get correct rtb.SelectionStart when pressing Keys.Up

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C#

I want to get rtb.SelectionStart when pressing the keys: up, down, left, right.I can get it when I press: Left, Right.But I just get it when I press up 2 times.

//store SelectionStart
public int ise=0;
//richTextBox1_TextChanged:
ise = richTextBox1.SelectionStart;

//keydown
private void richTextBox1_KeyDown(object sender, KeyEventArgs e)
{
            if (e.KeyCode == Keys.Left || e.KeyCode==Keys.Right)
            {
                ise = richTextBox1.SelectionStart;
                textBox1.Text = string.Format("select start:{0}", ise.ToString());
            }
            if (e.KeyCode == Keys.Up)
            {
                ise = richTextBox1.SelectionStart;
                textBox1.Text = string.Format("select start:{0}", ise.ToString());
            }
            if (e.KeyCode == Keys.Down)
            {
                ise = richTextBox1.SelectionStart;
                textBox1.Text = string.Format("select start:{0}", ise.ToString());
            }
}