How to fix Error 6264 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: SQL Server Error 6264 — “.NET Framework Routine Data Access” Failure

Why This Error Happens

This error occurs when a CLR (Common Language Runtime) stored procedure, function, or trigger attempts to access data without being properly decorated with the correct data access attribute. Specifically, the managed code method is missing the DataAccessKind.Read attribute (for stored procedures) or SystemDataAccessKind.Read attribute (for functions) in its SqlProcedure or SqlFunction declaration. This typically happens after a .NET assembly is recompiled, redeployed, or upgraded without the attribute being explicitly set, causing SQL Server 2025 to block the data access call at runtime as a security and integrity safeguard.

Step-by-Step Resolution Guide

Step 1: Locate the Problematic CLR Assembly

  • Identify which SQLCLR object is throwing Error 6264 by checking SQL Server error logs or Extended Events.
  • Run the following query to find the offending assembly and module:
SELECT a.name, m.assembly_class, m.assembly_method
FROM sys.assembly_modules m
JOIN sys.assemblies a ON m.assembly_id = a.assembly_id;
  • Note the assembly name, class, and method associated with the failing routine.

Step 2: Correct the Attribute in the Source Code

  • Open the original Visual Studio or .NET SDK project that generated the assembly.
  • Locate the method decorated with [SqlProcedure] or [SqlFunction].
  • Update the attribute declaration to explicitly include the correct data access kind:
[SqlProcedure(DataAccessKind = DataAccessKind.Read)]
public static void MyStoredProc()
{
    // data access logic
}
  • For scalar or table-valued functions, use:
[SqlFunction(SystemDataAccess = SystemDataAccessKind.Read)]

Step 3: Recompile and Redeploy the Assembly

  • Rebuild the project using the .NET SDK matching your SQL Server 2025 CLR integration version (verify via sys.dm_clr_properties).
  • Drop and re-register the assembly in SQL Server:
ALTER ASSEMBLY MyAssembly
FROM 'C:\Deploy\MyAssembly.dll'
WITH PERMISSION_SET = SAFE;
  • Ensure the DLL path is accessible from the SQL Server service account on Windows Server 2025.

Step 4: Verify CLR Integration and Permission Set

  • Confirm CLR integration is enabled:
EXEC sp_configure 'clr enabled', 1;
RECONFIGURE;
  • Check that the assembly’s PERMISSION_SET (SAFE, EXTERNAL_ACCESS, or UNSAFE) matches the level of data access required by the routine.
  • If the routine needs external data access, redeploy with PERMISSION_SET = EXTERNAL_ACCESS and ensure the assembly is signed or the database is marked TRUSTWORTHY only if organizational policy allows it.

Alternative or Verification Steps

  • Re-execute the stored procedure or function that previously failed and confirm Error 6264 no longer appears in sys.dm_exec_requests or the SQL Server error log.
  • If the error persists, use sn.exe (Strong Name Tool) to verify the assembly signature integrity, or temporarily redeploy with PERMISSION_SET = UNSAFE in a test environment to isolate whether the issue is attribute-related or permission-related.