How to fix Error 6571 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6571 – “Class is Generic, Generic Types Are Not Supported”

Why This Error Happens

This error occurs when registering a SQLCLR (Common Language Runtime) assembly in SQL Server that contains a class marked as a UDT (User-Defined Type), UDF, or user-defined aggregate using generic type parameters (e.g., MyClass<T>). SQL Server’s CLR integration strictly requires non-generic, sealed classes for these objects because the SQL Server type system has no mechanism to resolve or bind generic type parameters at the T-SQL level. This typically results from a developer compiling a .NET assembly with generics enabled against SQL Server CLR attributes without accounting for SQL Server 2025’s stricter assembly validation during CREATE ASSEMBLY or CREATE TYPE operations.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Assembly and Class

  • Open SQL Server Management Studio (SSMS) and check the full error message for the assembly name and class name referenced in the %ls placeholders.
  • Run the following query to locate the problematic assembly in your database:
  SELECT name, assembly_id, clr_name FROM sys.assemblies WHERE name = 'YourAssemblyName';
  • Use a decompiler tool (such as ILSpy or dotPeek) on the .dll file to inspect the class definition and confirm it uses a generic type parameter (e.g., public class MyUDT<T> : INullable).

Step 2: Refactor the Source Code to Remove Generics

  • Open the Visual Studio (2022/2026) project that generated the SQLCLR assembly.
  • Locate the class decorated with [Serializable] and [SqlUserDefinedType] or [SqlFunction] attributes.
  • Remove the generic type parameter and replace it with a concrete, sealed, non-generic implementation. For example, change:
  public class MyUDT<T> : INullable

to:

  public sealed class MyUDT : INullable
  • Recompile the project to produce a clean, non-generic .dll.

Step 3: Drop and Re-register the Corrected Assembly

  • In SSMS, drop the old assembly and any dependent objects (functions, types, triggers) that reference it:
  DROP TYPE IF EXISTS dbo.MyUDT;
  DROP ASSEMBLY IF EXISTS YourAssemblyName;
  • Re-register the newly compiled assembly using CREATE ASSEMBLY, specifying the correct file path (e.g., C:\SQLAssemblies\YourAssembly.dll) or binary hex value:
  CREATE ASSEMBLY YourAssemblyName
  FROM 'C:\SQLAssemblies\YourAssembly.dll'
  WITH PERMISSION_SET = SAFE;

Step 4: Validate CLR Integration Settings

  • Confirm CLR integration is enabled on the instance, as SQL Server 2025 may reset this during major upgrades:
  EXEC sp_configure 'clr enabled', 1;
  RECONFIGURE;
  • Verify the clr strict security setting (enabled by default since SQL Server 2017+) isn’t blocking assembly loading due to unsigned/untrusted binaries, and sign the assembly with a certificate if required.

Alternative or Verification Steps

  • To verify the fix, re-run the CREATE TYPE or CREATE FUNCTION statement that previously failed; if successful, query sys.assembly_types or sys.assembly_modules to confirm the object is bound correctly without errors.
  • If refactoring the source code is not immediately possible, consider using a non-generic wrapper class that internally handles the generic logic in a separate, non-SQLCLR-exposed internal library, exposing only a concrete type to the SQL Server assembly registration.