Implementing the Singleton Pattern in ASP.NET Core with Simple Example

The structure of project is shown as below screenshot.

Step 1: Create a class with the name 'Applicant.cs' and copy and paste the following code.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

namespace SingletonPatternCore.Model

{

    public class Applicant

    {

        public int ApplicantId { get; set; }

        public string FName { get; set; }

        public string LName { get; set; }

        public int Dept { get; set; }

    }

}

Step 2: Create an interface with the name 'IApplicant.cs' and copy and paste the following code.

using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

namespace SingletonPatternCore.Model

{

    public interface IApplicant

    {

        Applicant SaveEmployee(Applicant emp);

        IEnumerable GetEmployees(int id);

        IEnumerable GetAllEmployees();

    }

}

Step 3: Create a class with the name 'MockApplicantRepository.cs', copy and paste the following code.

using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

namespace SingletonPatternCore.Model

{

    public class MockApplicantRepository : IApplicant

    {

        private List<Applicant> _lst;

        public MockApplicantRepository()

        {

          _lst = new List<Applicant>()

          {

           new Applicant(){ApplicantId = 1, FName = "Ashok", LName = "Kumar", Dept = 10 },

           new Applicant() { ApplicantId = 2, FName = "Mukesh", LName = "Kumar", Dept = 10 },

           new Applicant() { ApplicantId = 3, FName = "Sanjay", LName = "Kumar", Dept = 10 }

          };

        }

        public IEnumerable GetAllEmployees()

        {

            return _lst;

        }

        public IEnumerable GetEmployees(int id)

        {

           return _lst.Where(w => w.ApplicantId == id);

        }

        public Applicant SaveEmployee(Applicant emp)

        {

            emp.ApplicantId = _lst.Max(e => e.ApplicantId) + 1;

            _lst.Add(emp);

            return emp;

        }

    }

}

Step 4: Create a  controller with the name ApplicantController, copy and paste the following code.

using Microsoft.AspNetCore.Mvc;

using SingletonPatternCore.Model;

using System.Collections;

namespace SingletonPatternCore.Controllers

{

    [Route("api/[controller]")]

    [ApiController]

    public class ApplicantController : ControllerBase

    {

        private IApplicant _emp;

        public ApplicantController(IApplicant emp)

        {

            _emp = emp;

        }

        [HttpGet]

        public IEnumerable Get()

        {

            return _emp.GetAllEmployees();

        }

        [HttpPost("SaveEmployee")]

        public Applicant Post([FromBody]Applicant emp)

        {

            return _emp.SaveEmployee(emp);

        }

        [HttpGet("GetEmployeeById/{Id}")]

        public IEnumerable GetEmployeeById(int Id)

        {

           return _emp.GetEmployees(Id);

        }

       

    }

}

Step 5: Following code, you have to add on Startup.cs file.

using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

using Microsoft.AspNetCore.Builder;

using Microsoft.AspNetCore.Hosting;

using Microsoft.AspNetCore.Mvc;

using Microsoft.Extensions.Configuration;

using Microsoft.Extensions.DependencyInjection;

using Microsoft.Extensions.Logging;

using Microsoft.Extensions.Options;

using SingletonPatternCore.Model;

namespace SingletonPatternCore

{

    public class Startup

    {

        public Startup(IConfiguration configuration)

        {

            Configuration = configuration;

        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.

        public void ConfigureServices(IServiceCollection services)

        {

            services.AddMvc();

            //You can add AddSingleton, AddTransient and AddScoped

            services.AddSingleton<IApplicant, MockApplicantRepository>();

        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.

        public void Configure(IApplicationBuilder app, IHostingEnvironment env)

        {

            if (env.IsDevelopment())

            {

                app.UseDeveloperExceptionPage();

            }

            app.UseMvc();

        }

    }

}

Following are the way to test API as shown in the screenshot.