You shouldn't use the control private tooltip, but the form one. This example works well:
public partial class Form1 : Form
{
private System.Windows.Forms.ToolTip toolTip1;
public Form1()
{
InitializeComponent();
this.components = new System.ComponentModel.Container();
this.toolTip1 = new System.Windows.Forms.ToolTip(this.components);
MyRitchTextBox myRTB = new MyRitchTextBox();
this.Controls.Add(myRTB);
myRTB.Location = new Point(10, 10);
myRTB.MouseEnter += new EventHandler(myRTB_MouseEnter);
myRTB.MouseLeave += new EventHandler(myRTB_MouseLeave);
}
void myRTB_MouseEnter(object sender, EventArgs e)
{
MyRitchTextBox rtb = (sender as MyRitchTextBox);
if (rtb != null)
{
this.toolTip1.Show("Hello!!!", rtb);
}
}
void myRTB_MouseLeave(object sender, EventArgs e)
{
MyRitchTextBox rtb = (sender as MyRitchTextBox);
if (rtb != null)
{
this.toolTip1.Hide(rtb);
}
}
public class MyRitchTextBox : RichTextBox
{
}
}