common Language Runtime (CLR) user-defined type (UDT) or user-defined aggregate whose binary serialization format does not match what the engine expects. This typically results from a mismatched or corrupted CLR assembly version, a database restored or attached from an incompatible SQL Server build, or manual tampering with the FORMAT property (Native vs. UserDefined) during type registration. It can also surface after an in-place upgrade where legacy assemblies were not recompiled against the current .NET runtime bundled with SQL Server 2025.

Step-by-Step Resolution Guide

Step 1: Identify the Affected Assembly and Type

  • Run the following query to locate the problematic type:
  • SELECT name, assembly_id, is_binary_ordered, is_fixed_length FROM sys.types WHERE is_assembly_type = 1;
  • Cross-reference the assembly using SELECT * FROM sys.assemblies WHERE assembly_id = <id>;
  • Note the exact %.*ls.%.*ls schema and type name reported in the error message.

Step 2: Validate and Rebuild the CLR Assembly

  • Open the original Visual Studio / .NET project source for the UDT.
  • Confirm the [Serializable] and SqlUserDefinedType attributes explicitly declare Format.Native or Format.UserDefined correctly matching the type’s structure.
  • Rebuild the assembly targeting the .NET version supported by SQL Server 2025 (check sys.dm_os_loaded_modules for runtime compatibility).
  • Drop and re-register the assembly using:
  • ALTER ASSEMBLY [YourAssemblyName] FROM 'C:\Assemblies\YourAssembly.dll' WITH PERMISSION_SET = SAFE;

Step 3: Repair or Recreate the User-Defined Type

  • If rebuilding does not resolve it, drop the dependent columns/tables referencing the UDT (back up data first).
  • Drop the corrupted type: DROP TYPE [dbo].[YourUDTName];
  • Re-create the type against the newly validated assembly:
  • CREATE TYPE [dbo].[YourUDTName] EXTERNAL NAME [YourAssemblyName].[YourClassName];

Step 4: Check for Version Mismatch After Upgrade/Restore

  • If this appeared after a database migration, verify compatibility level:
  • SELECT compatibility_level FROM sys.databases WHERE name = 'YourDB';
  • Update it to the SQL Server 2025 supported level: ALTER DATABASE [YourDB] SET COMPATIBILITY_LEVEL = 170;
  • Re-run DBCC CHECKDB('YourDB') WITH DATA_PURITY; to confirm no residual structural corruption remains.

Alternative or Verification Steps

  • Verify the fix by inserting/selecting test data into a table using the repaired UDT column (INSERT INTO TestTable (UDTColumn) VALUES (...)) and confirm no Error 6245 is thrown.
  • If the issue persists, restore the database from a known-good backup taken prior to the assembly corruption, then reapply only the transaction log changes made after validating the assembly separately in a test environment.