How to fix Add-Migration problem?

The error message "Unable to create an object of type 'ApplicationDbContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728" typically indicates that Entity Framework Core is unable to create an instance of your ApplicationDbContext class. This can occur for several reasons, including issues with the constructor of your ApplicationDbContext or missing configuration.

Here are the steps to resolve this issue:

1. Ensure Correct Constructor

Make sure your ApplicationDbContext class has a constructor that takes DbContextOptions<ApplicationDbContext> as a parameter and passes it to the base class:

public class ApplicationDbContext : IdentityDbContext

{

    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options)

        : base(options)

    {

    }

    public DbSet<Category> Categories { get; set; }

    public DbSet<Product> Products { get; set; }

    public DbSet<ProductPrice> ProductPrices { get; set; }

    public DbSet<OrderHeader> OrderHeaders { get; set; }

    public DbSet<OrderDetail> OrderDetails { get; set; }

    public DbSet<ApplicationUser> ApplicationUsers { get; set; }

    public DbSet<Refund> Refunds { get; set; }

}

2. Add a Design-Time Factory

If you are using dependency injection and the DbContext is configured in Program.cs, you may need to provide a design-time factory for migrations. This factory is used to create instances of your DbContext at design time.

Create a class that implements IDesignTimeDbContextFactory<ApplicationDbContext>:

using Microsoft.EntityFrameworkCore;

using Microsoft.EntityFrameworkCore.Design;

using Microsoft.Extensions.Configuration;

using System.IO;

public class ApplicationDbContextFactory : IDesignTimeDbContextFactory<ApplicationDbContext>

{

    public ApplicationDbContext CreateDbContext(string[] args)

    {

        var optionsBuilder = new DbContextOptionsBuilder<ApplicationDbContext>();

        // Read the connection string from appsettings.json or environment variables

        IConfigurationRoot configuration = new ConfigurationBuilder()

            .SetBasePath(Directory.GetCurrentDirectory())

            .AddJsonFile("appsettings.json")

            .Build();

        var connectionString = configuration.GetConnectionString("DefaultConnection");

        optionsBuilder.UseSqlServer(connectionString);

        return new ApplicationDbContext(optionsBuilder.Options);

    }

}

3. Ensure Proper Configuration in Program.cs

Ensure your DbContext is properly configured in Program.cs:

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddDbContext<ApplicationDbContext>(options =>

    options.UseSqlServer(builder.Configuration.GetConnectionString("DefaultConnection")));

// Other service configurations...

var app = builder.Build();

// Configure the HTTP request pipeline...

app.Run();

4. Run the Migration Command Again

After implementing the design-time factory, try running the migration command again:

Add-Migration Add_Refund

Summary

By ensuring that your ApplicationDbContext has the correct constructor, adding a design-time factory, and properly configuring your DbContext in Program.cs, you should be able to resolve the issue and create the migration successfully.

Additional Resources

For more information on design-time DbContext creation, refer to the official Microsoft documentation.

I have the DbContext in the program.cs of the server application.

First, make sure the solution builds without an error.

Verify the server project is set as the the start up project. Use Visual Studio to set the startup project or set the startup project in the command line.

Add-Migration Add_Refund -StartupProject "../Path/to/the/startup/project"

If the above does not work then use the verbose argument. The details should help pinpoint the problem.

Add-Migration Add_Refund -verbose

Lastly, it is always good to review the official documentation.

https://video2.skills-academy.com/en-us/ef/core/cli/powershell

Isn't ApplicationDbContext duplicated?

It was due to EF Core version mismatch in the projects of the solution.