C# Inject DbContext to Class

I work in an environment where applications are typically already stood up and so I have never had to setup the "guts" of a C# web application. However, wanting to learn this, I decided to create a .NET 6 Web API project, a .NET 6 Class Library project, and a .NET 4.8.1 Database project.

My Web API project only has the controllers using the MVC pattern. My Class Library project is my "domain" layer where I have my models, services (aka domains), and database queries. My Database project allows me to manage my database schema and any migrations.

In my class library I have setup my DbContext as well as a query, service, and model for an "Example" entity (which only has id and name fields) that look like this:

// DbContext
using MyApplication.Domain.Models;
using Microsoft.EntityFrameworkCore;
namespace MyApplication.Domain
{
    public class MyApplicationDbContext : DbContext
    {
        public DbSet<ExampleModel> Examples { get; set; }

        MyApplicationDbContext(DbContextOptions<MyApplicationDbContext> options) : base(options) { }
    }
}

// Query
using MyApplication.Domain.Models;
namespace MyApplication.Domain.Queries
{
    public class ExampleQuery
    {
        private readonly MyApplicationDbContext _context;
        public ExampleQuery(MyApplicationDbContext context)
        {
            _context = context;
        }

        public IEnumerable<ExampleModel> Query(Dictionary<string, dynamic>? filter = null)
        {
            filter ??= new Dictionary<string, dynamic>();
            var records = _context.Examples.AsQueryable();
            if (filter.ContainsKey("ExampleId"))
            {
                Guid exampleId = filter["ExampleId"];
                records = records.Where(record => record.ExampleId == exampleId);
            }
            return records;
        }
    }
}

// Service
using MyApplication.Domain.Exceptions;
using MyApplication.Domain.Models;
using MyApplication.Domain.Queries;
namespace MyApplication.Domain.Services
{
    public class ExampleService
    {
        private readonly ExampleQuery _exampleQuery;

        public ExampleService(ExampleQuery exampleQuery)
        {
            _exampleQuery = exampleQuery;
        }

        public IEnumerable<ExampleModel> GetAll()
        {
            return _exampleQuery.Query();
        }

        public ExampleModel Get(Guid id)
        {
            var records = _exampleQuery.Query(new Dictionary<string, dynamic>
            {
                { "ExampleId", id }
            });

            if (!records.Any())
            {
                throw new NotFound404("There aren't any records by that id");
            }
            if (records.Count() > 1)
            {
                throw new Conflict409("There are multiple records with that id");
            }
            return records.Single();
        }
    }
}

// Model
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
namespace MyApplication.Domain.Models
{
    public class ExampleModel
    {
        [Key]
        [Required]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public Guid ExampleId { get; set; }

        [Required]
        [MaxLength(64)]
        public string? ExampleName { get; set; }
    }
}

Then in my Web API project, I setup the dependency injection using the following in my Program.cs (using top level statements):

builder.Services.AddDbContext<MyApplicationDbContext>(dbContextOptions => dbContextOptions.UseSqlServer(builder.Configuration.GetValue<string>("ConnectionStrings:MyApplication")));
builder.Services.AddSingleton<ExampleQuery>();
builder.Services.AddSingleton<ExampleService>();

The issue is that my Web API project fails to run because ExampleQuery injects the MyApplicationDbContext with the error:

A suitable constructor for type
‘MyApplication.Domain.MyApplicationDbContext’ could not be located.
Ensure the type is concrete and services are registered for all
parameters of a public constructor.

Again, I’m not very familiar with setting up projects from scratch so I’m not certain how to resolve this dependency issue.

>Solution :

Your MyApplicationDbContext has private constructor, make it public

public class MyApplicationDbContext : DbContext
{
    public DbSet<ExampleModel> Examples { get; set; }

    public MyApplicationDbContext(DbContextOptions<MyApplicationDbContext> options) : base(options) { }
}