How to fix Error 4986 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Error 4986 — ALTER TABLE SWITCH CLR Type Mismatch

Why This Error Happens

This error occurs when a column defined with a CLR user-defined type (UDT) in the source table does not precisely match the assembly version, type definition, or binary layout of the same-named column in the target table during a partition switch operation. SQL Server 2025 performs strict metadata validation on CLR types during ALTER TABLE...SWITCH, so even a minor discrepancy—such as a different assembly GUID, version mismatch, or a UDT registered from a different assembly build—will trigger this failure. This is commonly caused by one table referencing a CLR assembly that was redeployed, updated, or registered independently of the other, resulting in two logically similar but technically distinct type definitions.

Step-by-Step Resolution Guide

Step 1: Identify the Mismatched Column and CLR Type

  • Run the following query to inspect the offending column’s type on both tables:
  • SELECT c.name, t.name AS type_name, t.system_type_id, t.user_type_id FROM sys.columns c JOIN sys.types t ON c.user_type_id = t.user_type_id WHERE c.object_id = OBJECT_ID('SourceTable') OR c.object_id = OBJECT_ID('TargetTable');
  • Confirm both columns reference the same user_type_id and assembly name.

Step 2: Compare CLR Assembly Versions Between Tables

  • Query sys.assemblies and sys.assembly_types to check the version, culture, and public key token of the assembly backing each UDT:
  • SELECT a.name, a.clr_name, at.name AS type_name FROM sys.assemblies a JOIN sys.assembly_types at ON a.assembly_id = at.assembly_id;
  • If the source and target databases show different clr_name version strings, this confirms the mismatch.

Step 3: Re-register the CLR Assembly Consistently

  • Drop and re-create the UDT and its underlying assembly using the exact same .dll file for both tables:
  • DROP TYPE dbo.YourUDT; DROP ASSEMBLY YourAssemblyName;
  • CREATE ASSEMBLY YourAssemblyName FROM 'C:\SQLAssemblies\YourAssembly.dll' WITH PERMISSION_SET = SAFE;
  • CREATE TYPE dbo.YourUDT EXTERNAL NAME YourAssemblyName.[YourUDTClass];
  • Re-apply this identical assembly binary across both the source and target table definitions.

Step 4: Rebuild the Column Using the Corrected Type

  • Alter the column on whichever table has the outdated or mismatched type reference:
  • ALTER TABLE TargetTable ALTER COLUMN YourColumn dbo.YourUDT;
  • Ensure no open transactions or schema locks are blocking the alteration before executing.

Step 5: Retry the Partition Switch Operation

  • Re-run the original command:
  • ALTER TABLE SourceTable SWITCH PARTITION x TO TargetTable PARTITION y;
  • Confirm the operation completes without raising Error 4986.

Alternative or Verification Steps

  • Verify the fix by querying sys.assembly_types on both tables and confirming identical assembly_id and user_type_id values post-fix.
  • If the error persists, drop both UDT columns entirely and recreate them from a single centralized assembly deployment script to guarantee binary-level consistency across environments.