CODEWITHSUNDEEP
wpfentity-framework.net-core
PawelC
PawelC

Reputation: 1226

WPF with entity framework on net core - unable to create and object of type AppDbContext

I have simple WPF Net Core app when i'm using entity framework, i have DbContext, but when i have add migration then i receive error:

PM> add-migration init
Build started...
Build succeeded.
Unable to create an object of type 'AppDbContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728

my appsettings.json

{
  "ConnectionStrings": {
    "SqlConnection": "Server=DESKTOP-K6R1EB3\\tester;Database=test;Trusted_Connection=True;MultipleActiveResultSets=true"
  }
}

AppDbContext

using Microsoft.EntityFrameworkCore;

namespace WpfApp1
{
    public class AppDbContext : DbContext
    {
        public DbSet<Person> Person { get; set; }
        public AppDbContext(DbContextOptions<AppDbContext> options) : base(options)
        {

        }


    }
}

App.xaml.cs

using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using System;
using System.IO;
using System.Windows;

namespace WpfApp1
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        public IServiceProvider ServiceProvider { get; private set; }

        public IConfiguration Configuration { get; private set; }

        protected override void OnStartup(StartupEventArgs e)
        {
            var builder = new ConfigurationBuilder()
             .SetBasePath(Directory.GetCurrentDirectory())
             .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true);

            Configuration = builder.Build();

            var serviceCollection = new ServiceCollection();
            ConfigureServices(serviceCollection);

            ServiceProvider = serviceCollection.BuildServiceProvider();

            var mainWindow = ServiceProvider.GetRequiredService<MainWindow>();
            mainWindow.Show();
        }

        private void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<AppDbContext>
        (options => options.UseSqlServer(
                    Configuration.GetConnectionString("SqlConnection")));

            services.AddTransient(typeof(MainWindow));
        }
    }
}

Of course i have simple model for database

namespace WpfApp1
{
    public class Person
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string Surname { get; set; }
    }
}

What is wrong? I search solution but i nothing found. Early i write web app in net core and entity framework, and i don't have any problem. This is my first WPF Net Core App, and i have problem with EF configuration.

Upvotes: 4

Views: 3241

Answers (1)

Kirk Horton
Kirk Horton

Reputation: 408

i was able to reproduce your issue and get past it.

i've ran into this before. this article guided me through what i needed to do, very thankful for it: https://blog.tonysneed.com/2018/12/20/idesigntimedbcontextfactory-and-dependency-injection-a-love-story/

from the article in the error message: https://go.microsoft.com/fwlink/?linkid=851728. specifically this section https://learn.microsoft.com/en-us/ef/core/miscellaneous/cli/dbcontext-creation#from-a-design-time-factory, we can use some duck typing and provide a class that Microsoft.EntityFrameworkCore.Tools will use to create the migrations, implementing IDesignTimeDbContextFactory<T>.

tony sneed's article provides a generic abstract implementation, which we can then derive very simply.

here is the generic abstract implementation (edit, sorry, just realized i copy-pasted my adaptation of tony sneed's code that i'd used when i ran into this before):

public abstract class DesignTimeDbContextFactoryBase<TContext> : IDesignTimeDbContextFactory<TContext> where TContext : DbContext
{
    public TContext CreateDbContext(string[] args)
    {
        return Create(Directory.GetCurrentDirectory(), Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT"));
    }
    protected abstract TContext CreateNewInstance(DbContextOptions<TContext> options);
    public TContext Create()
    {
        var environmentName = Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT");
        var basePath = AppContext.BaseDirectory;
        return Create(basePath, environmentName);
    }
    TContext Create(string basePath, string environmentName)
    {
        var builder = new ConfigurationBuilder()
            .SetBasePath(basePath)
            .AddJsonFile("appsettings.json")
            .AddJsonFile($"appsettings.{environmentName}.json", true)
            .AddEnvironmentVariables();
        var config = builder.Build();
        var connstr = config.GetConnectionString("default");
        if (string.IsNullOrWhiteSpace(connstr) == true)
            throw new InvalidOperationException("Could not find a connection string named 'default'.");
        return Create(connstr);
    }
    TContext Create(string connectionString)
    {
        if (string.IsNullOrEmpty(connectionString))
            throw new ArgumentException($"{nameof(connectionString)} is null or empty.", nameof(connectionString));
        var optionsBuilder = new DbContextOptionsBuilder<TContext>();
        Console.WriteLine($"MyDesignTimeDbContextFactory.Create(string): Connection string: {connectionString}");
        optionsBuilder.UseSqlServer(connectionString, s => s.CommandTimeout((int)TimeSpan.FromMinutes(10).TotalSeconds));
        DbContextOptions<TContext> options = optionsBuilder.Options;
        return CreateNewInstance(options);
    }
}

you provide an implementation for AppDbContext:

public class AppDbContextFactory : DesignTimeDbContextFactoryBase<AppDbContext>
{
    protected override AppDbContext CreateNewInstance(DbContextOptions<AppDbContext> options)
    {
        return new AppDbContext(options);
    }
}

that Add-Migration (or maybe technically Microsoft.EntityFrameworkCore.Tools) will detect.

there were a lot of nuget packages to add:

<ItemGroup>
  <PackageReference Include="Microsoft.EntityFrameworkCore" Version="3.1.1" />
  <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="3.1.1" />
  <PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="3.1.1">
    <PrivateAssets>all</PrivateAssets>
    <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
  </PackageReference>
  <PackageReference Include="Microsoft.Extensions.Configuration" Version="3.1.1" />
  <PackageReference Include="Microsoft.Extensions.Configuration.EnvironmentVariables" Version="3.1.1" />
  <PackageReference Include="Microsoft.Extensions.Configuration.FileExtensions" Version="3.1.1" />
  <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="3.1.1" />
</ItemGroup>

once you get past your error, i got another one: Could not find a connection string named 'default'. so i changed appsettings SqlConnection to default.

Upvotes: 8

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