KeyEventArgs not found

Question

My Key Event is not working on asp.net. I haven't really got an idea on how to fix it. I searched the internet or answers but haven't found them. So maybe you could help me.

Error code:

The type or namespace name 'keyeventargs' could not be found (are you missing a using directive or an assembly reference?

This is the code I am using.

private void TextBox1_KeyDown(object sender, KeyEventArgs e)
{
    Keys iCode = e.KeyCode;
    String expr = TextBox1.Text;

    switch (iCode)
    {
        case Keys.Return:
            {
                Calc(expr);

                if (expr.Length >= 0)
                    m_history.Add(expr);
                else
                    TextBox1.Text = "";

                meHistory.SelectionStart = meHistory.TextLength;
                meHistory.ScrollToCaret();
            }
            break;

        case Keys.Up:
        case Keys.Down:
            {
                m_histLine = Math.Max(0, Math.Min(m_history.Count - 1, m_histLine));

                if (m_history.Count == 0)
                    break;

                TextBox1.Text = m_history[m_histLine];
                TextBox1.Select(TextBox1.Text.Length, 0);

                m_histLine += (iCode == Keys.Up) ? -1 : 1;
            }
            break;
    } // switch keyvode
}