c# - 添加迁移时用于提取数据库连接字符串时，ConfigurationManager返回null

Question

我在.NET Core控制台应用程序中使用Entity Framework Core。我将连接字符串存储在App.config文件中，并在上下文类中使用ConfigurationManager来访问连接字符串。

当我想向项目中添加新的迁移时，出现以下错误:

System.NullReferenceException: Object reference not set to an instance of an object.

at EF_tut.Context.OnConfiguring(DbContextOptionsBuilder optionsBuilder) in C:\..\Context.cs:line 12
at Microsoft.EntityFrameworkCore.DbContext.get_InternalServiceProvider()
at Microsoft.EntityFrameworkCore.Internal.InternalAccessorExtensions.GetService[TService](IInfrastructure1 accessor)
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func1 factory) 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.OperationBase.<>c__DisplayClass3_0`1.b__0() at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)

Object reference not set to an instance of an object.

这是我的App.config文件:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <connectionStrings>
         <add name="EfTutDb" 
              connectionString="Data Source=.;Initial Catalog=EF_tut;Integrated Security=True;"/>
    </connectionStrings>
</configuration>

这是我的上下文类:

class Context : DbContext
{
    public DbSet<Student> Students { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer(ConfigurationManager.ConnectionStrings["EfTutDb"].ConnectionString); // Line 12.
    }
}

当我尝试在main方法中使用ConfigurationManger获取连接字符串时，我得到了连接字符串，但是当我添加迁移时，我得到了Null Reference错误。

Answer 1

也许这是一种解决方法，但是它可以工作:

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        var configuration = new ConfigurationBuilder()
            .SetBasePath(AppDomain.CurrentDomain.BaseDirectory)
            .AddXmlFile("App.config")
            .Build(); 

        optionsBuilder.UseSqlServer(configuration.GetValue<string>("connectionStrings:add:EfTutDb:connectionString"));
    }
}