How to fix Error 6941 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Microsoft SQL Server Error 6941 – “Invalid Type Definition”

Why This Error Happens

Error 6941 typically occurs when SQL Server encounters a corrupted or malformed CLR (Common Language Runtime) user-defined type (UDT) definition within the database metadata. This can result from an incomplete assembly deployment, a mismatch between the assembly version registered in SQL Server and the actual DLL on disk, or corruption in the system catalog views (sys.assemblies, sys.types) following an improper upgrade from a previous SQL Server version. In Windows Server 2025 environments, this is also frequently triggered by .NET runtime version conflicts after a Windows Update patches the CLR hosting environment.

Step-by-Step Resolution Guide

Step 1: Verify the CLR Assembly Integrity

  • Open SQL Server Management Studio (SSMS) and run SELECT * FROM sys.assemblies WHERE name = 'YourAssemblyName'; to confirm the assembly is registered.
  • Check the physical DLL path (commonly under C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Binn\) to ensure the file has not been deleted, renamed, or replaced with an incompatible version.
  • Run SELECT assembly_class, assembly_method FROM sys.assembly_types WHERE assembly_id = <id>; to cross-reference the type definitions against the assembly.

Step 2: Rebuild or Redeploy the User-Defined Type

  • Drop the affected type using DROP TYPE dbo.YourTypeName; (ensure no dependent objects exist first by checking sys.sql_dependencies).
  • Drop and re-add the assembly: DROP ASSEMBLY YourAssemblyName; CREATE ASSEMBLY YourAssemblyName FROM 'C:\Path\To\YourAssembly.dll' WITH PERMISSION_SET = SAFE;
  • Recreate the type definition: CREATE TYPE dbo.YourTypeName EXTERNAL NAME YourAssemblyName.[Namespace.ClassName];

Step 3: Check .NET Runtime Compatibility on Windows Server 2025

  • Navigate to Control Panel > Programs > Programs and Features and confirm the correct .NET Framework version (4.8.1 or higher) is installed alongside .NET 9 runtime components required by SQL Server 2025.
  • If a mismatch exists, repair the .NET installation via dotnet repair or reinstall the specific runtime from the Microsoft .NET download page.
  • Restart the SQL Server service via services.msc or run net stop MSSQLSERVER && net start MSSQLSERVER in an elevated Command Prompt.

Step 4: Run Database Consistency Checks

  • Execute DBCC CHECKDB('YourDatabaseName') WITH NO_INFOMSGS, ALL_ERRORMSGS; to scan for underlying catalog corruption.
  • If corruption is found in system tables, restore the database from the last known good backup rather than attempting an in-place repair, as REPAIR_ALLOW_DATA_LOSS does not reliably fix CLR type metadata.

Alternative or Verification Steps

  • To verify the fix, run the original query or stored procedure that triggered Error 6941 and confirm it executes without exceptions; also re-check sys.assembly_types to ensure the type resolves correctly.
  • If the error persists, as an alternative, export the data using bcp or SSIS to a new database with freshly created (non-CLR) native data types, bypassing the corrupted UDT entirely.