EF Core Add-Migration 因 System.MissingMethodException 失败：未定义无参数构造函数

Question

I am using Ef Core 3.1 and I have the following simple DBContext.

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

    }
    public DbSet<User> Users{ get; set; }
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
    }
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
    }
}

And here is my Startup:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddOptions();
        services.Configure<Settings>(Configuration);
        var appSettings = services.BuildServiceProvider().GetService<IOptions<Settings>>().Value;
        services.AddDbContext<DbContext>(options => 
            options.UseSqlServer(appSettings.DataStoreSettings.ConnectionString));
        services.AddControllers();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        app.UseHttpsRedirection();

        app.UseRouting();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
        using (var serviceScope = app.ApplicationServices.GetService<IServiceScopeFactory>().CreateScope())
        {
            var context = serviceScope.ServiceProvider.GetRequiredService<DbContext>();
            context.Database.Migrate();
        }
    }
}

When starting my website, the database gets created successfully with only dbo.__EFMigrationsHistory table.

When I try to add a migration using:

Add-Migration Initial -c UserDbContext -Verbose

I get the following Error:

Microsoft.EntityFrameworkCore.Design.OperationException: Unable to create an object of type 'UserDbContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728 ---> System.MissingMethodException: No parameterless constructor defined for type 'UserDbContext'.

Adding a parameterless constructor to my context results:

No database provider has been configured for this DbContext. A provider can be configured by overriding the DbContext.OnConfiguring method or by using AddDbContext on the application service provider. If AddDbContext is used, then also ensure that your DbContext type accepts a DbContextOptions object in its constructor and passes it to the base constructor for DbContext.

I am not sure why Add-Migration is not working, can't I use Dependency Injection with EF core, if I need to use migrations?

Answer 1

The issue was in registering DbContext in IOC.

Replacing:

services.AddDbContext<DbContext>(options => 
            options.UseSqlServer(appSettings.DataStoreSettings.ConnectionString));

With:

services.AddDbContext<UserDbContext>(options => 
            options.UseSqlServer(appSettings.DataStoreSettings.ConnectionString));

Fixed it.