How to fix Error 3862 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 3862 — CLR Type Disabled Due to On-Disk Format Change

Why This Error Happens

This error occurs when a CQRS/CLR user-defined type (UDT) was serialized to disk using an older version of its assembly, and a subsequent assembly update (via ALTER ASSEMBLY) changed the binary serialization format, rendering existing on-disk data incompatible. SQL Server disables the type to prevent data corruption or misinterpretation of the stored binary representation. This commonly happens after a CLR assembly redeployment, a Windows Server 2025 cumulative update, or an in-place upgrade where the underlying .NET CLR integration libraries were modified without first exporting dependent data.

Step-by-Step Resolution Guide

Step 1: Identify the Disabled Type and Dependent Objects

  • Run the following query to locate affected types:
  SELECT name, is_binary_format_disabled 
  FROM sys.types 
  WHERE is_binary_format_disabled = 1;
  • Check sys.assembly_types and sys.columns to find every table/column referencing this CLR type.
  • Note the schema and type name exactly as reported in Error 3862 (case-sensitive).

Step 2: Export Data Before Making Changes

  • Before dropping anything, export all affected column data to a staging table or flat file using BCP or SELECT INTO:
  SELECT * INTO dbo.Staging_CLRBackup FROM YourTable;
  • Verify row counts match the source table to confirm a complete backup.
  • Store the export on a separate volume (e.g., D:\SQLBackups\CLRExport\) in case rollback is needed.

Step 3: Drop and Recreate the Disabled Type

  • Remove all dependent columns/objects referencing the disabled type first.
  • Drop the type using:
  DROP TYPE dbo.YourCLRType;
  • Reload the correct assembly version using CREATE ASSEMBLY with WITH PERMISSION_SET = SAFE (or UNSAFE if required) from the validated .dll path, e.g., C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Binn\.
  • Recreate the type with CREATE TYPE dbo.YourCLRType EXTERNAL NAME YourAssembly.[YourClass];.

Step 4: Restore Data and Rebuild Dependencies

  • Re-import the staged data from Step 2 into the newly created columns using the restored CLR type.
  • Recreate any indexes, constraints, or stored procedures that referenced the original type.
  • Run DBCC CHECKDB on the affected database to confirm structural integrity post-migration.

Alternative or Verification Steps

  • Verify the fix by re-running the sys.types query from Step 1 — the is_binary_format_disabled column should now return 0 for the type.
  • If data loss is unacceptable and the original assembly binary is unavailable, restore the database from a pre-upgrade backup taken before the ALTER ASSEMBLY operation, then reapply schema changes in a staged (non-production) environment first.