Implement Repository Pattern with ASP.NET Core Web API

This blog explains how to implement a Repository Pattern with ASP.NET Web API, AutoMapper, Entity Framework, CRUD operations, and async operations.

Repository Pattern

A repository pattern is an abstraction that isolates business logic from the database access layer resulting in a decoupled architecture. Repositories hide logic that involves communication with the data store. Repository Pattern provides low-level abstraction.

You would usually have one repository per entity, for example, to perform CRUD operations on a Category entity you will have CategoryRepository.

Advantages of Repository Pattern

  • Dependency Injection: you can inject interface dependency and mock repository, this increases the testability of your code. Directly using DBContext of EF Core in API Controller is not testable. For more details on Dependency Injection
  • Decouples Persistence Framework

    Using Repository Pattern, it is easy to change the persistence framework for example from EF Core to Dapper or something else. So DataAccess Layer and Business Layer can be tested separately.

  • Minimize Duplicate Query Logic

    If you have this code to get a list of products. Whenever you need a list of products you will have to repeat the same four lines of code.

       .Where(p => p.Id = categoryId)

    With Repository, you can encapsulate this code to Repository and from controller just use it like this.


Steps to Implement Repository Pattern

  1. Setting up Application

    1. Create Database

      For this tutorial, we will be using a SQL database having Category as a table. Web API will perform CRUD operations on this database. Create a SQL database with the name GeekStore. You can download Create GeekStore database SQL script.

      Update the connection string in appsettings.json

      "ConnectionStrings": {
          "GeekStoreConnection": "Data Source=<servername>
              Initial Catalog=GeekStore;Integrated Security=True"
    2. Create ASP.NET Core Web API

      Create ASP.NET Core Web API application with the name GeekStore, you can use any version of Visual Studio and .NET. The code in this blog uses Visual Studio 2022 and .NET 7. For more detailed steps please visit Create ASP.NET Core Web API with Entity Framework.

      Create folders as shown

      • Configurations: for AutoMapper Configuration.
      • Contracts: for Repository Contracts.
      • Controllers: for API Controllers.
      • Data.Models: for Domain Models generated by EF Core.
      • DTOs: for Data Transfer Objects.
      • Repository: for repository classes.

      ASP.NET Core Web API Repository Pattern folder Structure

      Register DbContext in program.cs.

      var builder = WebApplication.CreateBuilder(args);
      var connectionString =
      builder.Services.AddDbContext<GeeksStoreContext>(options => {
    3. Install NuGet Packages

      As we are using Entity Framework and AutoMapper to perform CRUD operations. Install the following required NuGet packages.

      1. Microsoft.EntityFrameworkCore
      2. Microsoft.EntityFrameworkCore.Sqlserver
      3. Microsoft.EntityFrameworkCore.Tools
      4. AutoMapper
      5. AutoMapper.Extensions.Microsoft.DependencyInjection
    4. Generate Data Model

      You can generate database models using EF Core Database first. Use this command in Package Manager Console to generate Data Models

      Scaffold-DbContext "Data Source=<servername>
              Initial Catalog=GeekStore;Integrated Security=True" 
          -Provider Microsoft.EntityFrameworkCore.SqlServer
          -OutputDir Data.Models

      Generated Models will be available in Data.Models folder

  2. Create Interface

    Having Interface contracts with a Repository helps for unit testing the business layer as the interface is not dependent on database or setup of the database structure.

    Also, you can have multiple implementations of Interface, you might have a separate Repository for the GeekStore online application and a back office of GeekStore that implements Interface contract.

    Add a new interface with the name ICategoryRepository to the folder Contracts. And add all required CRUD operation definitions.

    using GeekStore.API.Core.Data.Models;
    namespace GeekStore.API.Core.Contracts
        public interface ICategoryRepository 
            Task<Category> GetAsync(int? categoryId);
            Task<List<Categorygt>> GetAllAsync();
            Task<Category> CreateAsync(Category category);
            Task DeleteAsync(int categoryId);
            Task UpdateAsync(Category category);
  3. Create Repository Class

    The repository class implements an interface contract and encapsulates the logic required to access data sources. It is responsible for retrieving data in the form of Entity models.

    Add a new class with name the CategoryRepository.cs to the folder Repository. This class implements the ICategoryRepository contract.

    namespace GeekStore.API.Core.Repository
        public class CategoryRepository : ICategoryRepository
            private readonly GeeksStoreContext _context;
            public CategoryRepository(GeeksStoreContext context)
                this._context = context;

    Add following methods to CategoryRepository

    1. GetAllAsync

      This method gets all categories available in the database and returns them to the controller in the form of a list.

      public async Task<List<Category>> GetAllAsync()
          return await _context.Set<Category>().ToListAsync();
    2. GetAsync

      This method accepts a integer parameter CategoryId and returns Category details.

      public async Task<Category?> GetAsync(int? categoryId)
          if(categoryId == null)
              return null;
          return await this._context.Categories.FindAsync(categoryId);            
    3. CreateAsync

      This method accepts a Category object and inserts it into the database as a new Category, and returns a newly created Category to the client.

      public async Task<Category< CreateAsync(Category category)
          await this._context.AddAsync(category);
          await this._context.SaveChangesAsync();
          return category;
    4. DeleteAsync

      This method accepts an integer parameter categoryId and deletes the category from the database.

      public async Task DeleteAsync(int categoryId)
          var category = await GetAsync(categoryId);
          if (category is null)
              throw new Exception($"CategoryID {categoryId} is not found.");
          await _context.SaveChangesAsync();
    5. UpdateAsync

      Updates Category values to the database as provided by the client/controller.

      public async Task UpdateAsync(Category category)
          await _context.SaveChangesAsync();           
  4. APIController

    The CategoryRepository is ready with all CRUD operations and can be used by the controller. Add a new API Controller with the name CategoryController to the folder Controllers.

    CategoryRepository returns domain models to controller however controller should return specific data in form of DTOs, so to map domain models to DTOs we are going to use AutoMapper. For this you will have to create a MappingProfile. Please check for detailed steps about How to use AutoMapper in ASP.NET Core Web API

    You can find all required DTO class definition here.

    Create a constructor and inject dependency of AutoMapper and ICategoryRepository interface.

    namespace GeekStore.API.Core.Controllers
        public class CategoryController : ControllerBase
            private readonly IMapper _mapper;
            private readonly ICategoryRepository _categoryRepository;
            public CategoryController(IMapper mapper,
                ICategoryRepository categoryRepository)
                this._mapper = mapper;
                this._categoryRepository = categoryRepository;

    Add following CRUD operations that uses Category Repository.

    1. GetAllCategories

      Get the list of categories from Repository and return it to the client application. Notice that the controller is getting a list of Category domain models however it is returning a list of GetCategoryDetailsDto DTO.

      Get list of category from Repository and return to client application. Notice that controller is getting list of Category domain model however it is returning list of GetCategoryDetailsDto DTO.

      public async Task<ActionResult<List<GetCategoryDetailsDto>>> GetAllCategories()
          var categories = await this._categoryRepository.GetAllAsync();
          var records = _mapper.Map<List<GetCategoryDetailsDto>>(categories);
          return Ok(records);
    2. GetCategory

      Get a single Category details

      public async Task<ActionResult<GetCategoryDetailsDto>> GetCategory(int categoryId)
          var category = await this._categoryRepository.GetAsync(categoryId);
          if (category == null)
              throw new Exception($"CategoryID {categoryId} is not found.");
          var categoryDetailsDto = _mapper.Map(category);
          return Ok(categoryDetailsDto);
    3. CreateCategory

      Create a new Category, HttpPost method.

      public async Task<ActionResult<Category>> 
          CreateCategory(CreateCategoryDto createCategoryDto)
          var category = _mapper.Map<Category>(createCategoryDto);
          await this._categoryRepository.CreateAsync(category);
          return CreatedAtAction("GetCategory", new { id = category.Id }, category);
    4. UpdateCategory

      This method updates a Category with required validation. This is the HttpPut method.

      public async Task<IActionResult> 
          UpdateCategory(int categoryId, UpdateCategoryDto updateCategoryDto)
          if (categoryId != updateCategoryDto.Id)
              return BadRequest("Invalid Category Id");
          var category = await _categoryRepository.GetAsync(categoryId);
          if (category == null)
              throw new Exception($"CategoryID {categoryId} is not found.");
          _mapper.Map(updateCategoryDto, category);
              await _categoryRepository.UpdateAsync(category);
          catch (Exception)
              throw new Exception($"Error occured while updating CategoryID {categoryId}.");
          return NoContent();
    5. DeleteCategory

      Call Category to delete Category having specific categoryId

      public async Task<IActionResult> DeleteCategory(int categoryId)
          await _categoryRepository.DeleteAsync(categoryId);
          return NoContent();
  5. Register Dependency of Repository

    IoC container will create an instance of a repository depending on how you have registered it. A new instance of CategoryRepository will be required for every request to perform a unique operation so register it as AddScoped. Scoped instances are created once per request, for more details check Dependency Injection lifetime.

    Add this code to program.cs file.

        <ICategoryRepository, CategoryRepository>();

    Run your Web API application, you will see all CRUD operations listed by Swagger. You can test it using Swagger UI.

    Implement Repository Pattern with ASP.NET Core Web API

Source code on Git hub Source Code on Github

Speak your mind
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  • geeksarray user
    09/21/2016 04:13 PM itortu

    Could you show how to test the CUD operations? Great article. Thank you!

  • geeksarray user
    06/27/2017 03:27 PM beyazcennet

    How is tokens added to this structure?

  • geeksarray user
    06/27/2017 03:27 PM beyazcennet

    How is tokens(security) added to this structure?

  • geeksarray user
    05/15/2018 05:11 AM sky

    While adding data its showing An exception of type 'System.NullReferenceException' occurred in CustomerApp.dll but was not handled in user code Additional information: Object reference not set to an instance of an object.

  • geeksarray user
    05/15/2018 05:12 AM sky

    While adding data its showing An exception of type 'System.NullReferenceException' occurred in CustomerApp.dll but was not handled in user code Additional information: Object reference not set to an instance of an object.

  • geeksarray user
    05/15/2018 08:34 PM Laxmikant

    Sky, have you added line - static readonly ICustomerRepository repository = new CustomerRepository(); ?

  • geeksarray user
    03/09/2021 11:14 AM GeeksArray

    @itortu - please check to test your CRUD operations