Collision detection between a circle and a rectangle

I need to implement collision detection algorithm for a program I'm working on. The objects i have here are a 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 but without success. This is my first program in C# so please bear with me.