Query interceptor in EF core allows developer to intercept the query before or after the execution. This provides the ability of interception, modification, or suppression of the query execution.

How to use Query Interceptor into code

EF core exposes DbCommandInterceptor base class, this base class is abstract class and has overridable methods which could be hooked into a class which is inherited DbCommandInterceptor class. Since Entity framework internally uses ADO.NET for database operations, DbCommandInterceptor class provides lifecycle methods for a query.

Code example

Create a class that inherits DbCommandInterceptor

public class QueryInterceptor :DbCommandInterceptor {
}

Override the lifecycle method which you are interested in

public class QueryInterceptor :DbCommandInterceptor {
    // runs before a query is executed
    public override InterceptionResult<DbDataReader> ReaderExecuting(DbCommand command,CommandEventData eventData, InterceptionResult<DbDataReader> result){
        Console.WriteLine($"Before Query execution. Query : {command.CommandText}");
        return result;
    }
    // runs after a query is excuted
    public override DbDataReader ReaderExecuted(DbCommand command, CommandExecutedEventData eventData, DbDataReader result) {
        Console.WriteLine($"After Query execution. Query : {command.CommandText}");
        return result;
    }        
}

DbCommandInterceptor provides async version of these methods to support asynchronous operations

public class QueryInterceptor :DbCommandInterceptor {
    public override ValueTask<InterceptionResult<DbDataReader>> ReaderExecutingAsync(DbCommand command, CommandEventData eventData, InterceptionResult<DbDataReader> result, CancellationToken cancellationToken = default){
        return base.ReaderExecutingAsync(command, eventData, result, cancellationToken);
    }
    public override ValueTask<DbDataReader> ReaderExecutedAsync(DbCommand command, CommandExecutedEventData eventData, DbDataReader result, CancellationToken cancellationToken = default){
        return base.ReaderExecutedAsync(command, eventData, result, cancellationToken);
    }
}

Register the interceptor with DbContext:

An interceptor could be added to context using two ways :

• register while adding DbContext to services

    services.AddDbContext<ExampleContext>(opt => 
      opt.UseNpgsql(cfg.GetConnectionString("DefaultConnectionString"))
         .AddInterceptors (new QueryInterceptor()));
  

• register inside OnConfiguring method of DbContext class

    public class ExampleContext : DbContext{
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder){
            base.OnConfiguring(optionsBuilder);
            optionsBuilder.AddInterceptors(new QueryInterceptor())
        }
    }
  

List of lifecycle methods available

Resources

• Query interception in Entity Framework Core - What’s keeping Lizzy busy? (lizzy-gallagher.github.io)

• Interceptors - EF Core | Microsoft Learn