What's the meaning of this.textBox2.MouseEnter += new System.EventHandler(this.textBox1_MouseEnter);

1. This means that you attached a delegate of the method this.textBox1_MouseEnter to the event textBox2.MouseEnter. 2. In this case, the += operator is the attachement operator used to attach delegates to events. 3. This means that whenever the textBox2.MouseEnter event is fired, the method this.textBox1_MouseEnter will be called via its delegate.

this.textBox2 is your text box. MouseEnter is a delegate. It is called when the mouse enters the focus of your control. += basically sets what is going to happen when this happens. Your function is: textBox1_MouseEnter. If you were to change this to be: this.textBox2.MouseEnter += new System.EventHandler(this.test); this.textBox2.MouseEnter += new System.EventHandler(this.textBox1_MouseEnter); it would call first the test function, then it would call the textBox1_MouseEnter function.