How to create a triangular shaped textbox in winforms using graphics path?

Yes actually iam trying out from morning how to do because as there is no method like addTriangle how to acheive this.as this was question asked in one interview place.as i couldn't able to answer this hence i was excited how to acheive this?

private void Form1_Load(object sender, EventArgs e)
{
System.Drawing.Drawing2D.GraphicsPath myGraphicsPath = new
System.Drawing.Drawing2D.GraphicsPath();
//myGraphicsPath.AddEllipse(new Rectangle(0, 0, 500, 700));
myGraphicsPath.AddLine(10,20,30,40);
//myGraphicsPath.AddEllipse(new Rectangle(75, 75, 20, 20));
//myGraphicsPath.AddEllipse(new Rectangle(120, 0, 125, 125));
//myGraphicsPath.AddEllipse(new Rectangle(145, 75, 20, 20));
//Change the button's background color so that it is easy
//to see.
textBox1.BackColor = Color.Chartreuse;
textBox1.Size = new System.Drawing.Size(256, 256);
textBox1.Region = new Region(myGraphicsPath);
}