ARC0003: Handle() must be on [Command] type

Rule

Reports a public instance Handle() method on a non-command type when a parameter is a [Command] type. Move that behavior onto the command itself. Default severity: Error.

Diagnostic example

Compile these alternatives separately with Cratis.Arc.Core. The external handler deliberately triggers ARC0003. The command already has Handle() to avoid an unrelated ARC0004 diagnostic; it returns sample data, not persisted state.

using Cratis.Arc.Commands.ModelBound;

[Command]
public record NormalizeName(string Name)
{
    public string Handle() => Name;
}

public class NormalizeNameHandler
{
    public string Handle(NormalizeName command) => command.Name.Trim();
}

Remove the external handler and put its behavior on the command:

using Cratis.Arc.Commands.ModelBound;

[Command]
public record NormalizeName(string Name)
{
    public string Handle() => Name.Trim();
}

These declarations illustrate handler placement; they are not complete host programs. See model-bound commands for the execution model.