COMPOSITE0003.md

COMPOSITE0003: Type must be partial

Property	Value
Rule ID	COMPOSITE0003
Category	CompositeKey.SourceGeneration
Severity	Error

Cause

The type annotated with [CompositeKey] (and/or its containing types) is not declared as partial. The source generator needs the partial modifier to emit additional members into the type.

How to fix

Add the partial modifier to the type and all containing types:

// Before (error)
[CompositeKey("{Id}#{Name}")]
public sealed record MyKey(Guid Id, string Name);

// After (fixed)
[CompositeKey("{Id}#{Name}")]
public sealed partial record MyKey(Guid Id, string Name);

If the type is nested, the containing type must also be partial:

public partial class Outer
{
    [CompositeKey("{Id}")]
    public sealed partial record MyKey(Guid Id);
}