Troubleshooting Guide: Error 4175 — “Nested Updates Cannot Be Performed on CLR Types That Are Not Format.Structured”

Why This Error Happens

This error occurs when a CLR user-defined type (UDT) column is updated using a nested UPDATE syntax (e.g., UPDATE table.column.Mutator()) while the type itself is serialized using Format.Native or Format.UserDefined instead of Format.Structured. SQL Server 2025 requires structured format to support mutator methods and nested update operations because non-structured formats are treated as atomic binary blobs that cannot be partially modified in place. This typically stems from a CLR assembly or UDT definition that was built incorrectly, or a legacy assembly migrated from an older SQL Server version without updating its serialization attributes.

Step-by-Step Resolution Guide

Step 1: Identify the Offending CLR Type and Assembly

• Run the following query to locate the type causing the issue:

  SELECT name, is_user_defined, is_binary_ordered 
  FROM sys.types 
  WHERE is_user_defined = 1;

• Cross-reference the assembly backing the type using sys.assembly_types and sys.assemblies.
• Confirm the type’s serialization format by inspecting the [Serializable] and SqlUserDefinedType attributes in the original C# or VB.NET source code.

Step 2: Modify the CLR Type Definition to Use Format.Structured

• Open the source project for the UDT in Visual Studio 2026 (or your CLR development environment).
• Update the attribute declaration on the class to explicitly set structured format:

  [Serializable]
  [SqlUserDefinedType(Format.Structured, 
      IsByteOrdered = true, 
      MaxByteSize = 8000)]
  public struct MyCustomType : INullable, IBinarySerialize

• Ensure the type implements IBinarySerialize with proper Read and Write methods, as Format.Structured requires this interface for serialization control.
• Rebuild the assembly as a .dll targeting .NET 9.0 (compatible with SQL Server 2025 CLR integration).

Step 3: Drop and Re-register the Assembly in SQL Server

• Disable CLR strict security checks temporarily if blocked by signing requirements:

  EXEC sp_configure 'clr strict security', 0;
  RECONFIGURE;

• Drop the existing type and assembly:

  DROP TYPE dbo.MyCustomType;
  DROP ASSEMBLY MyAssembly;

• Re-register the updated assembly:

  CREATE ASSEMBLY MyAssembly 
  FROM 'C:\CLR_Assemblies\MyAssembly.dll'
  WITH PERMISSION_SET = SAFE;

  CREATE TYPE dbo.MyCustomType 
  EXTERNAL NAME MyAssembly.[MyCustomType];

• Restore clr strict security to its original value (typically 1) once registration succeeds.

Step 4: Rebuild Dependent Tables and Indexes

• Any table columns referencing the old UDT must be altered or recreated to bind to the newly structured type.
• Run DBCC CHECKDB against the affected database to confirm no corruption occurred during the type swap.
• Rebuild clustered indexes on tables containing the UDT column to refresh internal metadata:

  ALTER INDEX ALL ON dbo.YourTable REBUILD;

Alternative or Verification Steps

• Verify the fix by executing a nested update against the UDT column (e.g., UPDATE YourTable SET UdtColumn.MutatorMethod(value) WHERE Id = 1;) — it should complete without raising Error 4175.
• If reworking the CLR assembly is not feasible, use a workaround by retrieving the full UDT value, modifying it in application code or T-SQL variables, and performing a full column replacement (UPDATE YourTable SET UdtColumn = @NewValue) instead of a nested mutator call.