EFCore 入门遇到的几个问题

1.  配置的GetConnectionString 怎么取不到链接数据库的字串？

   public void ConfigureServices(IServiceCollection services)
   {
      services.AddDbContext<EntityContext>(options => options.UseSqlServer(_Configuration.GetConnectionString("DbConnection"))); }

json 文件：

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "ConnectionString": {
    "DbConnection": "Server=(localdb)\\mssqllocaldb;Database=EntityDb;Trusted_Connection=True;"
  }
}

 查询官方文档得知 json 配置的字串少一个s, 

 ConnectionString->ConnectionStrings

还遇到一次，加了s怎么还是取不到？仔细看配置文件，别把配置写到Logging 节点里面去了。

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft": "Warning",
      "Microsoft.Hosting.Lifetime": "Information"
    }
  },
  "ConnectionStrings": {
    "DbConnection": "Server=(localdb)\\mssqllocaldb;Database=ForestryDB;Trusted_Connection=True;"
  }
}

2. 在PMC 中执行Add-Migration 不能识别，可能你没有安装Microsoft.EntityFrameworkCore.Tools 包

PM> Add-Migration
Add-Migration : The term 'Add-Migration' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:1
+ Add-Migration
+ ~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (Add-Migration:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException
 

总结下需要安装的包，

Repository 需要安装两个包

 1.Microsoft.EntityFrameworkCore.SqlServer

 2.Microsoft.EntityFrameworkCore.Tools

项目启动文件需要安装

 1.Microsoft.EntityFrameworkCore.SqlServer

 2.Microsoft.EntityFrameworkCore.Design

配置中还可以指定生成的迁移文件在哪个程序集里面

  services.AddDbContext<EntityDBContext>(options =>
            {
                options.UseSqlServer(Configuration.GetConnectionString("DbConnection"), builder=> { builder.MigrationsAssembly("DL.Forestry.Repository"); });
            });

 引入Serilog 后重写了加载配置的方法，发现不能EF 不能migration，一直报错执行不成功，赶快打开调试模式Verbose查看详细错误  add-migration "modified user" -verbose

Using environment 'Development'.
System.ArgumentNullException: Value cannot be null. (Parameter 'config')
   at Microsoft.Extensions.Configuration.ChainedBuilderExtensions.AddConfiguration(IConfigurationBuilder configurationBuilder, IConfiguration config, Boolean shouldDisposeConfiguration)
   at Microsoft.Extensions.Configuration.ChainedBuilderExtensions.AddConfiguration(IConfigurationBuilder configurationBuilder, IConfiguration config)
   at DL.Forestry.WebApi.Program.<>c.<CreateHostBuilder>b__3_0(IConfigurationBuilder configurationBuilder) in D:\Forestry\DL.Forestry.WebApi\Program.cs:line 59
   at Microsoft.Extensions.Hosting.HostBuilder.BuildHostConfiguration()
   at Microsoft.Extensions.Hosting.HostBuilder.Build()
An error occurred while accessing the Microsoft.Extensions.Hosting services. Continuing without the application service provider. Error: Value cannot be null. (Parameter 'config')
No application service provider was found.
Finding DbContext classes in the project...
Found DbContext 'EntityDBContext'.
Microsoft.EntityFrameworkCore.Design.OperationException: Unable to create an object of type 'EntityDBContext'. 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[DL.Forestry.Repository.EntityDBContext]' while attempting to activate 'DL.Forestry.Repository.EntityDBContext'.
   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.AddMigration(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType, String namespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>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)
Unable to create an object of type 'EntityDBContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728

错误在59 行，先以为是没有执行到config 里面的appsetting.json. 于是手动addjson 还是无效，打断点，断点也没进来，奇怪怎么回事？仔细看代码 59 行我们重写了系统的委托 ConfigureHostConfiguration

后来查资料得知，原来EFCore在执行迁移的时候不执行Main方法，直接调用 ConfigureHostConfiguration 这个默认委托的实现（core 3.1 里是这样，后续版本不清楚是不是会更新），这里我们重写了，所以加载不到系统配置，注释掉写的委托，OK了，记得migration 完得还原。