cratis-arc-command

Define an Arc command

A command is a record that carries the user's intent and owns its own handler. Arc discovers it, runs authorization and validation, calls Handle(), and turns whatever Handle() returns into appended events, a response, or both.

Verified product sources

Package	Version	Purpose
Cratis.Arc.Core	22.10.4	Cratis.Arc.Commands.ModelBound.CommandAttribute, CommandResult, CommandValidator<T>
Cratis.Arc.Chronicle	22.10.4	event append handling, Cratis.Arc.Chronicle.Commands.NotAuditedAttribute
Cratis.Arc.ProxyGenerator.Build	22.10.4	CratisProxiesOutputPath MSBuild integration
Cratis.Fundamentals	7.18.2	Cratis.Monads.Result<TResult, TError>
Cratis.Chronicle	16.39.1	EventTypeAttribute, EventForEventSourceId, ICanProvideEventSourceId
@cratis/arc	22.10.4	ICommand, CommandResult, ValidationResult

Reverify before claiming support for another version. Arc without Chronicle is a supported setup; everything on this page that appends events needs Chronicle.

Declare the command

// Copyright (c) Cratis. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

using Cratis.Arc.Commands.ModelBound;
using Cratis.Chronicle.Events;

namespace <NamespaceRoot>.<Feature>;

[Command]
public record <ImperativeName>(<ConceptType> <Name>, <ConceptType> <Name>)
{
    public <EventType> Handle() => new(<Name>);
}

[EventType]
public record <EventType>(<ConceptType> <Name>);

Rules the framework actually enforces:

• [Command] is Cratis.Arc.Commands.ModelBound.CommandAttribute and applies to a class declaration only. A type counts as a command when it carries [Command] and declares a Handle method; either one alone is not enough.
• Handle is found by name on the command type itself. ARC0003 is an error when another class defines Handle for a command, and ARC0004 is an error when a [Command] type has no public instance Handle.
• ARC0002 warns when a type has Handle but no [Command]; ARC0007 warns when a command is a class rather than a record.
• Name the command as the action — OpenDebitAccount, not OpenDebitAccountCommand.
• Handle parameters are resolved from DI, so ask for services there rather than through a constructor.
• Never inject IEventLog into Handle to append the command's own events — ARCCHR0007 warns, and it bypasses Arc's append pipeline. Express the append through the return type.