Collision detection between a circle and rectangle

[Qobacha]

Hi,
I need to implement collision detection algorithm for the small game I'm developing. The objects i have here is ball/circle and a rectangle. The balls are moving vertically, while the rectangle is moving horizontally. The condition is that if the ball and rectangle touch each other then an event should be raised. I have been trying to do that for a while without success. This is my first program in C# so please bear with me.

Here is my code for the two objects that i want to check their collision

class Ball
    {        
        public float x, y, yvel, radius;
        public Brush brush;
        public Ball(int gamewidth,int gameHeight,Random r)
        {
           
            x = r.Next(gamewidth);
            y = r.Next(gameHeight);

            yvel = r.Next(2) + 5;

            radius = r.Next(10) + 5;
            brush = new SolidBrush(Color.Blue);            
        }
      
        public void Move(int gameHeight)
                       
        {               
            if (y + radius >= gameHeight)
            {
                y =  0;             
            }
            y += yvel;
        }
        public void Draw(Graphics g)
        {
            g.FillEllipse(brush, new RectangleF(x-radius,y - radius, radius * 2, radius * 2));
        }
    }

class Player
    {
        
        public float x, y, xvel;
        public Brush brush;

        public Player()
        {
        
        }

        public void DrawPlayer(Graphics g)
        {
            
            g.FillRectangle(brush, new RectangleF(x, y, 30,30));       
        }

        public void MovePlayerLeft(int gameWidth)
        {
            if (x > 0)
            {
                x -= xvel;
            }
        }
   

    }