尝试创建 C# EF 核心迁移时出错，System.NullReferenceException: Object 引用未设置为 object 的实例

Question

我正在 C# Winforms.Net 6.0 中练习依赖注入。 我有 DAL、BAL、服务和 WinForms 应用程序的单独项目。 在 Program.cs 中注入依赖项时，一切运行正常，但是当我想从控制台创建迁移时，出现此错误：

PM> Add-Migration InitialLoad

构建开始...构建成功。 System.NullReferenceException：Object 引用未设置为 object 的实例。 在 MaravilClient.Program.<>c.b__5_1(DbContextOptionsBuilder options) in D:\Repos\MaravilClient\MaravilClient\Program.cs:第 36 行 Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass1_0 2.<AddDbContext>b__0(IServiceProvider p, DbContextOptionsBuilder b) at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.CreateDbContextOptions[TContext](IServiceProvider applicationServiceProvider, Action 2 optionsAction) 在 Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass17_0 1.<AddCoreServices>b__0(IServiceProvider p) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, RuntimeResolverContext context) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument) 在 Microsoft.Extensions.DependencyInjection.ServiceLoo kup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSite(ServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass2_0.<RealizeService>b__0(ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType) at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType) at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider provider) at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__17 2.VisitCallSite(ServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass2_0.<RealizeService>b__0(ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceProvider.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType) at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType) at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider provider) at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__17 1.b__17_1(IServiceProvider p) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, RuntimeResolverContext context) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSite(ServiceCallSite callSite, TArgument argument) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitIEnumerable(IEnumerableCallSite en RuntimeResolverContext context) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<> 2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite callSite, RuntimeResolverContext context) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor (ServiceProviderEngineScope范围）在 Microsoft.Extensions.DependencyInjection.ServiceProvider.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType) 在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider provider, Type serviceType) 在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider provider) 在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetServices[ T]（IServiceProvider provider）在 Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextTypes() 在 Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextType(String name) 在 Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType) 在 Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType, String namespace) 在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType, String namespace)在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0() 在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.b__0() 在 Microsoft.EntityFrameworkCore.Design.OperationExecutor .OperationBase.Execute(Action action) Object 引用未设置为 ZA8CFDE6 的实例 331BD59EB2AC96F8911C4B666Z。

这是我的 Program.cs 代码：

using DAL.DataContext;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Services.ClientActions;
using Services.UserActions;
using System.Configuration;
using System.Windows.Forms;

namespace MaravilClient
{
    internal static class Program
    {
        /// <summary>
        ///  The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.SetHighDpiMode(HighDpiMode.SystemAware);
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            var host = CreateHostBuilder().Build();
            ServiceProvider = host.Services;
            var dataContext = ServiceProvider.GetRequiredService<MaravilContext>();
            dataContext.Database.Migrate();
            Application.Run(ServiceProvider.GetRequiredService<Inicio>());
        }
        public static IServiceProvider ServiceProvider { get; private set; }
        static IHostBuilder CreateHostBuilder()
        {
            return Host.CreateDefaultBuilder()
                .ConfigureServices((context, services) => {
                    services.AddDbContext<MaravilContext>(options =>
                                                   options.UseSqlServer(ConfigurationManager.ConnectionStrings["ConnString"].ConnectionString),ServiceLifetime.Scoped,ServiceLifetime.Scoped);
                    services.AddTransient<IUserActions, UserActions>();
                    services.AddTransient<IClientActions, ClientActions>();
                    services.AddTransient<Inicio>();
                });
        }
      }
}

此图像显示了 MessageBox 中连接字符串的读取。

我究竟做错了什么？

Answer 1

你能分享更多关于错误的细节吗？ 就像任何屏幕截图或任何东西一样。

Answer 2

您需要在运行命令之前设置相关项目（您的 dbcontext 所在的位置），使用终端设置该特定项目。之后您可以运行迁移命令。

Answer 3

我找到了解决方案。

我添加了一个 json 配置文件，我将属性设置为：复制到 Output 目录 => 始终复制。 我还将连接字符串添加到此 json 文件中。 CreateHostBuilder 代码如下所示：

static IHostBuilder CreateHostBuilder()
    {
       
        return Host.CreateDefaultBuilder()
            .ConfigureAppConfiguration(x=>x.AddJsonFile("appConfig.json"))
            .ConfigureServices((context, services) => {
                services.AddDbContext<MaravilContext>(options =>
                                               options.UseSqlServer(context.Configuration.GetConnectionString("SQLConnection")));
                services.AddTransient<IUserActions, UserActions>();
                services.AddTransient<IClientActions, ClientActions>();
                services.AddTransient<Inicio>();
            });
    }

最后，要从控制台/PowerShell 执行迁移，需要指定迁移上下文的 csproj 地址，如下所示：

dotnet ef migrations add InitialModification -p ..\DAL\DAL.csproj

感谢大家的贡献。