为类库创建实体框架核心迁移

Question

I'm trying to generate migration in EF Core class library but to no avail so far. I'm using Package Manager Console (Visual Studio 2017 Community) to write a command like this :

PM> Add-Migration Update37

but I always get an error:

Unable to create an object of type 'DBContext'. Add an implementation of 'IDesignTimeDbContextFactory' to the project, or see https://go.microsoft.com/fwlink/?linkid=851728 for additional patterns supported at design time.

My solution structure :

As you can see I have 2 projects where MesDBModels is class library for which I would like to generate migration to Migrations folder. I have added a DummyProject in order to provide "Startup project" on .NETcore runtime. DummyProject also contains implementations of IDesignTimeDBContextFactory<> interface in DBContextFactory class. Also DummyProject contains reference to MesDBModels project.

MesDBModels.Database.DBContext :

using MesDBModels.FluentConfig;
using MesDBModels.Models;
using Microsoft.EntityFrameworkCore;

namespace MesDBModels.Database
{
public class DBContext : DbContext
{
    public DBContext(DbContextOptions<DBContext> options) : base(options) { }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.ApplyConfiguration(new AccessCardConfig());
        modelBuilder.ApplyConfiguration(new AlarmConfig());
        modelBuilder.ApplyConfiguration(new BitPositionConfig());
        modelBuilder.ApplyConfiguration(new MachineConfig());
        modelBuilder.ApplyConfiguration(new ParameterTemplateConfig());
        modelBuilder.ApplyConfiguration(new ReferenceConfig());
        modelBuilder.ApplyConfiguration(new ReferenceTemplateConfig());
        modelBuilder.ApplyConfiguration(new UserConfig());
        modelBuilder.ApplyConfiguration(new UserGroupConfig());
        modelBuilder.ApplyConfiguration(new UserPermissionConfig());

        base.OnModelCreating(modelBuilder);
    }

    public DbSet<AccessCard> AccessCards { get; set; }
    public DbSet<Alarm> Alarms { get; set; }
    public DbSet<AlarmDefinition> AlarmDefinitions { get; set; }
    public DbSet<AlarmPriority> AlarmPriorities { get; set; }
    public DbSet<BitPosition> BitPositions { get; set; }
    public DbSet<EventLog> EventLogs { get; set; }
    public DbSet<EventType> EventTypes { get; set; }
    public DbSet<Group> Groups { get; set; }
    public DbSet<Machine> Machines { get; set; }
    public DbSet<MachineState> MachineStates { get; set; }
    public DbSet<MachineStateDefinition> MachineStateDefinitions { get; set; }
    public DbSet<Permission> Permissions { get; set; }
    public DbSet<ParameterTemplate> ParameterTemplates { get; set; }
    public DbSet<ReferenceParameterHistory> ReferenceParameterHistory { get; set; }
    public DbSet<ParameterValueLimit> ParameterValueLimits { get; set; }
    public DbSet<Reference> References { get; set; }
    public DbSet<ReferenceParameterType> ReferenceParameterTypes { get; set; }
    public DbSet<ReferenceTemplate> ReferenceTemplates { get; set; }
    public DbSet<Session> Sessions { get; set; }
    public DbSet<Unit> Units { get; set; }
    public DbSet<UnitStatus> UnitStatuses { get; set; }
    public DbSet<User> Users { get; set; }
    public DbSet<UserGroup> UserGroups { get; set; }
    public DbSet<UserPermission> UsersPermissions { get; set; }
}

}

DummyProject.DBContextFactory :

using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Design;

namespace MesDBModels
{
public class DBContextFactory : IDesignTimeDbContextFactory<DbContext>
{ 
    public DbContext CreateDbContext(string[] args)
    {
        var builder = new DbContextOptionsBuilder<DbContext>();
        builder.UseSqlServer(
            "Server=(local)\\serverName;Database=dbName;Trusted_Connection=True;MultipleActiveResultSets=true");

        return new DbContext(builder.Options);
    }
}

}

DummyProject.Program class :

using System;

namespace DummyProject
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");
        }
    }
}

So my question is : what am I doing wrong ? I have tried to put DBContextFactory class in MesDBModels project along with DBContext class but the result is the same. I have followed this : Using Entity Framework Core migrations for class library project and this : https://garywoodfine.com/using-ef-core-in-a-separate-class-library-project/ but maby I misunderstood something or done something wrong.

Please help.

Answer 1

You have made a typo in your

public class DBContextFactory : IDesignTimeDbContextFactory<DbContext>
{ 
    public DbContext CreateDbContext(string[] args)
    {
        var builder = new DbContextOptionsBuilder<DbContext>();
        builder.UseSqlServer(
            "Server=(local)\\serverName;Database=dbName;Trusted_Connection=True;MultipleActiveResultSets=true");

        return new DbContext(builder.Options);
    }
}

When trying to construct your DBContext , the framework looks for IDesignTimeDbContextFactory for DBContext , but you are instead implementing a factory for the base DbContext . Modify DbContext to DBContext in this code and it should work.