How to fix Error 4159 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Error 4159 in Microsoft SQL Server 2025

Why This Error Happens

Error 4159 occurs when a query references a CLR user-defined type (UDT) in a computed context—such as a computed column, check constraint, or indexed view—where SQL Server defers (delays) the evaluation of the CLR instance, but that delayed instance implicitly depends on a column from the local base table that was not explicitly passed as an argument. This typically happens after upgrading to SQL Server 2025 or Windows Server 2025, where stricter CLR binding validation exposes previously tolerated schema-binding gaps in assemblies. It can also surface due to an outdated or improperly registered CLR assembly (.dll) that no longer aligns with the updated .NET runtime integration in SQL Server 2025.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Object

  • Run the following query to locate the computed column, constraint, or view causing the error:
  • SELECT object_name(object_id), name, definition FROM sys.computed_columns WHERE definition LIKE '%YourCLRType%';
  • Cross-reference the result with sys.assembly_types to confirm the CLR type involved.
  • Check SQL Server Error Log (C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG) for the exact object and line number tied to Error 4159.

Step 2: Correct the CLR Method Invocation

  • Open the definition of the computed column or constraint in SQL Server Management Studio (SSMS) 21.
  • Ensure the base table column is explicitly passed as a parameter to the CLR method, rather than relying on implicit binding:
  • Example fix: ALTER TABLE dbo.YourTable ADD ComputedCol AS dbo.YourCLRFunction(BaseColumn) PERSISTED;
  • Avoid referencing this or instance-level CLR state that implicitly pulls from the table without explicit parameterization.

Step 3: Rebuild and Redeploy the CLR Assembly

  • Recompile the SQLCLR assembly using the .NET 9 SDK (required baseline for SQL Server 2025 compatibility).
  • Drop and re-register the assembly:
  • ALTER ASSEMBLY YourAssembly FROM 'C:\SQLCLR\YourAssembly.dll' WITH PERMISSION_SET = SAFE;
  • Verify clr strict security is enabled and the assembly is properly signed with a certificate trusted in master.

Step 4: Rebuild Dependent Computed Columns/Indexes

  • Drop and recreate any persisted computed columns or indexed views that reference the corrected CLR type:
  • DROP INDEX IX_YourView ON dbo.YourView;
  • ALTER TABLE dbo.YourTable DROP COLUMN ComputedCol; then re-add with the corrected definition.
  • This forces SQL Server to re-validate the delayed CLR binding against the updated assembly.

Alternative or Verification Steps

  • Verify the fix by running DBCC CHECKCONSTRAINTS and executing a test SELECT against the affected computed column to confirm no Error 4159 is raised.
  • If the error persists, as a fallback, replace the CLR-based computed column with an equivalent native T-SQL scalar function (non-CLR), which bypasses delayed CLR instance evaluation entirely.