---
title: 'ARC0022: Invalid generated value parameter type'
editUrl: https://github.com/Cratis/Arc/edit/main/Documentation/backend/csharp/code-analysis/ARC0022.md
description: Use a non-nullable type implementing IGeneratable for a generated Handle parameter.
---


## 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.

```csharp
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:

```csharp
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](/arc/backend/csharp/commands/generated-values/) for resolution and validation behavior.