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Design-time DbContext Creation

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Some of the EF Core Tools commands (for example, the Migrations commands) require a derived DbContext instance to be created at design time in order to gather details about the application's entity types and how they map to a database schema. In most cases, it is desirable that the DbContext thereby created is configured in a similar way to how it would be configured at run time.

There are various ways the tools try to create the DbContext:

From application services

If your startup project uses the ASP.NET Core Web Host or .NET Core Generic Host, the tools try to obtain the DbContext object from the application's service provider.

The tools first try to obtain the service provider by invoking Program.CreateHostBuilder(), calling Build(), then accessing the Services property.

public class Program
{
    public static void Main(string[] args)
        => CreateHostBuilder(args).Build().Run();

    // EF Core uses this method at design time to access the DbContext
    public static IHostBuilder CreateHostBuilder(string[] args)
        => Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(
                webBuilder => webBuilder.UseStartup<Startup>());
}

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
        => services.AddDbContext<ApplicationDbContext>();

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
    }
}

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

Note

When you create a new ASP.NET Core application, this hook is included by default.

The DbContext itself and any dependencies in its constructor need to be registered as services in the application's service provider. This can be easily achieved by having a constructor on the DbContext that takes an instance of DbContextOptions<TContext> as an argument and using the AddDbContext<TContext> method.

Using a constructor with no parameters

If the DbContext can't be obtained from the application service provider, the tools look for the derived DbContext type inside the project. Then they try to create an instance using a constructor with no parameters. This can be the default constructor if the DbContext is configured using the OnConfiguring method.

From a design-time factory

You can also tell the tools how to create your DbContext by implementing the Microsoft.EntityFrameworkCore.Design.IDesignTimeDbContextFactory<TContext> interface: If a class implementing this interface is found in either the same project as the derived DbContext or in the application's startup project, the tools bypass the other ways of creating the DbContext and use the design-time factory instead.

public class BloggingContextFactory : IDesignTimeDbContextFactory<BloggingContext>
{
    public BloggingContext CreateDbContext(string[] args)
    {
        var optionsBuilder = new DbContextOptionsBuilder<BloggingContext>();
        optionsBuilder.UseSqlite("Data Source=blog.db");

        return new BloggingContext(optionsBuilder.Options);
    }
}

A design-time factory can be especially useful if you need to configure the DbContext differently for design time than at run time, if the DbContext constructor takes additional parameters are not registered in DI, if you are not using DI at all, or if for some reason you prefer not to have a CreateHostBuilder method in your ASP.NET Core application's Main class.

Args

Both IDesignTimeDbContextFactory<TContext>.CreateDbContext and Program.CreateHostBuilder accept command line arguments.

You can specify these arguments from the tools:

dotnet ef database update -- --environment Production

The -- token directs dotnet ef to treat everything that follows as an argument and not try to parse them as options. Any extra arguments not used by dotnet ef are forwarded to the app.