不调用构造函数

Question

public class Unit
{
    public int UnitId { get; set; }
    public Engine EngineStuff { get; set; }

}
public class Engine
{
    public int PS { get; set; }
    public int MaxSpeed { get; set; }
}

var unit = new Unit();
unit.UnitId = 3; //OK because Unit-constructor was called
unit.EngineStuff.PS = 200; //error, because EngineStuff-constructor obviously wasn't called.

如何调用这个“内部”构造函数？ 我以为它会自动初始化它？

我怎样才能简单地为属性“EngineStuff.PS”赋值？

Answer 1

您必须显式实例化对象的属性：

public class Unit
{
    public Unit()
    {
        EngineStuff = new Engine();
    }
}

---

或者，如果要手动控制何时实例化：

var unit = new Unit();
unit.UnitId = 3; 
unit.EngineStuff = new Engine();
unit.EngineStuff.PS = 200;

或者更简单：

var unit = new Unit { UnitId = 3 };
unit.EngineStuff = new Engine { PS = 200 };

Answer 2

您只需在Unit构造函数中或在使用它之前初始化EngineStuff 。

public class Unit
{
    public Unit()
    {
        EngineStuff = new Engine();
    }
    public int UnitId { get; set; }
    public Engine EngineStuff { get; set; }

}

要么 ：

var unit = new Unit();
unit.UnitId = 3;
unit.EngineStuff = new Engine():
unit.EngineStuff.PS = 200; 

Answer 3

EngineStuff只是一个引用，您需要将其值设置为您在Unit构造函数，属性初始值设定项或普通代码中创建的Engine对象。

所以要么：

// Constructor
public class Unit
{
    public Unit() { EngineStuff = new Engine(); }
    public int UnitId { get; set; }
    public Engine EngineStuff { get; set; }
}

要么

// Property initializer
var unit = new Unit
{
    UnitId = 3,
    EngineStuff = new Engine { PS = 200 }
};

要么

// Plain code
var unit = new Unit();
unit.UnitId = 3;
unit.EngineStuff = new Engine { PS = 200 };

Answer 4

不，你需要像下面那样明确地设置它，因为EngineStuff是Engine类型，所以你在访问它的任何成员之前首先实例化它。

 unit.EngineStuff = new Engine { PS = 10, MaxSpeed = 2000 };

（或者）您可以更改Unit类以将EngineStuff作为getter属性并在下面创建实例

    public Engine EngineStuff 
    { 
        get 
        {
            return new Engine();
        }
    }

然后你可以访问它

        var unit = new Unit();
        unit.UnitId = 3;
        unit.EngineStuff.PS = 100;

Answer 5

除了其他答案之外，您还可以定义getter属性，以便在第一次需要时创建实例：

public class Unit
{
    public int UnitId { get; set; }

    private Engine engineStuff;
    public Engine EngineStuff
    {
        get
        {
            if (engineStuff == null) engineStuff = new EngineStuff();
            return engineStuff;
        }
    }
}