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 'MyContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728

I've added: var context = host.Services.GetRequiredService<MyContext>(); Also I've added private readonly DbContextOptions<MyContext> _opts; in my Post Class:

using (MyContext db = new MyContext(_opts))
{
    db.Posts.Add(postData);
    db.SaveChanges();
}

This how I added service:

.ConfigureServices((context, services) =>
{
    services.Configure<DataOptions>(opts =>
        context.Configuration.GetSection(nameof(DataOptions)).Bind(opts)
    );
    services.AddDbContext<MyContext>((provider, builder) =>
        builder.UseSqlite(provider.GetRequiredService<IOptions<DataOptions>>().Value.ConnectionString)
    );
});

And this is my Context:

public sealed class MyContext : DbContext
{
    private readonly DbContextOptions<MyContext> _options;
    
    public DbSet<PostData> Posts { get; set; }
    public DbSet<VoteData> Votes { get; set; }
    
    
    public MyContext(DbContextOptions<MyContext> options) : base(options)
    {
        _options = options;
    }
    
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        if (!optionsBuilder.IsConfigured)
        {
            optionsBuilder.UseSqlite("ConnectionString");
        }
    }
}

I tried add-migration and has this error

What I do wrong?

Breannebrear

Answer 1

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);            
    }
}

Polly

Answer 2

I came across this problem today. In my case, the SqlDbContext was in a separate ASP.Net Core 3.1 class library project, and I was trying to setup migrations using the dotnet CLI from that project's root folder. The main web application, which is the default project, contains the connection string configuration inside the appsettings.json and the startup configurations therefore I had to specify the startup project path using the -s switch as follows.

>dotnet ef migrations add initialcreation -s ..\MyWebApp\MyWebApp.csproj

-s, short for startup project, is a quick alternative to implementing IDesignTimeDbContextFactory when the DbContext is in a different project than the web application project.

Giaimo

Answer 3

The quick solution to the problem is to implement the default constructor in your context

 public MyContext() : base()
 {
 }

The problem with this solution is that you will have to enter the connection string in the 'OnConfiguring' function explicitly, which is not recommended

 protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
 {
        if (!optionsBuilder.IsConfigured)
        {
            optionsBuilder.UseSqlite("ConnectionString");
        }
 }

Disherison

Answer 4

This is not an answer for this specific question's code, but this is for the error message.

A possible fix would be the order of constructors. If your DbContext has multiple constructors, make sure that the empty constructor is on top

Pewit

Answer 5

I've had same problem as You. Maybe it was not for a Console Application but error was the same. So i thought that it is worth to share with my answer. I was using NET Core 3.0 and to fix the problem I have to change the IHostBuilder into IWebHost and then everything was fine. The problem was in class Program.

public static void Main(string[] args)
    {
        CreateHostBuilder(args).Build().Run();
    }

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>();
            });

into

public static void Main(string[] args)
    {
        BuildWebHost(args).Run();
    }

    public static IWebHost BuildWebHost(string[] args) =>
    WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>()
        .Build();

Piquant

Answer 6

In my case problem was that I has multiple startup projects selected. selected one and it was fixed

Kirstiekirstin

Answer 7

Kindly check your connection string also in appsetting,json.

Folklore

Answer 8

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.

Swiftlet

Answer 9

Hade the same problem with NET Core 3.1.

Needed to add one more constructor too solev this. Why I do not know.

Never hade to do it in the past.

public DataContext()
{            
}

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

Outage

Or remove both as they do nothing ;-) (EF Tools / Migration uses DI behind scenes) – Apostasy

Answer 10

Ensure you have added DbContext to your services in Startup class

Wein

Answer 11

In my case at .Net 7 the problem was at the program.cs.

AddDbContext was after WebApplication.Build() and it should be before Build (just a mistake).

Incorrect:

using var app = builder.Build();

services.AddDbContext<DatabaseContext>(opt => opt
            .UseSqlServer("<Connection-String>"));

Correct:

services.AddDbContext<DatabaseContext>(opt => opt
            .UseSqlServer("<Connection-String>"));

using var app = builder.Build();

Tightwad

Answer 12

if you using the macOS platform, you must set up the startup project.

 dotnet ef --startup-project ../NLayer.API/ migrations add Initial

For Database Update

dotnet ef --startup-project ../NLayer.API/ database update

Lobar

Answer 13

In my case, I had two startup project in my solution, so setting the project that has the connection string as the only startup project fixed the issue

Peisch

Answer 14

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);
    }

Breannebrear

Answer 15

In my case with Net Core 3.1 (PostgreSQL) Solved it by removing "AddUserStore"

 services.AddIdentity<User, IdentityRole<int>>()
                .AddEntityFrameworkStores<MyDBContext>()
                //.AddUserStore<MyDBContext>()
                .AddDefaultTokenProviders();

Taxi

Answer 16

I had this problem in .NET 6. In program.cs, you should put var app = builder.Build(); after Services.AddDbContext and connectionString.

Luben

Answer 17

1. In my case, my database connection string was wrong, I gave wrong database password, which caused this problem. So, check your connection string in appsettings, json as well.
2. Make sure to set your web or api project as Set as Startup Project. Do not use multiple startup project.
3. In Package Manager Console, set your data access layer (if any) as a default project
4. Then run the command again

Mooned

Answer 18

If someone still uses this alternative to know why it gives them this error.

It is because there is an error in the configurations of the keys or constraints of the tables.

You can use dotnet ef database update --verbose, to know the error that was generated with the migrations.

Helaine

Answer 19

You need to add this in the Program.cs file

builder.Services.AddDbContext<DataContext>(options => {
options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection")); });

and this is appsettings.json

"ConnectionStrings": {
"DefaultConnection" :  "server=localhost\\sqlexpress;database=superherodb;trusted_connection=true"  },

Vocational

Answer 20

I had the same problem in my project. I forgot to add the DbContext in my Program.cs file. I added the following code and it worked for me:

builder.Services.AddDbContext<DataContext>(options =>
{
    options.UseSqlServer(builder.Configuration
        .GetConnectionString("DefaultConnection"));
});

Screenshot of Program.cs 1

And in my "appsettings.json" file:

"ConnectionStrings": {
"DefaultConnection": "Data Source=YOUR_SERVER_HERE;Initial Catalog=YOUR_DB_HERE;Integrated Security=True"

}

Kyte

Answer 21

I encountered a similar issue and had a similar project structure, as mentioned by @Rajeesh. In my setup, the main web application, which serves as the default project, contains the connection string configuration inside the appsettings.json file. Additionally, it references the Infrastructure project, where the partial class DataContext : DbContext class is located.

There are two ways to handle migrations:

1. Using Command Prompt:

C:\\MyWebAppFolder\MyWebAppProject.Infrastructure > dotnet ef migrations add MigrationName -s ..\MyWebAppFolder\MyWebAppProject.csproj

Important is to mention that you start your Command Prompt in Admin mode, and execute this on the path in your project where "DataContext : DbContext" class is. In my case it was ProjectName.Infrastructure project

Run command to update DB if you want on the same Path:

C:\xxx\xx\xxx.Infrastructure> dotnet ef database update -s ..\MyWebAppFolder\MyWebAppProject.csproj

2. Nuget Package Manager

Set StandardProject where your partial class DataContext : DbContext class is. My case Infrastructure Project:

Important: Set your MyWebAppProject.csproj as startup project in Visual Studio other way AddMigration won't work.

Run PM> Update-Database if you want to update DB in Package Manager Console

Marga