为 CLI 实体框架迁移的 DbContext 建立连接字符串时遇到问题答案

【问题标题】：Trouble establishing connection string for DbContext for CLI entity framework migration为 CLI 实体框架迁移的 DbContext 建立连接字符串时遇到问题
【发布时间】：2020-04-22 02:04:11
【问题描述】：

在 ASP.NET Core 3.0 Web 应用程序中，尝试建立到 VS 本地数据库的连接字符串。

我正在关注一个在 2.1 中完成的在线项目，并且没有遇到很多问题。以下是我当前的代码，与讲师匹配。尝试通过 CLI 开始进行迁移时发生错误。

在 appsettings.JSON 中；

"ConnectionStrings": {
    "ProjectDb": "Data Source=(localdb)\\MSSQLLocalDB;Initial Catalog=Project;Integrated Security=True;"
}

在 Startup.cs 中；

public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContextPool<OdeToFoodDbContext>(options =>
            {
                options.UseSqlServer(Configuration.GetConnectionString("ProjectDb"));
            });

在 ProjectDbContext.cs 中（在单独的项目中）；

public class ProjectDbContext : DbContext
    {
        public ProjectDbContext(DbContextOptions<ProjectDbContext> options)
            : base(options)
        {

        }

当我尝试在 CLI 中使用以下命令访问它时；

dotnet ef dbcontext info --project <project.data> --startup-project <project.web>

我收到以下错误；

System.ArgumentNullException: Value cannot be null. (Parameter 'connectionString')
   at Microsoft.EntityFrameworkCore.Utilities.Check.NotEmpty(String value, String parameterName)
   at Microsoft.EntityFrameworkCore.SqlServerDbContextOptionsExtensions.UseSqlServer(DbContextOptionsBuilder optionsBuilder, String connectionString, Action`1 sqlServerOptionsAction)
   at <project.web>.Startup.<ConfigureServices>b__4_0(DbContextOptionsBuilder options) in <project.web>..\Startup.cs:line 30
   at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass3_0`2.<AddDbContextPool>b__0(IServiceProvider _, DbContextOptionsBuilder ob)
   at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass5_0`2.<AddDbContextPool>b__0(IServiceProvider sp, DbContextOptionsBuilder ob)
   at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.CreateDbContextOptions[TContext](IServiceProvider applicationServiceProvider, Action`2 optionsAction)
   at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass10_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)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitRootCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at 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__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.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__10`1.<AddCoreServices>b__10_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.VisitRootCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitIEnumerable(IEnumerableCallSite enumerableCallSite, 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.VisitRootCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at 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__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.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.ServiceProviderServiceExtensions.GetServices[T](IServiceProvider provider)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextTypes()
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextType(String name)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.GetContextInfo(String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.GetContextInfoImpl(String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.GetContextInfo.<>c__DisplayClass0_0.<.ctor>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
Value cannot be null. (Parameter 'connectionString')

我在错误中看到这一行；

   ... in <project.web>..\Startup.cs:line 30

这行代码是什么；

options.UseSqlServer(Configuration.GetConnectionString("ProjectDb"));

我尝试将连接字符串直接输入到启动文件中，但同样的错误。

本地数据库中尚不存在该数据库，但已针对连接字符串中的“初始”标记进行了指示和注释。

这是不正确的，数据库现在需要先存在吗？或者您能否看到我尝试使用 CLI 开始迁移的代码/方法的任何其他问题？

【问题讨论】：

通知 OdeToFoodDbContext 与 ProjectDbContext。这是问题中的错误还是您的代码中的错误？
问题。尝试交换通用名称失败...

标签： c# entity-framework asp.net-core .net-core entity-framework-core

【解决方案1】：

我不知道根本的变化是什么，但现在正在工作。

我重建了项目。要检查连接字符串是否实际上为空，请执行在随机页面上打印的步骤。

打印连接字符串后，我复制了 page.html.cs 的工作代码并粘贴到 startup.cs 部分。

现在工作，但我没有发现任何实质性差异？

【讨论】：