How to fix Error 6255 on Windows Server 2025 / Windows 11

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Why This Error Happens

This error occurs when SQL Server’s CLR integration engine attempts to load a .NET assembly containing a User-Defined Type (UDT) or User-Defined Aggregate (UDA), but the target class is missing a required custom attribute (such as SqlUserDefinedType or SqlUserDefinedAggregate). SQL Server 2025 enforces strict validation against the Format.Native or Format.UserDefined specification, and without this metadata attribute, the CLR host cannot determine how to serialize or marshal the type. This is typically caused by a recompiled or corrupted DLL where the attribute decoration was stripped, commented out, or lost during a build pipeline update targeting .NET 8/9 runtimes.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Assembly and Type

  • Open the SQL Server Error Log via SSMS or C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG to capture the exact type name referenced in the Error 6255 message.
  • Run the following query to cross-reference the assembly:
SELECT a.name, att.* FROM sys.assemblies a
JOIN sys.assembly_types att ON a.assembly_id = att.assembly_id
WHERE att.name = 'YourTypeNameHere';

Step 2: Inspect and Repair the Source Code Attribute

  • Locate the original C# or VB.NET source project for the custom type.
  • Verify the class definition includes the mandatory attribute above the class declaration. For example, a UDT must look exactly like this:
[Serializable]
[Microsoft.SqlServer.Server.SqlUserDefinedType(Format.Native)]
public struct MyCustomType : INullable
  • If the attribute is missing or malformed (e.g., incorrect Format.UserDefined casing), add it back and ensure the using Microsoft.SqlServer.Server; namespace is referenced at the top of the file.

Step 3: Recompile and Re-register the Assembly

  • Rebuild the DLL using Visual Studio 2026 or the dotnet build CLI, ensuring the target framework matches the SQL CLR compatibility level (net9.0 or the specific LTS version supported by SQL Server 2025).
  • Drop the old assembly and type definitions in SQL Server to clear the corrupted metadata:
DROP TYPE dbo.MyCustomType;
DROP ASSEMBLY MyAssemblyName;
  • Re-import the newly compiled DLL using CREATE ASSEMBLY with the WITH PERMISSION_SET = SAFE clause.

Step 4: Validate CLR Strict Security Settings

  • Since SQL Server 2025 enforces clr strict security by default, ensure the assembly is signed with a certificate or asymmetric key and that a corresponding login has been granted UNSAFE ASSEMBLY permission if the type requires external access.
  • Run sp_configure 'clr strict security' to confirm the value is set to 1 (default) and that your assembly trust chain is valid.

Alternative or Verification Steps

  • Verify the fix by executing a SELECT statement or function call that invokes the specific UDT/UDA; if the attribute is correctly applied, the query will execute without throwing Error 6255.
  • If the error persists after redeployment, use ildasm.exe (IL Disassembler) on the DLL to manually confirm the custom attribute metadata was actually compiled into the binary, as sometimes conditional compilation flags (#if DEBUG) accidentally exclude the attribute in Release builds.