Can't encode/decode textboxValue
<p>how to display html tags with textbox ?
 now i give this error : </p>

<blockquote>
  <p>A potentially dangerous Request.Form value was detected from the
  client (ctl00$MainContent$TextBoxDescription="<b>OP</b>").</p>
</blockquote>

<pre><code>  public void InsertCost()
                {

                string connectionString = cs.getConnection();
                DateTime timeNow = DateTime.Now;
                string insertSql = "INSERT INTO costs(Value,Description,DateCreate,IdCategory,IdUser) VALUES(@Value,@Description, @DateCreate,@CategoryId,@UserId)";

                using (SqlConnection myConnection = new SqlConnection(connectionString))
                {
                    myConnection.Open();
                    SqlCommand command = new SqlCommand(insertSql, myConnection);

                    command.Parameters.AddWithValue("@Value", Server.HtmlEncode(TextBoxValue.Text));
                    command.Parameters.AddWithValue("@Description", Server.HtmlDecode(TextBoxDescription.Text));
                    command.Parameters.AddWithValue("@DateCreate", timeNow);
                    command.Parameters.AddWithValue("@CategoryId", DropDownListCategory.SelectedValue);
                    command.Parameters.AddWithValue("@UserId", cui.getCurrentId());
                    command.ExecuteNonQuery();

                    Response.Redirect("~/Cost.aspx");
                    myConnection.Close();
                }

            }
</code></pre>
