ARC0022: Invalid generated value parameter type

Rule

A [GeneratedValue] parameter must implement Cratis.Concepts.IGeneratable<TSelf> or IGeneratableInContext<TSelf> with its own type as TSelf. A static New() method alone is not enough. Arc rejects a reference parameter whose runtime nullable read state is Nullable, matching NullabilityInfoContext. Nullable<T> and by-reference parameters do not satisfy the generation contract. Default severity: Error.

Read state includes nullable flow attributes: [MaybeNull] can make a non-nullable reference parameter invalid, while [NotNull] can override a nullable reference annotation. [AllowNull] and [DisallowNull] affect write state instead and do not change this check. An oblivious reference parameter with unknown read state is not rejected for nullability. Prefer a plainly non-nullable declaration rather than relying on these overrides.

Diagnostic example

Compile these alternatives separately with Cratis.Arc.Core. The first intentionally produces ARC0022; Guid does not implement the generatable contract.

using System;
using Cratis.Arc.Commands.ModelBound;

[Command]
public record AllocateCartId
{
    public Guid Handle([GeneratedValue] Guid cartId) => cartId;
}

Fix

Declare the generation strategy on a non-nullable identifier type:

using System;
using Cratis.Arc.Commands.ModelBound;
using Cratis.Concepts;

public record CartId(Guid Value) : ConceptAs<Guid>(Value), IGeneratable<CartId>
{
    public static CartId New() => new(GenerateValue.Uuid());
}

[Command]
public record AllocateCartId
{
    public CartId Handle([GeneratedValue] CartId cartId) => cartId;
}

Explicit implementations of IGeneratable<CartId>.New() are also supported. Keep client-supplied identifiers on command properties rather than marking them generated. See generated values for resolution and validation behavior.