How to fix Error 6213 on Windows Server 2025 / Windows 11

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SQL Server 2025 Error 6213: CLR Assembly Synchronized Attribute Failure

Environment: Microsoft SQL Server 2025 on Windows Server 2025 / Windows 11

Why This Error Happens

SQL Server’s CLR integration (SQLCLR) hosts .NET assemblies inside the Database Engine process using a custom application domain model that manages thread scheduling directly through SQLOS. Explicit synchronization via the [MethodImpl(MethodImplOptions.Synchronized)] attribute (or the C# lock/VB SyncLock compiler-generated monitor wrapper applied at the method level) is blocked because it allows a thread to hold an OS-level monitor lock while yielding control back to SQLOS, which can create unpredictable blocking and deadlock conditions inside the engine. This restriction applies regardless of the assembly’s PERMISSION_SET (SAFE, EXTERNAL_ACCESS, or UNSAFE).

When the engine binds the assembly during CREATE ASSEMBLY or ALTER ASSEMBLY, it walks the metadata of every type and method. If it finds the Synchronized bit set in the method’s IL attributes, it raises Error 6213 and aborts the operation. Because the error text is a templated string, the actual method name, type name, and assembly name are substituted at runtime by the engine — you must capture the fully rendered message from your own deployment output (SSMS, sqlcmd, or SqlPackage log) to identify the exact offending member.

Step-by-Step Resolution Guide

Step 1: Identify the Offending Method

  • Capture the complete rendered error text from the deployment output (SSMS Messages pane, sqlcmd console output, or SqlPackage.exe deployment log) and record the exact method name, type name, and assembly name reported.
  • Open the Visual Studio SQL Server Database Project or class library solution that produces the assembly, typically located under a path such as C:\Source\Repos\<ProjectName>\<ProjectName>.csproj.
  • Use Find in Files (Ctrl+Shift+F) in Visual Studio to search the solution for MethodImplOptions.Synchronized and for the lock ( / SyncLock keyword, then cross-reference the results against the type and method names from the error text.

Step 2: Remove the Synchronized Attribute From the Method

  • Remove the [MethodImpl(MethodImplOptions.Synchronized)] attribute from the method declaration, or remove the lock (object) / SyncLock block if the compiler generated the synchronized method wrapper.
  • Replace the removed synchronization with a pattern supported inside SQLCLR, such as:
  • Avoiding shared mutable state entirely within the method.
  • Using System.Threading.Interlocked operations for simple counters/flags.
  • Using System.Threading.Monitor.TryEnter / Monitor.Exit wrapped in a manual try/finally block instead of the compiler’s synchronized method attribute (note: EXTERNAL_ACCESS/UNSAFE assemblies have more latitude here, but SAFE assemblies should avoid threading constructs altogether).
  • Save the file, then re-run the Find in Files search to confirm no other method in the assembly still carries the synchronized attribute.

Step 3: Rebuild the Assembly

  • From a Visual Studio Developer Command Prompt on Windows Server 2025 / Windows 11, rebuild the project with:
  msbuild "C:\Source\Repos\<ProjectName>\<ProjectName>.csproj" /t:Rebuild /p:Configuration=Release
  • Confirm the compiled output DLL is written to C:\Source\Repos\<ProjectName>\bin\Release\<ProjectName>.dll and that the MSBuild output reports 0 Error(s).
  • Optionally confirm the attribute is gone at the IL level using ildasm:
  ildasm "C:\Source\Repos\<ProjectName>\bin\Release\<ProjectName>.dll"

and verify the target method no longer shows .method flagged with synchronized in its IL signature.

Step 4: Redeploy the Assembly to SQL Server 2025

  • Connect to the target instance using SSMS or sqlcmd, for example:
  sqlcmd -S <ServerName>\<InstanceName> -d <DatabaseName> -E
  • If the assembly already exists in the database, update it in place with ALTER ASSEMBLY (this preserves dependent object bindings):
  ALTER ASSEMBLY [<AssemblyName>]
  FROM 'C:\Source\Repos\<ProjectName>\bin\Release\<ProjectName>.dll'
  WITH PERMISSION_SET = SAFE;
  • If this is the first deployment, create it instead:
  CREATE ASSEMBLY [<AssemblyName>]
  FROM 'C:\Source\Repos\<ProjectName>\bin\Release\<ProjectName>.dll'
  WITH PERMISSION_SET = SAFE;
  • Confirm the statement completes successfully with Command(s) completed successfully. and no Error 6213 is raised.

Step 5: Confirm Dependent Objects Still Bind

  • Query sys.assemblies and sys.assembly_modules to confirm the assembly registered correctly and to list every CLR procedure, function, or trigger bound to it:
  SELECT a.name AS assembly_name, am.assembly_class, am.assembly_method, o.name AS sql_object_name
  FROM sys.assembly_modules am
  JOIN sys.assemblies a ON am.assembly_id = a.assembly_id
  JOIN sys.objects o ON am.object_id = o.object_id
  WHERE a.name = '<AssemblyName>';
  • Force each dependent module to re-resolve against the updated assembly metadata:
  EXEC sp_refreshsqlmodule N'dbo.<DependentProcedureName>';
  • Execute each affected stored procedure/function with representative test parameters to confirm functional behavior matches expectations post-fix.

Alternative or Verification Steps

  • Verify the fix: Re-run the original deployment script, SSDT publish, or SqlPackage.exe publish action end-to-end. A successful completion with no Error 6213 in the output log confirms the synchronized attribute has been fully removed from the deployed assembly.
  • Alternative if the primary steps fail: If Error 6213 persists after redeployment, confirm the DLL actually loaded into SQL Server is the newly built one and not a stale copy from a prior build or deployment script. Compare file hashes between the build output and the binary SQL Server has on record:
  certutil -hashfile "C:\Source\Repos\<ProjectName>\bin\Release\<ProjectName>.dll" SHA256

against:

  SELECT a.name, af.content
  FROM sys.assemblies a
  JOIN sys.assembly_files af ON a.assembly_id = af.assembly_id
  WHERE a.name = '<AssemblyName>';

Also check for an automated CI/CD pipeline, DACPAC, or legacy deployment script that might be silently redeploying an older, unmodified version of the assembly after your manual fix, and update or disable that stale deployment source before retrying.