IQueryHandler

IQueryHandler is a software interface used in the CQRS (Command Query Responsibility Segregation) pattern to separate the reading of data from its presentation. It defines how a query object is processed and a result is produced, allowing the query’s structure to be independent from the logic that executes it.

A typical IQueryHandler is generic and pairs a query type with a result type. A common form is a declaration such as: public interface IQueryHandler<TQuery, TResult> where TQuery : IQuery { Task<TResult> HandleAsync(TQuery query, CancellationToken cancellationToken); }. The TQuery represents the read operation to perform, while TResult is the data or data structure returned. Many implementations also use a non-generic variant in some frameworks, but the generic form is the most widely used in strongly typed languages.

In practice, IQueryHandler is often used with a mediator or dispatcher that routes each query to its corresponding handler. Queries are typically plain data objects that describe what information is requested, while the handlers contain the orchestration and data access logic required to fulfill the request. This enables decoupling, easier testing, and the ability to apply cross-cutting concerns through pipeline behaviors such as validation, logging, and caching.

Common considerations include supporting asynchronous processing, cancellation, and stateless, thread-safe handlers. While the pattern improves separation of concerns, it can introduce indirection and boilerplate, so teams balance the benefits against complexity. IQueryHandler implementations are central to query-centric architectures and are often paired with patterns like Mediator to streamline dispatch and execution.