Unable to create an object of type 'MyContext'. For the different patterns supported at design time

Question

I have ConsoleApplication on .NET Core and also i added my DbContext to dependencies, but howewer i have an error:

Unable to create an object of type \'My

Answer 1

I resolve this issue by this way:

    public DbSet<PostData> Posts { get; set; }
    public DbSet<VoteData> Votes { get; set; }

    public MyContext(DbContextOptions options) : base(options) { Database.EnsureCreated(); }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.ApplyConfigurationsFromAssembly(typeof(MyContext).Assembly);
    }

Answer 2

I Resolved this by just adding a plain constructor to my Context

public class DataContext : DbContext
{
    public DataContext()
    {
    }

    public DataContext(DbContextOptions options) : base(options)
    {
    }

    protected override void OnConfiguring(DbContextOptionsBuilder options)
    {
        if (!options.IsConfigured)
        {
            options.UseSqlServer("A FALLBACK CONNECTION STRING");
        }
    }
    
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);            
    }
}

Answer 3

This applies to ASP .NET or .NET console applications using .NET Core 3.1.

In my case it was ASPNET Core and I had the same reported problem after upgrading my application from 2.1 to 3.1. The answer provided by @Matt lead me to a solution that works and allows me to continue using the new Generic Host. The Web Host remains only for backward compatibility.

The documentation for Generic Host and Design-time DbContext Creation both state what needs to happen.

Your program.cs must have a CreateHostBuilder method with a signature exactly as documented. This is because the framework attempts to resolve it using Program.CreateHostBuilder(). The signature must be:

public static IHostBuilder CreateHostBuilder(string[] args)

This is what caught me out, I initially had it named CreateWebHostBuilder, a 2.1 convention; and then I didn't have the args parameter defined. Fixing these two issues immediately resolved my add-migration ... error.

The Design-time DbContext Creation documentation is quite helpful detailing how the framework attempts to resolve the DbContext and explains why other suggestions here work the way they do, e.g. parameterless constructor.