ASP.NET Core 2 Unable to resolve service for type Microsoft EntityFrameworkCore DbContext

Question

When I run my asp.net core 2 projects I get the following error message:

InvalidOperationException: Unable to resolve service for type 'Microsoft.EntityFrameworkCore.DbContext' while attempting to activate 'ContosoUniversity.Service.Class.StudentService'.

Here is my project structure:

-- solution 'ContosoUniversity'
----- ContosoUniversity
----- ContosoUniversity.Model
----- ContosoUniversity.Service

IEntityService (related code) :

public interface IEntityService<T> : IService
 where T : BaseEntity
{
    Task<List<T>> GetAllAsync();      
}

IEntityService (related code) :

public abstract class EntityService<T> : IEntityService<T> where T : BaseEntity
{
    protected DbContext _context;
    protected DbSet<T> _dbset;

    public EntityService(DbContext context)
    {
        _context = context;
        _dbset = _context.Set<T>();
    }

    public async virtual Task<List<T>> GetAllAsync()
    {
        return await _dbset.ToListAsync<T>();
    }
}

Entity :

public abstract class BaseEntity { 

}

public abstract class Entity<T> : BaseEntity, IEntity<T> 
{
    public virtual T Id { get; set; }
}

IStudentService :

public interface IStudentService : IEntityService<Student>
{
    Task<Student> GetById(int Id);
}

StudentService :

public class StudentService : EntityService<Student>, IStudentService
{
    DbContext _context;

    public StudentService(DbContext context)
        : base(context)
    {
        _context = context;
        _dbset = _context.Set<Student>();
    }

    public async Task<Student> GetById(int Id)
    {
        return await _dbset.FirstOrDefaultAsync(x => x.Id == Id);
    }
}

SchoolContext :

public class SchoolContext : DbContext
{
    public SchoolContext(DbContextOptions<SchoolContext> options) : base(options)
    {
    }

    public DbSet<Course> Courses { get; set; }
    public DbSet<Enrollment> Enrollments { get; set; }
    public DbSet<Student> Students { get; set; }
}

And finally here is my Startup.cs class :

public class Startup
{
    public Startup(IConfiguration configuration, IHostingEnvironment env, IServiceProvider serviceProvider)
    {
        Configuration = configuration;

        var builder = new ConfigurationBuilder()
        .SetBasePath(env.ContentRootPath)
        .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
        .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true);


        Configuration = builder.Build();

    }

    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.AddDbContext<SchoolContext>(option =>
            option.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));


        services.AddScoped<IStudentService, StudentService>();

        services.AddMvc();
    }

    // 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.UseBrowserLink();
        }
        else
        {
            app.UseExceptionHandler("/Home/Error");
        }

        app.UseStaticFiles();

        app.UseMvc(routes =>
        {
            routes.MapRoute(
                name: "default",
                template: "{controller=Home}/{action=Index}/{id?}");
        });
    }
}

What should I do to resolve this problem?

Answer 1

StudentService expects DbContext but the container does not know how to resolve it based on your current startup.

You would need to either explicitly add the context to the service collection

Startup

services.AddScoped<DbContext, SchoolContext>();
services.AddScoped<IStudentService, StudentService>();

Or update the StudentService constructor to explicitly expect a type the container knows how to resolve.

StudentService

public StudentService(SchoolContext context)
    : base(context)
{ 
    //...
}