Im very newbie and making a 2d spaceship game, similar to Stargunner.
I have target(enemies) coming from right side of screen and heading left, where my ship is at.
My targets are circles who randomly chooses Y coords from 0, 640.
The thing is. When one of my targets hit the upper "wall" i want them to bounce down.
And my problem is that i cant modify the value of Points. What can i do, is it even possible to modify Points values?

I have my properties in GameItem class.

"Cannot modify the return value of 'SpaceAttack.GameItem.Position' because it is not a variable"

 if (target.Position.Y < 0)
            {
                target.Position = new Point(this.target.Position.X, this.target.Position.Y = 0);

                target.Velocity = new Point(this.target.Velocity.X, this.target.Velocity.Y *= -1);
            }

If your enemy is less than the screen min or greater than the screen max you would reverse the speed of x and/or y.

xSpeed = 1; ySpeed = 1; If (target.x < screenMinX || target.x > screenMaxX) xSpeed = -xSpeed; //reverse if(target.y < screenMinY || target.y > screenMaxX) ySpeed = -ySpeed; //reverse</pre> target.x += xSpeed; target.y += ySpeed;

you mean that?

Why can you not modify the point?

The code you posted in the OP was trying to replace the point, which could be no good for any number of reasons depending on the class. Can't you just

if(target.Position.Y < 0) {
  target.Position.Y = 0;
  target.Velocity.Y = -target.Velocity.Y;
}

If target.Velocity is a C# Property then you won't be able to modify its members, since a Point in C# is of the struct data type. What this means is that target.velocity returns a temporary copy of your data, so modifying that has little meaning. The solution is to either just make velocity public, without any get:ers and set:ers, or create a method that allow you to set velocity, like so

public void SetVelocity(int velX, int velY)
{
      velocity.X = velX;
      velocity.Y = velY;
}

Why are you using Points by the way? I would suggest using Vector2 for things like speed and velocity. It's better suited for that kind of stuff

Hope that helps

Well, Vector2 aint possible to modify like variables either?

Yeah, but what you say, is that Vector2 is better to use than Point in games? Well I fixed it. And here is my new code. Thanks alot for the help guys! Realy amazing community here on gamedev!

            if (target.Position.Y < 0)
            {
                Vector2 vv;
                Vector2 vp;
                vv = target.Velocity;
                vv.Y *= -1;
                target.Velocity = vv;
                
                vp = target.Position;
                vp.Y = 0;
                target.Position = vp;            
            }
            else if (target.Position.Y > SCREEN_HEIGHT - target.Origin.Y)
            {
                Vector2 v2v;
                Vector2 v2p;
                v2v = target.Velocity;
                v2v.Y *= -1;
                target.Velocity = v2v;

                v2p = target.Position;
                v2p.Y = SCREEN_HEIGHT - target.Origin.Y;
                target.Position = v2p;      
            }