public void Form1_Load(Object sender, EventArgs e)
{
... // Other initialization code
maskedTextBox1.Mask = "00/00/0000";
maskedTextBox1.MaskInputRejected += new MaskInputRejectedEventHandler(maskedTextBox1_MaskInputRejected)
}
void maskedTextBox1_MaskInputRejected(object sender, MaskInputRejectedEventArgs e)
{
toolTip1.ToolTipTitle = "Invalid Input";
toolTip1.Show("We're sorry, but only digits (0-9) are allowed in dates.", maskedTextBox1, maskedTextBox1.Location, 5000);
}