英文:
ValueType as property in class
问题
我对类中值类型属性的概念有点困惑。
据我理解，当调用getter时，会返回值类型的副本。
让我们来看一个例子：
public struct Point
{
public int X;
public int Y;
public void Set(int x, int y)
{
X = x;
Y = y;
}
}
public class Test
{
public Int32 X { get; set; }
public void SetX(int x) => X = x;
public Point Point { get; set; }
}
static void Main(string[] args)
{
var test = new Test();
test.SetX(10);
test.Point.Set(20, 30);
Console.WriteLine(test.X);
Console.WriteLine($"{test.Point.X} {test.Point.Y}");
}
Point 没有被修改，因为它是值类型，getter 返回给我们一个副本，这是有道理的。我不明白的是，为什么在这种情况下 X 被修改？它也是一个值类型。这对我来说看起来有矛盾之处。
请帮助我理解我漏掉了什么。
英文:
I am a bit confused with a concept of value types as properties in a class.
As far as I understand, when getter is called - a copy of value type is returned.
Let's have an example:
public struct Point
{
public int X;
public int Y;
public void Set(int x, int y)
{
X = x;
Y = y;
}
}
public class Test
{
public Int32 X { get; set; }
public void SetX(int x) => X = x;
public Point Point { get; set; }
}
static void Main(string[] args)
{
var test = new Test();
test.SetX(10);
test.Point.Set(20, 30);
Console.WriteLine(test.X);
Console.WriteLine($"{test.Point.X} {test.Point.Y}");
}
Point is not modified, since it is a value type, and getter gives us a copy, which makes sense. The thing I don't understand - why in this case X is modified? It is a value type also. This looks like a contradiction to me.
Please help me to understand what I am missing
答案1
得分: 2
Point is not modified, since it is a value type, and getter gives us a copy, which makes sense
这里 Point 没有被修改，因为它是一个值类型，而 getter 给了我们一个副本，这是有道理的。
The thing I don't understand - why in this case X is modified?
我不明白的是 - 为什么在这种情况下 X 被修改了？
Because you are modifying object containing X not the copy of X. X = x invokes setter (there is no "returns copy" happening here) for auto-property which will write corresponding value to corresponding memory location.
因为你正在修改包含 X 的对象，而不是 X 的副本。X = x 调用了setter（这里没有发生“返回副本”），对于自动属性，它会将相应的值写入相应的内存位置。
Code for Point simulating this behavior can look for example like:
模拟这种行为的 Point 代码可以如下所示：
public void SetPoint(int x, int y) => Point = new Point{X= x, Y =y};
And in both cases the following will work:
而在两种情况下，以下代码都会起作用：
test.X = 42;
test.Point = new Point {X = 7, Y = 42}
英文:
> Point is not modified, since it is a value type, and getter gives us a copy, which makes sense
Yes, because you get a make changes to local copy of Point returned by auto-property getter.
> The thing I don't understand - why in this case X is modified?
Because you are modifying object containing X not the copy of X. X = x invokes setter (there is no "returns copy" happening here) for auto-property which will write corresponding value to corresponding memory location.
Code for Point simulating this behavior can look for example like:
public void SetPoint(int x, int y) => Point = new Point{X= x, Y =y};
And in both cases the following will work:
test.X = 42;
test.Point = new Point {X = 7, Y = 42}
答案2
得分: 0
修改 'Test.Point' 代码如下：
static void Main(string[] args)
{
var test = new Test();
test.SetX(10); // 你给属性赋了一个新值
// test.Point.Set(20, 30); - 你改变了一个值的副本，没有将结果分配给属性
test.Point = new Point(){ X = 20, Y = 30 }
Console.WriteLine(test.X);
Console.WriteLine($"{test.Point.X} {test.Point.Y}");
}
在值类型中，结构体只能拥有（用户定义的）属性或方法来修改自身。这让人感到困惑。你可以遵循建议，使用只读结构体以避免出现难以识别的行为可能性。
英文:
To modify 'Test.Point' code this:
static void Main(string[] args)
{
var test = new Test();
test.SetX(10); // you assign a new value to the property
// test.Point.Set(20, 30); - you change a copy of the value without assignment of the result to the property
test.Point = new Point(){ X = 20, Y = 30 }
Console.WriteLine(test.X);
Console.WriteLine($"{test.Point.X} {test.Point.Y}");
}
Among ValueTypes the struct only can have (user defined) properties or methods which can modify itself. It confuses a lot. You can follow recommendation to use the read-only structs to avoid any possibility of such behavior which is hard to identify.
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