Queries

To be able to read data, this library has support for queries that are fetched into a single and required query handler using generic variance.

Components

The following interfaces and abstract classes are used to implement the query pattern:

Interface/Class	Description
IQuery<TResult>	interface that represents a query that will have an operation result
Query<TResult>	abstract class that implements the IQuery<TResult> interface
IQueryHandler<TQuery,TResult>	interface that will handle asynchronously a TQuery instance extending from IQuery<TResult> and fetching the query data
QueryHandlerNotFoundException	exception thrown if no handler is found by the mediator when fetching a query

Query properties

The following properties are included in the query interfaces:

Property	Description
Id	required unique identifier for each query. It may be used to detect query duplication, allowing to fetch the data from a cache or just for logging purposes. The abstract query classes will assign a Guid.NewGuid() value
CreatedOn	required date and time in which the query was created. The abstract query classes will assign a DateTimeOffset.Now value
CreatedBy	a string identifier for the user who created the query

Creating a query

To create a query just implement the IQuery<TResult> interface or corresponding abstract class:

public class UserByIdQuery<User> : Query<User>, IQuery<User> {
  public Guid UserId { get; set; }
}

public class User {
  public string Email { get; set; }
}

Handling a query

To handle a query just implement the IQueryHandler<TQuery,TResult> interface:

public class UserQueryHandler : IQueryHandler<UserByIdQuery,User> {
  public async Task<User> HandleAsync(UserByIdQuery query, CancellationToken ct) {
    User user = null;
    // try to get the user from the store
    return user;
  }
}

Fetching a query

To fetch the data from a query, you only need the mediator instance and use the FetchAsync method:

var user = await _mediator.FetchAsync<UserByIdQuery,User>(new UserByIdQuery {
  UserId = Guid.Parse("6ca84be0-d509-4263-9e06-f255ecb4144c")
}, ct);