How to fix Error 6258 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6258 – FillRow Method Signature Mismatch

Why This Error Happens

This error occurs when the FillRow method referenced in a SqlFunctionAttribute.FillRowMethodName declaration has parameter types or an ordering that does not precisely match the columns defined in the RETURNS TABLE clause of the CLR table-valued function’s T-SQL declaration. SQL Server performs strict type-checking between the .NET method signature and the SQL schema at registration or execution time, and any mismatch—such as a SqlString parameter mapped to a SQL INT column, or columns listed in the wrong order—triggers this failure. This is typically caused by outdated assembly builds, manual editing of the CREATE FUNCTION script without updating the corresponding C#/VB.NET code, or a corrupted/mismatched DLL deployed to SQL Server 2025.

Step-by-Step Resolution Guide

Step 1: Review the FillRow Method Signature

  • Open the CLR project source code and locate the method specified in FillRowMethodName.
  • Confirm the number, order, and data types of parameters exactly match the RETURNS TABLE (...) column list in the T-SQL function definition.
  • Pay close attention to nullable types—SqlInt32 vs int, SqlString vs string, etc.—since implicit CLR types are not always compatible with SQL Server’s expected wrapper types.

Step 2: Correct the Mismatched Column Definition

  • Edit either the T-SQL CREATE FUNCTION script or the C#/VB.NET FillRow method so both sides align column-for-column.
  • Example fix in T-SQL:
RETURNS TABLE (ID INT, Name NVARCHAR(100))
  • Ensure the corresponding method signature reads:
public static void FillRow(object obj, out SqlInt32 ID, out SqlString Name)

Step 3: Rebuild and Redeploy the Assembly

  • Recompile the CLR project in Visual Studio targeting .NET Framework/assembly compatibility supported by SQL Server 2025.
  • Drop the existing assembly and function using:
DROP FUNCTION dbo.YourFunctionName;
DROP ASSEMBLY YourAssemblyName;
  • Re-register the updated assembly:
CREATE ASSEMBLY YourAssemblyName FROM 'C:\Path\To\YourAssembly.dll' WITH PERMISSION_SET = SAFE;
  • Re-create the function referencing the corrected FillRow method.

Step 4: Validate CLR Integration Settings

  • Confirm CLR integration is enabled on the instance:
EXEC sp_configure 'clr enabled', 1;
RECONFIGURE;
  • Check SQL Server error logs (C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG) for additional assembly load errors that may indicate versioning conflicts.

Alternative or Verification Steps

  • After redeployment, execute a test query against the table-valued function (e.g., SELECT * FROM dbo.YourFunctionName()) to confirm it returns data without raising Error 6258.
  • If the error persists, use sys.assembly_modules and sys.assembly_types to cross-verify that SQL Server’s cached metadata matches the newly deployed assembly, and restart the SQL Server service to clear any stale plan cache referencing the old signature.