Handler Duplicates

Header

• ID: analyzers.duplicate-command-handler, analyzers.duplicate-query-handler
• Diagnostics: LB1001 (Error), LB1010 (Error)
• Maturity: GA
• Summary: Reports when more than one main handler is discovered for the same command, query, or stream query message type.

Trigger Conditions

LB1001 (Duplicate Command Handler)

Reports when two or more handlers are registered for one command message type in the command pipeline:

• ICommandHandler<TCommand>
• ICommandHandler<TCommand, TResult>

LB1010 (Duplicate Query Handler)

Reports when two or more handlers are registered for one query type in either pipeline:

• Query pipeline: IQueryHandler<TQuery, TResult>
• Stream query pipeline: IStreamQueryHandler<TQuery, TResult>

Bad Example

using System.Threading;
using System.Threading.Tasks;
using LiteBus.Commands.Abstractions;

public sealed record CreateUserCommand(string Name) : ICommand;

public sealed class FirstCreateUserCommandHandler : ICommandHandler<CreateUserCommand>
{
    public Task HandleAsync(CreateUserCommand command, CancellationToken cancellationToken = default) => Task.CompletedTask;
}

public sealed class SecondCreateUserCommandHandler : ICommandHandler<CreateUserCommand>
{
    public Task HandleAsync(CreateUserCommand command, CancellationToken cancellationToken = default) => Task.CompletedTask;
}

Expected diagnostic:

• LB1001 on the second registration point.

Good Example

using System.Threading;
using System.Threading.Tasks;
using LiteBus.Queries.Abstractions;

public sealed record GetUserQuery(int UserId) : IQuery<string>;

public sealed class GetUserQueryHandler : IQueryHandler<GetUserQuery, string>
{
    public Task<string> HandleAsync(GetUserQuery query, CancellationToken cancellationToken = default)
        => Task.FromResult("Ada");
}

Suppression Guidance

• Do not suppress by default, this rule prevents runtime MultipleHandlerFoundException style ambiguity.
• If multiple handlers are intentional, split responsibilities into pre, post, or error handlers, or use event handlers.
• Keep one main handler per command and one per query type.

Test Coverage

Source: tests/LiteBus.Analyzers.UnitTests/DuplicateCommandHandlerAnalyzerTests.cs, tests/LiteBus.Analyzers.UnitTests/DuplicateQueryHandlerAnalyzerTests.cs

Untested in this suite:

• Duplicate stream query handler fixture for LB1010 (logic is implemented, dedicated stream duplicate fixture is not present).