How to fix Error 6273 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6273 — ALTER ASSEMBLY Type Mismatch

Why This Error Happens

This error occurs when the updated assembly (.DLL) being registered via ALTER ASSEMBLY no longer contains a type definition that is referenced by an existing SQLCLR object (such as a User-Defined Type, Function, or Aggregate) in the database. This typically happens due to a mismatch between the compiled assembly version and the schema metadata, often caused by renaming a class, changing a namespace, or deploying a stale/corrupted DLL build. SQL Server validates strict binary dependency mapping, so even a minor change in the type’s signature without updating dependent objects will trigger this failure.

Step-by-Step Resolution Guide

Step 1: Verify the Assembly Metadata and Dependencies

  • Query the system catalog to identify which SQL objects depend on the specific type mentioned in the error message.
  • Run the following T-SQL to check dependencies: SELECT * FROM sys.assembly_types WHERE assembly_id = (SELECT assembly_id FROM sys.assemblies WHERE name = 'YourAssemblyName');
  • Confirm if the class name or namespace in your Visual Studio project matches exactly what is listed in sys.assembly_types.

Step 2: Rebuild and Validate the Source DLL

  • Open the original Visual Studio project (ensure it targets the correct .NET framework compatible with SQL Server 2025).
  • Check for accidental renaming of public classes or namespace changes that would break the [Serializable] or [SqlUserDefinedType] attribute linkage.
  • Perform a Clean and Rebuild of the solution to eliminate any cached or corrupted intermediate build files in the /bin/Debug or /bin/Release folder.

Step 3: Drop and Recreate Dependent Objects

  • If the type structure has legitimately changed, you cannot simply “ALTER” the assembly; you must remove the dependency chain first.
  • Execute DROP TYPE [YourTypeName]; and DROP AGGREGATE [YourAggregateName]; for all objects dependent on the specific CLR type.
  • Re-run the CREATE ASSEMBLY command with the WITH PERMISSION_SET = SAFE (or appropriate set) followed by CREATE TYPE to rebind the objects correctly.

Step 4: Check for Byte-Code Corruption

  • If the error persists despite a successful local rebuild, the binary transferred to the server might be corrupted.
  • Use PowerShell on the Windows Server 2025 host to verify the file hash of the DLL matches the local build: Get-FileHash -Path "C:\Deploy\YourAssembly.dll" -Algorithm SHA256.
  • Ensure no antivirus or Windows Defender policy on the server is quarantining or altering the DLL file during the transfer/deployment process.

Alternative or Verification Steps

  • Verification: After re-creating the objects, run SELECT * FROM sys.assembly_types; again to confirm the assembly_id and type_name now correctly correlate with the new DLL version.
  • Alternative: If dropping objects is not feasible due to heavy production dependencies, export the data, perform a full DROP ASSEMBLY ... WITH NO DEPENDENTS cleanup, and restore the assembly from scratch in a maintenance window.