How to fix Error 6565 on Windows Server 2025 / Windows 11

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Troubleshooting Guide: Error 6565 – Invalid Assembly Source Parameter

Why This Error Happens

Error 6565 occurs when SQL Server’s CREATE ASSEMBLY, ALTER ASSEMBLY, or similar DDL command receives a binary source that fails validation as a proper .NET assembly. This typically happens because the file path points to a corrupted DLL, a non-assembly file (such as a text file or incorrect binary), or a file compiled with an incompatible .NET runtime version. It can also occur if the byte stream was truncated during transfer, encoding was altered (e.g., opened and resaved as text), or the assembly was built targeting a CLR version not supported by SQL Server 2025’s hosted CLR integration.

Step-by-Step Resolution Guide

Step 1: Verify the Assembly File Integrity

  • Confirm the file path referenced in the CREATE ASSEMBLY statement actually points to a valid .dll file and not a shortcut, placeholder, or renamed file.
  • Open the file in a hex editor or use PowerShell to confirm it starts with the MZ header (PE executable signature):
  Format-Hex -Path "C:\Assemblies\MyAssembly.dll" -Count 2
  • If the header is missing or corrupted, re-copy the original compiled binary from your build server.

Step 2: Rebuild and Re-Target the Assembly Correctly

  • Recompile the source project in Visual Studio 2022/2025, ensuring the target framework is compatible with SQL Server 2025’s CLR hosting (typically .NET Framework 4.8 or the designated supported runtime for SQL CLR).
  • Avoid targeting .NET Core/.NET 5+ unless explicitly supported in your SQL Server 2025 build, as these produce assemblies SQL Server cannot load natively.
  • Ensure “Any CPU” or the correct platform target matches your SQL Server instance architecture (x64).

Step 3: Validate the Binary Transfer Method

  • If loading via FROM 'C:\Path\Assembly.dll', ensure the file was copied in binary mode (not FTP ASCII mode, which corrupts binaries).
  • If loading via a hex-encoded byte string in T-SQL, confirm no truncation occurred by checking the string length matches the original file size in bytes (multiplied by 2 for hex characters, plus the 0x prefix).
  • Re-run the command using the corrected path or byte array:
  CREATE ASSEMBLY MyAssembly
  FROM 'C:\Assemblies\MyAssembly.dll'
  WITH PERMISSION_SET = SAFE;

Step 4: Check CLR Strict Security and Trust Settings

  • Confirm clr strict security is properly configured, as SQL Server 2025 enforces stricter assembly trust validation by default.
  • Verify the assembly is signed and registered via sys.sp_add_trusted_assembly if using SAFE or EXTERNAL_ACCESS permission sets without a certificate-based trust.
  • Query trusted assemblies to confirm registration:
  SELECT * FROM sys.trusted_assemblies;

Alternative or Verification Steps

  • After re-running CREATE ASSEMBLY, verify success by querying SELECT * FROM sys.assemblies WHERE name = 'MyAssembly'; and confirm no error is returned.
  • If the issue persists, test loading the same DLL on a separate SQL Server instance or via sqlcmd to isolate whether the problem is file-specific or environment-specific (e.g., Windows Server 2025 file system permissions blocking binary reads).