C＃-如何修改由吸气剂给定的结构？

Question

I'm quite new to C# and I'm still figuring things out. My problem is that I've got a class like this:

class Sprite {
    private Vector3 _position;

    public Vector3 Position {
        get { return _position; }
        set { _position = value;
              HandleEvent(); }
    }

    public Sprite() {
        _position = new Vector3();
    }
}

With a Vector3 object and a get/set block. Here is a simplified version of the Vector3 struct:

struct Vector3 {
    float X, Y, Z;
}

Now my problem is that if I do this:

Sprite sprite = new Sprite();
sprite.Position.X += 4.0F;

It gives me an error because Position is not a variable but given to me by a getter. But I don't want to make _position public, because I want to handle an event when the variable is changed.

(In Java this works perfectly, that's why I'm confused :/)

Could you give me a hint what I'm doing wrong or what you would do differently?

Edit:

Yes, Vector3 is a struct and not a class. Sorry for the confusion, but I'm pretty new to C#.

Answer 1

(In Java this works perfectly, that's why I'm confused :/)

Java does not have what C# calls struct .

In C#, struct creates a value type . When you return a value type from a property getter, you don't return a reference to an already existing object somewhere, you return a brand new copy of the value. And because of that, it does not make sense to modify that copy of the value: no one would see the modification.

In Java, you would use class for this. In C#, you can do the same. That creates a reference type. When you return a reference type from a property getter, you don't return a copy of all of the object's fields, you just return a reference to the same object. Here, modifications do make sense. You have multiple references to the same object, and the modifications are still visible through any of the other references.

Answer 2

It should work fine. This sample compiles:

namespace ConsoleApplication8
{
    class Sprite
    {
        private Vector3 _position;

        public Vector3 Position
        {
            get { return _position; }
            set
            {
                _position = value;
                // HandleEvent();
            }
        }

        public Sprite()
        {
            _position = new Vector3();
        }
    }

    class Vector3
    {
        public float X, Y, Z;
    }

    class Program
    {
        static void Main()
        {
            Sprite sprite = new Sprite();

            sprite.Position.X += 4.0F;
        }
    }
}

As your vector seems to be a struct, because a struct is a value-type (which means your getter that is called will return a copy of your position) you cannot operate on what your getter returns. You will need to make a new Vector and assign that one as a whole. Please note that this seems to be an optimization somebody made for you. If the Vector object definition belongs to you, just make it a class so it works like you are used to from Java.

If you want this to be a struct, you will have to create a new one and assign it:

            sprite.Position = new Vector 
                              {
                                  X = sprite.Position.X + 4.0F,
                                  Y = sprite.Position.Y,
                                  Z = sprite.Position.Z
                              };

Answer 3

Because Vector3 is a struct, it gets copied around implicitly. For example, the following code creates two vectors, instead of one:

Vector3 tmp1 = new Vector3();
Vector3 tmp2 = tmp1;

This can be confusing if you're mostly used to classes, where an instance is not copied and not created unless you explicitly say so (using new , for example).

Combine the above with the fact that C# properties are syntactic sugar for methods, and you'll have your answer. This:

Sprite sprite = new Sprite();
sprite.Position.X += 4.0F;

Is seen like this:

Sprite sprite = new Sprite();
sprite.get_Position().X += 4.0F;

Which would be executed like this:

Sprite sprite = new Sprite();
Vector3 tmp1 = sprite.get_Position();
tmp1.X += 4.0F;

Notice that tmp1 is a brand new Vector3 . It's unrelated to sprite.Position in every way, except for the fact that they contained equal coordinates at some point.

Changing that temporary Vector3 won't have an effect on the Vector3 returned by the property. The C# compiler is aware of this unfortunate pattern and refuses to compile it because it's almost certainly not what you want.

To work around this, you can try taking the position explicitly, like above, and then explicitly put it back:

Sprite sprite = new Sprite();
Vector3 tmp1 = sprite.Position;
tmp1.X += 4.0F;
sprite.Position = tmp1;

In general, you should avoid mutable structs for reasons like this.

---

Java doesn't have this problem because it doesn't have structs at all.

Answer 4

Try this:

 Sprite sprite = new Sprite();
        Vector3 v = sprite.Position;
        v.X += 0.4F;
        sprite.Position = v;

Cause of Error:

The problem is that sprite.Position returns a struct, so when you access it, it creates a copy of it due to the value semantics. += attempts to modify that value, but it'll be modifying a temporary copy - not the actual struct stored in your sprite .

You have to take the value from reading the Position and put it into a local variable, change that local variable, and then assign the local variable back into Position.