执行 add-migration net core 3 时出错答案

【问题标题】：Error to execute add-migration net core 3执行 add-migration net core 3 时出错
【发布时间】：2019-11-15 15:13:30
【问题描述】：

我正在尝试使用带有 net core 3 的 Entity Framework Core 设置一个新的数据库/迁移，但是出现了下一个错误。带有添加迁移初始你能帮帮我吗？

程序.cs

public class Program
    {
        public static void Main(string[] args)
        {
            CreateHostBuilder(args).Build().Run();
        }

        public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    webBuilder.UseStartup<Startup>();
                });
    }

appsettings.json

{
  "connectionString": {
    "MyConnection": "data source=Localhost;Database=MyDataBase;User Id=MyUser;Password=MyPass;MultipleActiveResultSets=True"
  },
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },

  "AllowedHosts": "*"
}

Startup.cs

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

        public IConfiguration Configuration { get; }
        private void ConfigureJson(MvcNewtonsoftJsonOptions obj)
        {
            obj.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();
            obj.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;

        }
        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            string contactConnection = Configuration.GetConnectionString("MyConnection");
            services.AddDbContext<ApplicationContactDbContext>(options =>
                    options.UseSqlServer(contactConnection)
            );
            services.AddMvc()
                   .SetCompatibilityVersion(CompatibilityVersion.Version_3_0)
                   .AddNewtonsoftJson(ConfigureJson);
           services.AddCors(options =>
            {
                options.AddPolicy("EnableCORS", builder => { builder.WithOrigins("*").AllowAnyHeader().AllowAnyMethod(); });
            });

            services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
            services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
             .AddJwtBearer(options =>
             {
                 options.TokenValidationParameters = new TokenValidationParameters
                 {
                     ValidateIssuer = true,
                     ValidateAudience = true,
                     ValidateLifetime = true,
                     ValidateIssuerSigningKey = true,
                     ValidIssuer = "http://localhost:5000",
                     ValidAudience = "http://localhost:5000",
                     IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("superSecretKeyYoSoyExitoso@345"))
                 };
             });
            services.AddControllers();

        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseRouting();

            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }

这是错误：

System.ArgumentNullException：值不能为空。参数名称：连接字符串在 Microsoft.EntityFrameworkCore.Utilities.Check.NotEmpty（字符串值，字符串参数名称）在 Microsoft.EntityFrameworkCore.SqlServerDbContextOptionsExtensions.UseSqlServer（DbContextOptionsBuilder optionsBuilder，字符串 connectionString，Action1 sqlServerOptionsAction) at Elapoint.ContactExtension.MicroService.Startup.<>c__DisplayClass5_0.<ConfigureServices>b__0(DbContextOptionsBuilder options) in E:\PersonalesX\Proyectos\PV5\NetCore\Extensions\Contacts\Api\Startup.cs:line 37 at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass1_02.b__0（IServiceProvider p，DbContextOptionsBuilder b）在 Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.DbContextOptionsFactory[TContext](IServiceProvider applicationServiceProvider, Action2 optionsAction) at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass10_01.b__0(IServiceProvider p) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory（FactoryCallSite factoryCallSite，RuntimeResolverContext 上下文）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.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 singletonCallSite, RuntimeResolverContext context) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite（ServiceCallSite callSite，TArgument 参数）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve（ServiceCallSite callSite，ServiceProviderEngineScope 范围）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.c__DisplayClass1_0.b__0（ServiceProviderEngineScope 范围）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService（类型 serviceType，ServiceProviderEngineScope serviceProviderEngineScope）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService（类型 serviceType）在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService（IServiceProvider 提供程序，类型 serviceType）在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T]（IServiceProvider 提供程序）在 Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.c__101.<AddCoreServices>b__10_1(IServiceProvider p) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, RuntimeResolverContext context) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSiteMain(ServiceCallSite callSite, TArgument 参数) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache（ServiceCallSite callSite，RuntimeResolverContext 上下文，ServiceProviderEngineScope serviceProviderEngine，RuntimeResolverLock lockType）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache（ServiceCallSite singletonCallSite，RuntimeResolverContext 上下文）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(ServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitIEnumerable(IEnumerableCallSite enumerableCallSite, RuntimeResolverContext context) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSiteMain(ServiceCallSite callSite, TArgument 参数) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache（ServiceCallSite callSite，RuntimeResolverContext 上下文，ServiceProviderEngineScope serviceProviderEngine，RuntimeResolverLock lockType）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache（ServiceCallSite singletonCallSite，RuntimeResolverContext 上下文）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.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.MigrationsOperations.AddMigration(String name, String outputDir, String contextType) at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType) at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_1.<.ctor>b__0() at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_01.b__0() 在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute（操作动作）值不能为空。参数名称：连接字符串

【问题讨论】：

标签： entity-framework-core asp.net-core-webapi .net-core-3.0

【解决方案1】：

在您的 appsettings 文件中，尝试将“connectionString”重命名为“ConnectionStrings”

【讨论】：

我也有同样的问题
根据您收到的错误，这似乎是一个配置问题，而不是实体框架的特定问题。从您的 appsettings 来看，这个答案看起来是正确的。缺少的特定部分将是 ConnectionStrings. 末尾的 s