How to fix Error 10331 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 10331 – CLR Type Generic Inheritance Not Supported

Why This Error Happens

This error occurs when a SQLCLR assembly registered in SQL Server 2025 contains a type marked with [SqlUserDefinedType] or [Serializable] that derives from a generic base class (e.g., MyType<T>). SQL Server’s CLR integration engine does not support generic types as the basis for User-Defined Types (UDTs) or User-Defined Aggregates (UDAs) because it cannot resolve the generic type arguments at the SQL metadata level. This typically happens after a developer updates a .NET assembly to use generics for code reuse, then attempts to register or alter that assembly in SQL Server without refactoring the type hierarchy.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Assembly and Type

  • Run the following query to locate the problematic assembly referenced in the error:
SELECT a.name AS AssemblyName, a.permission_set_desc, m.definition
FROM sys.assemblies a
JOIN sys.assembly_modules m ON a.assembly_id = m.assembly_id;
  • Cross-reference the %ls type name from the error message with your Visual Studio SQLCLR project to find the exact class definition.
  • Open the source .cs or .vb file and confirm the class signature includes a generic base, such as public class SensorData : BaseReading<T>.

Step 2: Refactor the Generic Type to a Concrete Implementation

  • Remove the generic parameter <T> from the class marked with [SqlUserDefinedType(Format.Native)] or [Serializable].
  • Create a concrete, non-generic class that either implements the required interface directly or inherits from a non-generic base class.
  • Example fix:
// Unsupported
public class MyUDT<T> : INullable { ... }

// Supported
public class MyUDT : INullable { ... }
  • Recompile the assembly in Visual Studio 2022/2026 targeting .NET Framework 4.8.1 or the .NET version supported by SQL Server 2025 CLR integration.

Step 3: Drop and Re-register the Corrected Assembly

  • In SQL Server Management Studio (SSMS), drop the existing broken assembly reference:
DROP TYPE IF EXISTS dbo.MyUDT;
DROP ASSEMBLY IF EXISTS MyAssemblyName;
  • Re-add the corrected, compiled DLL using CREATE ASSEMBLY, pointing to the updated binary path:
CREATE ASSEMBLY MyAssemblyName
FROM 'C:\SQLCLR\MyAssembly.dll'
WITH PERMISSION_SET = SAFE;
  • Re-create the UDT definition:
CREATE TYPE dbo.MyUDT EXTERNAL NAME MyAssemblyName.[Namespace.MyUDT];

Step 4: Validate CLR Integration Settings

  • Ensure CLR integration is enabled on the instance, as Windows Server 2025 security defaults may disable it post-patching:
EXEC sp_configure 'clr enabled', 1;
RECONFIGURE;
  • Verify the clr strict security setting (enabled by default since SQL Server 2017+) is not blocking the load due to an unsigned assembly, and sign the assembly with a certificate if required.

Alternative or Verification Steps

  • Verification: Execute SELECT * FROM sys.assembly_types WHERE name = 'MyUDT'; to confirm the type now appears in the system catalog without throwing a registration error.
  • Alternative: If refactoring the generic class is not feasible due to legacy dependencies, implement the Adapter design pattern—wrap the generic class inside a non-generic “shell” class that exposes only the specific type (e.g., int or string) required by the SQL Server schema, then register the shell class as the UDT instead.