How to fix Error 6214 on Windows Server 2025 / Windows 11

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Why This Error Happens

Error 6214 occurs when the SQL Server CLR host attempts to load an assembly that contains an unmanaged (native) entry point, which it is architecturally incapable of hosting. The SQLCLR execution engine only hosts pure MSIL (Microsoft Intermediate Language) assemblies. This error is raised when the target DLL is actually a mixed-mode assembly (commonly produced by C++/CLI projects without the /clr:pure equivalent), a native Win32 DLL mistakenly registered as a SQLCLR assembly, or a managed assembly that was corrupted/altered during build or transfer in a way that damaged its CLR metadata header. The failure is isolated to the specific assembly named in the message and does not indicate a broader SQL Server CLR subsystem fault.

Step-by-Step Resolution Guide

Step 1: Identify the Failing Assembly

  • Note the exact assembly name reported in the error text (the value substituted for %.*ls).
  • Locate the source DLL on disk. If you do not know the original path, query the existing (failed or partial) registration metadata using:
  SELECT name, permission_set_desc, create_date, clr_name
  FROM sys.assemblies
  WHERE name = 'YourAssemblyName';
  • If the assembly was never successfully registered, check your deployment script or Visual Studio project’s build output directory, typically C:\Projects\<ProjectName>\bin\Release\<AssemblyName>.dll or C:\Projects\<ProjectName>\bin\Debug\<AssemblyName>.dll.

Step 2: Confirm Whether the Assembly Is Managed

  • Use the corflags.exe utility, included with the Windows SDK (installed with Visual Studio), to inspect the CLR header of the DLL:
  corflags "C:\Projects\YourProject\bin\Release\YourAssembly.dll"
  • Review the output. If ILONLY reports 0, the assembly contains native code and is mixed-mode — this is the direct cause of Error 6214. A pure managed assembly eligible for SQLCLR hosting will report ILONLY : 1.
  • As a secondary check, open the DLL in ildasm.exe (ships with the .NET SDK / Windows SDK). If ildasm fails to load it as a managed module or shows native thunks, the file is not a valid SQLCLR candidate.

Step 3: Rebuild the Assembly as a Pure Managed .NET Assembly

  • If the source project is C++/CLI, mixed-mode compilation cannot produce a pure-IL output with modern MSVC toolchains (the old /clr:pure and /clr:safe switches were removed starting with Visual Studio 2015 and are not available in current compilers). The supported path is to port the logic to C# (or VB.NET) as a Class Library targeting .NET Framework, since SQL Server’s SQLCLR host does not support .NET Core/.NET 5+ assemblies.
  • In the project file, confirm the target framework is a supported .NET Framework version (e.g., net48) and that “Allow unsafe code” and any native interop/P-Invoke-only entry points are removed if they are the sole purpose of the assembly.
  • Rebuild from the command line using MSBuild:
  msbuild "C:\Projects\YourProject\YourProject.csproj" /p:Configuration=Release /t:Rebuild
  • Confirm the newly produced file at C:\Projects\YourProject\bin\Release\YourAssembly.dll has an updated timestamp matching the rebuild, not a stale cached copy from a prior build.

Step 4: Remove the Old Assembly and Redeploy

  • Drop the previously registered (or failed) assembly from the target database:
  DROP ASSEMBLY YourAssemblyName;
  • If the assembly has dependent CLR objects (functions, procedures, types, triggers) you must drop those first:
  DROP FUNCTION dbo.YourClrFunction;
  DROP ASSEMBLY YourAssemblyName;
  • Register the rebuilt, verified-managed assembly, specifying the correct permission set for your environment:
  CREATE ASSEMBLY YourAssemblyName
  FROM 'C:\Projects\YourProject\bin\Release\YourAssembly.dll'
  WITH PERMISSION_SET = SAFE;
  • If the assembly requires EXTERNAL_ACCESS or UNSAFE, note that SQL Server 2025 enforces clr strict security by default, meaning the assembly must be signed and the signing certificate (or an asymmetric key) must have a corresponding login granted UNSAFE ASSEMBLY permission:
  CREATE ASYMMETRIC KEY YourAssemblyKey
  FROM EXECUTABLE FILE = 'C:\Projects\YourProject\bin\Release\YourAssembly.dll';

  CREATE LOGIN YourAssemblyLogin FROM ASYMMETRIC KEY YourAssemblyKey;

  GRANT UNSAFE ASSEMBLY TO YourAssemblyLogin;

Step 5: Verify CLR Integration Is Enabled

  • Confirm the CLR setting on the SQL Server 2025 instance:
  EXEC sp_configure 'clr enabled';
  • If the run_value column shows 0, enable it:
  EXEC sp_configure 'clr enabled', 1;
  RECONFIGURE;
  • This setting takes effect immediately and does not require a service restart. Also verify clr strict security is set as expected for your security posture:
  EXEC sp_configure 'clr strict security';

Alternative or Verification Steps

  • Verify the fix: Re-run the original CREATE ASSEMBLY or deployment statement and confirm it completes without Error 6214. Then confirm registration succeeded:
  SELECT name, permission_set_desc, create_date
  FROM sys.assemblies
  WHERE name = 'YourAssemblyName';
  • Alternative if the primary steps fail: Check whether the file was modified or corrupted during transfer by comparing its hash against the original build output using PowerShell:
  Get-FileHash -Path "C:\Projects\YourProject\bin\Release\YourAssembly.dll" -Algorithm SHA256

Compare this value against the hash captured immediately after the original build (from your build server artifact log or a hash taken right after compilation). If the hashes differ, redeploy from a clean, untouched build output rather than a copied or transferred file.