Instead of text changed event handle the key up event.
private void Form1_Load(object sender, EventArgs e)
{
textBox1.KeyUp +=new KeyEventHandler(textBox1_KeyUp);
}
protected void textBox1_KeyUp(object sender, KeyEventArgs e)
{
if (e.KeyCode == Keys.Enter)
{
MessageBox.Show("Enter key pressed");
}
}