Open Generic Handlers

• ID: mediator.open-generic-handlers
• Name: Open generic handlers
• Maturity: GA
• Summary: Registers one-parameter open generic handlers and closes them for each compatible concrete message type.

What It Does

When a type like OpenGenericPreHandler<TCommand> : ICommandPreHandler<TCommand> is registered, message registry stores it as an open generic handler definition. As concrete message types are registered, LiteBus closes the generic handler with MakeGenericType if generic constraints match.

Closed descriptors are inserted into normal handler collections and participate in priority, filtering, and pipeline semantics like non-generic handlers.

Unsupported open generic arity is rejected (UnsupportedOpenGenericHandlerException), and analyzer rule LB1005 covers the same shape at compile-time.

Public Surface

public sealed class CommandLoggingPreHandler<TCommand> : ICommandPreHandler<TCommand>
    where TCommand : ICommand
{
    public Task PreHandleAsync(TCommand message, CancellationToken cancellationToken = default)
        => Task.CompletedTask;
}

// Registration
builder.Register(typeof(CommandLoggingPreHandler<>));

Packages

• LiteBus.Messaging
• LiteBus.Messaging.Abstractions
• Semantic packages for concrete handler interfaces

Requires

• mediator.module-registration
• mediator.handler-pipeline
• mediator.generic-messages

Invariants

• Open generic handler shape supports exactly one type parameter.
• Generic constraints are enforced before closing.
• Registration order does not matter; handlers close for prior and future message registrations.
• Closed handlers receive the same routing and priority behavior as explicit handlers.

Non-Goals

• Multi-parameter open generic handler support.
• Automatic external library integration (for example FluentValidation adapters).
• Runtime re-closing after module composition completes.

Observability

No dedicated metrics for open generic close count or close latency exist.

Available signals:

• Runtime startup failure with UnsupportedOpenGenericHandlerException.
• Analyzer signal LB1005 in LiteBus.Analyzers.

Test Coverage

Covered

Untested

• Isolated open generic error-handler behavior for multiple message families.
• Open generic main handler scenarios with complex constraints and interface inheritance.

Out-of-Scope

• Multi-parameter open generic handler support.
• Hot path generic reflection (closing is startup/registration concern).

Deep Docs

• Open generic handlers
• Generic messages and handlers