c# - 带有中间变量的构造函数链接

Question

我遇到以下重载构造函数的情况，我正在努力寻找一个好的解决方案。我看不出如何使用带有构造函数链的中间赋值。

以下内容无效，但显示了我想做的事情

public MyThing(IServiceLocator services, int? userId)
{
    // blah....
}

public MyThing(IServiceLocator services, string userName)
{
    User user = services.UserService.GetUserByName(userName);
    int userId = user == null ? null : (int?)user.Id;
    // call the other constructor   
    this(services, userId);
}

我知道用有效代码编写上面的唯一方法是

public MyThing(IServiceLocator services, string userName)
    : this(services,
           services.UserService.GetUserByName(userName) == null ?
              null : (int?)services.UserService.GetUserByName(userName).Id)

这不仅是丑陋的代码，而且还需要调用两次数据库(除非编译器足够聪明来解决这个问题，我对此表示怀疑)。

有没有更好的写法？

Answer 1

这个怎么样:

public MyThing(IServiceLocator services, string userName)
{
    User user = services.UserService.GetUserByName(userName);
    int? userId = user == null ? null : (int?)user.Id;

    Initialize(services, userId);
}


public MyThing(IServiceLocator services, int? userId)
{
    Initialize(services, userId);
}

private void Initialize(IServiceLocator services, int? userId)
{
    // initialization logic
}

编辑

如果我是你，我会用这样的工厂方法替换构造函数:

private MyThing(IServiceLocator services, int? userId)
{
    // blah....
} 

public static Create(IServiceLocator services, int? userId)
{
    return new MyThing(services, userId);
}

public static Create(IServiceLocator services, string userName)
{
    User user = services.UserService.GetUserByName(userName);
    int userId = user == null ? null : (int?)user.Id;

    return new MyThing(services, userId);
}

用法:

var myThing = MyThing.Create(services, 123);
var myOtherThing = MyThing.Create(services, "userName");

Replace Constructor with Factory Method (refactoring.com)