insert null value in a textbox

Question

i have a textbox which have a decimal value 0.5.

this code to enable or disable the textbox.enable textbox to put 0.5 in the textbox.

when the textbox is disabled value null should be inserted in the database tablle

private void checkBox1_CheckedChanged(object sender, EventArgs e)
        {
            if (!checkBox1.Checked)
            {
                textBox4.Enabled = false;

            }
            else
            {
                textBox4.Enabled = true;
            }
        }

this code for inserting both null or 0.5 value from the textbox

  
SqlParameter pIn;
                    if (checkBox1.Checked == true)
                    {
decimal half = Convert.ToDecimal(textBox4.Text);

                        command.Parameters.Add("@HalfDay", SqlDbType.Decimal).Value = half;

                        textBox4.Text = String.Format("{0:0.0}", half);
                    }
                    else

                  if (checkBox1.Checked == false)
{
if (String.IsNullOrEmpty(textBox4.Text))
                                {
pIn = command.Parameters.Add("@HalfDay", SqlDbType.Decimal);
                                    pIn.Precision = 18;
                                    pIn.Scale = 2;//string s = Convert.ToString(radioButton2.Text);
                                    pIn.Value = DBNull.Value;
}
}

i can insert 0.5 via textbox but unable to insert null.

---

emmy