Building a C# Web API with Onion Architecture, CQRS, MediatR & Quartz.NET CRON Jobs

 

Introduction

In modern C# applications, clean architecture plays a crucial role in maintaining scalability and maintainability. In this blog, we will build a C# Web API using Onion Architecture, CQRS, MediatR, and Quartz.NET for scheduling background jobs. This guide will walk you through structuring a project with best practices and SOLID principles.

Why Use Onion Architecture?

Onion Architecture provides a clear separation of concerns by organizing the application into different layers:

1. Domain Layer → Contains core business logic and entities

2. Application Layer → Contains CQRS commands, queries, and interfaces

3. Infrastructure Layer → Handles database, repository pattern, and external integrations

4. Presentation Layer (API Layer) → Exposes the application as an API

Tech Stack

• ASP.NET Core Web API for building RESTful APIs

• Entity Framework Core for database operations

• CQRS Pattern for separating read & write operations

• MediatR for in-memory messaging

• Quartz.NET for CRON Job scheduling

• SOLID Principles to make the application scalable
  
  

  📌 Project Structure

  Your project will be structured as follows:
  
  

  Architecture Snippet

  Solution 'OnionArchitectureCQRS' │── Onion.Application (Application Layer) │ ├── Features/ │ │ ├── Students/ │ │ │ ├── Commands/ │ │ │ │ ├── CreateStudentCommand.cs │ │ │ ├── Queries/ │ │ │ │ ├── GetAllStudentsQuery.cs │ │ │ ├── Handlers/ │ │ │ │ ├── CreateStudentCommandHandler.cs │ │ │ │ ├── GetAllStudentsQueryHandler.cs │ ├── Interfaces/ │ │ ├── IStudentRepository.cs │── Onion.Domain (Domain Layer) │ ├── Models/ │ │ ├── Student.cs │── Onion.Infrastructure (Infrastructure Layer) │ ├── Repositories/ │ │ ├── StudentRepository.cs │ ├── Scheduler/ │ │ ├── CronJobService.cs │── Onion.API (Presentation Layer - Web API) │ ├── Controllers/ │ │ ├── StudentsController.cs │ ├── Program.cs
  Step 1: Create the Student Model (Domain Layer)

  📍 Location: Onion.Domain/Models/Student.cs


  Model Snippet

  public class Student { public int Id { get; set; } public string Name { get; set; } public string Description { get; set; } public DateTime CreatedOn { get; set; } }
  Step 2: Create CQRS Commands & Queries (Application Layer)

  ✅ Command to Create a Student

  📍 Location: Onion.Application/Features/Students/Commands/CreateStudentCommand.cs


  Code Snippet

  using MediatR; public class CreateStudentCommand : IRequest<int> { public string Name { get; set; } public string Description { get; set; } }
  ✅ Query to Get All Students

  📍 Location: Onion.Application/Features/Students/Queries/GetAllStudentsQuery.cs



  Code Snippet

  using Onion.Domain.Models; using MediatR; using System.Collections.Generic; public class GetAllStudentsQuery : IRequest<List<Student>> { }

  Step 3: Implement Repository (Infrastructure Layer)

  📍 Location: Onion.Infrastructure/Repositories/StudentRepository.cs


  Code Snippet

  using Onion.Domain.Models; using Microsoft.EntityFrameworkCore; using System.Collections.Generic; using System.Threading.Tasks; public class StudentRepository : IStudentRepository { private readonly AppDbContext _context; public StudentRepository(AppDbContext context) { _context = context; } public async Task<int> AddStudentAsync(Student student) { _context.Students.Add(student); await _context.SaveChangesAsync(); return student.Id; } }
  Step 4: Implement CRON Job Using Quartz.NET

  📍 Location: Onion.Infrastructure/Scheduler/CronJobService.cs


  Code Snippet

  using Quartz; using System; using System.Threading; using System.Threading.Tasks; public class CronJobService : IHostedService { private readonly IScheduler _scheduler; public CronJobService(IScheduler scheduler) { _scheduler = scheduler; } public async Task StartAsync(CancellationToken cancellationToken) { await _scheduler.Start(); var job = JobBuilder.Create<AddStudentsJob>() .WithIdentity("AddStudentsJob") .Build(); var trigger = TriggerBuilder.Create() .WithIdentity("AddStudentsTrigger") .WithCronSchedule("*/30 * * * * ?") // ✅ Every 30 seconds .Build(); await _scheduler.ScheduleJob(job, trigger); } public async Task StopAsync(CancellationToken cancellationToken) { if (_scheduler != null) { await _scheduler.Shutdown(); } } }
  Step 5: Register Everything in Program.cs (API Layer)

  📍 Location: Onion.API/Program.cs


  Code Snippet

  builder.Services.AddMediatR(cfg => cfg.RegisterServicesFromAssembly(typeof(CreateStudentCommand).Assembly)); builder.Services.AddSingleton<ISchedulerFactory, StdSchedulerFactory>(); builder.Services.AddSingleton(provider => provider.GetRequiredService<ISchedulerFactory>().GetScheduler().Result); builder.Services.AddHostedService<CronJobService>();

  Conclusion

  By following this guide, you have successfully built a C# Web API with Onion Architecture, CQRS, MediatR, and Quartz.NET CRON Jobs.
  This approach ensures:
  ✅Scalability using Onion Architecture
  ✅Separation of Concerns (SoC)
  ✅Background Job Execution with Quartz.NET
  ✅Better Maintainability with CQRS & MediatR
  

  🚀 Download Full Project
  Github