entity framework 6 - 播种数据库抛出 System.NullReferenceException

Question

我正在使用 .net core 2 和 entityframework 6.2。我正在尝试使用一些需要的基础数据来为数据库播种。

我没有找到 Database.SetInitializer 方法。所以我用谷歌搜索并找到了另一种在 Program class 的 Main 方法中播种我的数据库的方法（如下所示）。
我使用下面的代码为我的数据库设置种子，但是在执行 DbContext 的 SaveChange 方法时，总是出现 NullReferenceException 错误。
我使用的数据库与 Identity 使用的数据库相同。对结果有影响吗？

程序class:

public class Program
{
    public static void Main(string[] args)
    {
        //BuildWebHost(args).Run();

        var host = BuildWebHost(args);

        using (var scope = host.Services.CreateScope())
        {
            var services = scope.ServiceProvider;
            try
            {
                var context = services.GetRequiredService<ApplicationDbContext>();
                context.Provinces.Add(new Models.Province
                {
                    ID = 1,
                    Name = "TEST"
                });
                context.SaveChanges(); // <= ERROR
                //DbInitializer.Initialize(context);
            }
            catch (Exception ex)
            {
                var logger = services.GetRequiredService<ILogger<Program>>();
                logger.LogError(ex, "An error occured while seeding the database.");
            }
        }

        host.Run();
    }

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

异常：

Message: "Object reference not set to an instance of an object."
Source: "Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore"
StackTrace:
   at Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.DatabaseErrorPageMiddleware.System.IObserver<System.Collections.Generic.KeyValuePair<System.String,System.Object>>.OnNext(KeyValuePair`2 keyValuePair)
   at System.Diagnostics.DiagnosticListener.Write(String name, Object value)
   at Microsoft.EntityFrameworkCore.Internal.CoreLoggerExtensions.SaveChangesFailed(IDiagnosticsLogger`1 diagnostics, DbContext context, Exception exception)
   at Microsoft.EntityFrameworkCore.DbContext.SaveChanges(Boolean acceptAllChangesOnSuccess)
   at Microsoft.EntityFrameworkCore.DbContext.SaveChanges()
   at Invitation.Program.Main(String[] args) in d:\Projects\Invitation\Invitation\Program.cs:line 29
InnerException: null

启动class:

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<ApplicationDbContext>(options =>
            options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

        services.AddCustomIdentity()
            .AddEntityFrameworkStores<ApplicationDbContext>()
            .AddDefaultTokenProviders();

        services.AddTransient<IdentityErrorDescriber, FaIdentityErrorDescriber>();

        services.ConfigureCustomApplicationCookie();

        services.AddAntiforgery(options => options.HeaderName = "X-XSRF-TOKEN");

        services.AddMvc()
            .AddRazorPagesOptions(options =>
            {
                options.Conventions.AuthorizeFolder("/Account/Manage");
                options.Conventions.AuthorizePage("/Account/Logout");
            });

        var connection = Configuration.GetConnectionString("DefaultConnection");
        services.AddDbContext<ApplicationDbContext>(options => options.UseSqlServer(connection));

        services.AddSingleton<IEmailSender, EmailSender>();
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseBrowserLink();
            app.UseDeveloperExceptionPage();
            app.UseDatabaseErrorPage();
        }
        else
        {
            app.UseExceptionHandler("/Error");
        }

        app.UseStaticFiles();

        app.UseAuthentication();

        app.UseMvc();
    }
}

ApplicationDbContext class:

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

    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
    }

    public DbSet<Province> Provinces { get; set; }
}

B.W. 我正在使用 EntityFramework 6.2.0 & Microsoft.EntityFrameworkCore.Tools 2.1.1（可能有用的信息）

感谢您抽出时间。

Answer 1

使用此代码：

public class Program
{
    public static void Main(string[] args)
    {
        //BuildWebHost(args).Run();

        var host = BuildWebHost(args);

        using (var scope = host.Services.CreateScope())
        {
            var services = scope.ServiceProvider;
            try
            {
                using (var context = new ApplicationDbContext(
                services.GetRequiredService<DbContextOptions<ApplicationDbContext>>()))
                {
                    context.Provinces.Add(new Models.Province
                    {
                        ID = 1,
                        Name = "TEST"
                    });
                    context.SaveChanges();
                }
            }
            catch (Exception ex)
            {
                var logger = services.GetRequiredService<ILogger<Program>>();
                logger.LogError(ex, "An error occured while seeding the database.");
            }
        }

        host.Run();
    }

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

代码来自：https://docs.microsoft.com/en-us/aspnet/core/tutorials/first-mvc-app/working-with-sql?view=aspnetcore-2.0&tabs=aspnetcore2x

Answer 2

好的！我找到了原因。在 Main 方法中，当我创建 Province 类型的新实体时，为其关键参数赋值会导致错误。这解决了问题：

context.Provinces.Add(new Models.Province
{
     //ID = 1,
     Name = "TEST"
});

但是我真的很生气，因为抛出的异常并没有显示真正的错误！

entity framework 6 - 播种数据库抛出 System.NullReferenceException

entity framework 6 - seeding database throws System.NullReferenceException

c#

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entity-framework-6

entity-framework-core