更新数据库（EF Core 5）失败（代码优先）

Question

我将环境升级到 ASP.NET Core 5.01，但在使用代码优先方法更新数据库时遇到了一些困难。 Add-Migration 命令按预期工作并生成正确的迁移源文件。但是，当我尝试运行更新数据库时，我遇到了错误。这是我的 PMC 的输出：

PM> add-migration Project
Build started...
Build succeeded.
To undo this action, use Remove-Migration.
PM> Update-Database
Build started...
Build succeeded.
Unable to create an object of type 'SaberDbContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728
PM> 

运行 Update-Database -verbose 时，我得到以下输出：

> C:\Program Files\dotnet\dotnet.exe exec --depsfile
> C:\TFS_VSTUDIO\SaberToothPortal\SaberTooth.Data\bin\Debug\net5.0\SaberTooth.Model.deps.json
> --additionalprobingpath C:\Users\valere\.nuget\packages --additionalprobingpath "C:\Program Files\dotnet\sdk\NuGetFallbackFolder" --runtimeconfig
> C:\TFS_VSTUDIO\SaberToothPortal\SaberTooth.Data\bin\Debug\net5.0\SaberTooth.Model.runtimeconfig.json
> C:\Users\valere\.nuget\packages\microsoft.entityframeworkcore.tools\5.0.1\tools\netcoreapp2.0\any\ef.dll
> database update --verbose --no-color --prefix-output --assembly
> C:\TFS_VSTUDIO\SaberToothPortal\SaberTooth.Data\bin\Debug\net5.0\SaberTooth.Model.dll
> --startup-assembly C:\TFS_VSTUDIO\SaberToothPortal\SaberTooth.Data\bin\Debug\net5.0\SaberTooth.Model.dll
> --project-dir C:\TFS_VSTUDIO\SaberToothPortal\SaberTooth.Data\ --language C# --working-dir C:\TFS_VSTUDIO\SaberToothPortal --root-namespace SaberTooth.Model Using assembly 'SaberTooth.Model'. Using startup assembly 'SaberTooth.Model'. Using application base
> 'C:\TFS_VSTUDIO\SaberToothPortal\SaberTooth.Data\bin\Debug\net5.0'.
> Using working directory
> 'C:\TFS_VSTUDIO\SaberToothPortal\SaberTooth.Data'. Using root
> namespace 'SaberTooth.Model'. Using project directory
> 'C:\TFS_VSTUDIO\SaberToothPortal\SaberTooth.Data\'. Remaining
> arguments: . Finding DbContext classes... Finding
> IDesignTimeDbContextFactory implementations... Finding application
> service provider in assembly 'SaberTooth.Model'... Finding
> Microsoft.Extensions.Hosting service provider... No static method
> 'CreateHostBuilder(string[])' was found on class 'Program'. No
> application service provider was found. Finding DbContext classes in
> the project... Found DbContext 'SaberDbContext'.
> Microsoft.EntityFrameworkCore.Design.OperationException: Unable to
> create an object of type 'SaberDbContext'. For the different patterns
> supported at design time, see
> https://go.microsoft.com/fwlink/?linkid=851728  --->
> System.InvalidOperationException: Unable to resolve service for type
> 'Microsoft.EntityFrameworkCore.DbContextOptions`1[SaberTooth.Model.SaberDbContext]'
> while attempting to activate 'SaberTooth.Model.SaberDbContext'.    at
> Microsoft.Extensions.DependencyInjection.ActivatorUtilities.ConstructorMatcher.CreateInstance(IServiceProvider
> provider)    at
> Microsoft.Extensions.DependencyInjection.ActivatorUtilities.CreateInstance(IServiceProvider
> provider, Type instanceType, Object[] parameters)    at
> Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetServiceOrCreateInstance(IServiceProvider
> provider, Type type)    at
> Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass13_4.<FindContextTypes>b__13()
> --- End of inner exception stack trace ---    at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass13_4.<FindContextTypes>b__13()
> at
> Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func`1
> factory)    at
> Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String
> contextType)    at
> Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.UpdateDatabase(String
> targetMigration, String connectionString, String contextType)    at
> Microsoft.EntityFrameworkCore.Design.OperationExecutor.UpdateDatabaseImpl(String
> targetMigration, String connectionString, String contextType)    at
> Microsoft.EntityFrameworkCore.Design.OperationExecutor.UpdateDatabase.<>c__DisplayClass0_0.<.ctor>b__0()
> at
> Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action
> action)

输出表明 Program.cs 类中没有 CreateHostBuilder，但它显然存在：

namespace SaberTooth.Model
{
    public class Program
    {
        public static void Main(string[] args)
        {
            CultureInfo.DefaultThreadCurrentCulture = new CultureInfo("en-US");
            CreateHostBuilder(args).Build().Run();
        }

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

如果我随后尝试发出 Remove-Migration 命令，我会得到相同的结果：

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

我查看了错误消息中的建议链接，但我没有看到我做错了什么。我还使用完全相同的步骤（在升级之前）毫无问题地执行了六次迁移，这些步骤是：

1.  Create the Model:  
2.  Create the Interface:
3.  Add a DBSet Declaration in the SaberDbContext.cs File
4.  Create the Repository
5.  Add a Scope to the Startup.cs File
6.  Run the add-migration command
7.  Run the update-database command

我对 EF 还很陌生。这是我第一次尝试将它用于项目。任何帮助将不胜感激。升级到 .Net Core 5.0.1 后，我是否做错了什么？

谢谢

--- 值

Answer 1

我终于让它工作了。这是我所做的：

我更新了 Startup 类中的 Configure Services 方法并更改了以下条目：

services.AddDbContext<SaberDbContext>();

到：

    string _connectionString;
    string appSettings = String.Concat(Directory.GetCurrentDirectory(), "/appsettings.json");
    var configurationSettings = new ConfigurationBuilder().AddJsonFile(appSettings).Build();
    _connectionString = configurationSettings["ConnectionStrings:DefaultConnection"];

    services.AddDbContext<SaberDbContext>(options =>
        options.UseSqlServer(_connectionString));

这就是诀窍。不知道为什么这适用于 .NET Core 3.x 而不适用于 5.x。就像我说的，我是实体框架和 Web 应用程序开发的新手（这是我的第一个项目），所以我确定在某个地方我没有使用正确的定义。不过现在它可以工作了！

--- 值