给一个属性赋值，改变另一个属性的值

Question

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为什么赋值给后的secondDog.breed属性(第三行)，两个属性获得相同的值(firstDog)。和secondDog.Breed).

似乎只有secondDog应该显示&拉布拉多"，而不是firstDog !!

static void Main(string[] args)
{
var firstDog = new Dog() { Breed = "Bulldog" };
var secondDog = firstDog;
secondDog.Breed = "Labrador";   // Magic happens here !!!
Console.WriteLine(firstDog.Breed);      // Labrador (why ???)
Console.WriteLine(secondDog.Breed);     // Labrador
}

class Dog
{
public string Breed { get; set; }
}

为什么在下面的代码中我得到的结果与上面的代码(狗的例子)不同

int num = 5;
int numb = num;
numb = 30;
Console.WriteLine(num);    // 5  (Not 30 like in the above Dog example)
Console.WriteLine(numb);   // 30

Answer 1

这就是引用类型和值类型在操作中的区别。

Value类型将数据值保存在自己的内存空间中。它表示一个变量包含一个值。

int num = 5; // value type. We have created a value in memory
int numb = num; // here numb is 5. It is not pointing to `num`
// it just copies value of `num`. `numb` has own value 5 in 
// memory. We created the second value in memory. Here we have 
// two values in memory.
numb = 30; // here num is 30

但是，引用类型并不直接存储其值。它存储存储值的地址。也就是说，引用类型有一个指向保存数据的另一个内存位置的指针。

var firstDog = new Dog() { Breed = "Bulldog" }; // reference type
// We've created one object in memory
var secondDog = firstDog; // here you are assigning reference of 
// "firstDog" to "secondDog". We still have one object on 
// memory. We have not created new object in memory
secondDog.Breed = "Labrador";   // Here you are reading the address 
// of object which points to the object created in the first row of 
// this code snippet

在这里阅读更多关于引用类型和值类型的区别