.net - 在 C# 4.0 中为可选参数提供默认值

Question

如果其中一个参数是自定义类型，我该如何设置默认值？

public class Vehicle
{
   public string Make {set; get;}
   public int Year {set; get;}
}

public class VehicleFactory
{
   //For vehicle, I need to set default values of Make="BMW", Year=2011
   public string FindStuffAboutVehicle(string customer, Vehicle vehicle)
   {
       //Do stuff
   }
}

Answer 1

你不能，真的。但是，如果您不需要 null 来表示其他任何内容，您可以使用:

public string FindStuffAboutVehicle(string customer, Vehicle vehicle = null)
{
    vehicle = vehicle ?? new Vehicle { Make = "BMW", Year = 2011 };
    // Proceed as before 
}

在某些情况下这很好，但这确实意味着您不会发现调用者意外传递 null 的情况。

改用重载可能会更干净:

public string FindStuffAboutVehicle(string customer, Vehicle vehicle)
{
    ...
}

public string FindStuffAboutVehicle(string customer)
{
    return FindStuffAboutVehicle(customer, 
                                 new Vehicle { Make = "BMW", Year = 2011 });
}

Eric Lippert 关于 optional parameters and their corner cases 的帖子也值得一读.