---
title: 'ARC0023: Generated value declared outside a command Handle method'
editUrl: https://github.com/Cratis/Arc/edit/main/Documentation/backend/csharp/code-analysis/ARC0023.md
description: Declare generated values only on instance Handle parameters of model-bound commands.
---


## Rule

ARC0023 reports `[GeneratedValue]` where the declaration is invalid regardless of
which commands exist in this or another assembly:

- The method is not named `Handle`, including `Provide` and other method names.
- The method is static, including a static `Handle`.
- The method belongs to a `[ReadModel]` type, regardless of its name.
- The method belongs to a controller: a type deriving from
  `Microsoft.AspNetCore.Mvc.ControllerBase`, or marked `[ApiController]` or `[Controller]`.
  An action named `Handle` is still reported.

Inherited read-model and controller declarations are recognized.
**Default severity: Error.**

An instance `Handle()` that no command uses is **not reported**. Its attribute is inert:
the runtime never reads it. This applies to public base types, sealed non-command types,
and inaccessible types alike. The analyzer does not try to predict whether another
assembly will define a command subclass.

Only a model-bound command's effective `Handle()` participates in generation. When a
command uses an inherited handler, ARC0022, ARC0024, and ARC0025 still check that command's
generated parameters.

## Diagnostic example

The examples use this identifier, the same declaration as in
[generated values](/arc/backend/csharp/commands/generated-values/):

```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());
}
```

Compile each alternative below together with it, one at a time; the first
intentionally produces ARC0023.

```csharp
[Command]
public record AllocateCartId
{
    public CartId Provide([GeneratedValue] CartId cartId) => cartId;

    public CartId Handle(CartId cartId) => cartId;
}
```

## Fix

Move generation to the handler parameter:

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

For an ordinary service or read model dependency, remove `[GeneratedValue]` instead.
Arc resolves those dependencies through the normal Provide and dependency injection path.