How to fix Error 6269 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Error 6269 — ALTER ASSEMBLY Fails Due to Undefined User-Defined Aggregate

Why This Error Happens

This error occurs when SQL Server attempts to reload an assembly during an ALTER ASSEMBLY operation but cannot locate a valid, correctly structured implementation of the user-defined aggregate (UDA) inside the new assembly binary. This typically happens because the updated DLL was compiled without the required aggregate class, the class is missing one or more mandatory methods (Init, Accumulate, Merge, Terminate), or the Serializable and SqlUserDefinedAggregate attributes were stripped or altered during a rebuild. It can also occur if the assembly metadata in SQL Server is out of sync with the actual compiled binary due to a partial deployment or versioning mismatch.

Step-by-Step Resolution Guide

Step 1: Verify the Aggregate Definition in Source Code

  • Open the .NET project (C# or VB.NET) containing the user-defined aggregate.
  • Confirm the class is decorated with [Serializable] and [SqlUserDefinedAggregate(Format.Native)] or Format.UserDefined attributes.
  • Ensure all four required methods exist with correct signatures: Init(), Accumulate(), Merge(), and Terminate().
  • Check that the class name in code exactly matches the name referenced in the CREATE AGGREGATE or ALTER ASSEMBLY T-SQL statement (case-sensitive).

Step 2: Rebuild and Validate the Assembly Binary

  • Recompile the project in Visual Studio 2022/2026 targeting the .NET Framework version supported by SQL Server 2025 (typically .NET Framework 4.8 or the CLR integration runtime specified in your instance).
  • Use ildasm.exe (IL Disassembler) on the compiled DLL to confirm the aggregate class and its methods are present in the IL output:
    ildasm.exe "C:\Builds\MyAggregates.dll"
  • Confirm there are no build warnings about stripped attributes or trimmed assemblies if using .NET trimming/publishing options.

Step 3: Re-deploy the Assembly Using Correct Syntax

  • Drop any dependent aggregate definitions before altering the assembly:
    DROP AGGREGATE dbo.MyAggregateName;
  • Re-run ALTER ASSEMBLY pointing to the newly built binary path:
    ALTER ASSEMBLY MyAssemblyName FROM 'C:\Builds\MyAggregates.dll' WITH PERMISSION_SET = SAFE;
  • Recreate the aggregate explicitly referencing the external name:
    CREATE AGGREGATE dbo.MyAggregateName (@value nvarchar(4000)) RETURNS nvarchar(4000) EXTERNAL NAME MyAssemblyName.[Namespace.ClassName];

Step 4: Check CLR Integration and Permission Set Configuration

  • Verify CLR integration is enabled on the instance:
    EXEC sp_configure 'clr enabled', 1; RECONFIGURE;
  • If using PERMISSION_SET = EXTERNAL_ACCESS or UNSAFE, confirm the assembly is signed and the login has UNSAFE ASSEMBLY permission or the database has TRUSTWORTHY set appropriately (only as a last resort, due to security implications).
  • On Windows Server 2025, confirm the SQL Server service account has read access to the folder containing the DLL if loading from a file path rather than binary stream.

Alternative or Verification Steps

  • Run SELECT * FROM sys.assembly_modules WHERE assembly_id = OBJECT_ID('MyAssemblyName'); to confirm the aggregate is now correctly linked to the assembly.
  • If the error persists, drop the assembly entirely (DROP ASSEMBLY MyAssemblyName;) and perform a clean CREATE ASSEMBLY followed by CREATE AGGREGATE instead of using ALTER ASSEMBLY, as this forces SQL Server to fully re-validate the binary structure rather than attempting an in-place metadata update.