Custom Validation

Question

I am using custom validation. my problem is on save button data is inserted in table. e.isvalid=false in this case also data is inserted and in other case also. how can i use custom validation. so that if validation occurs data should not enter database.

.aspx
<asp:CustomValidator ID="cv_login" runat="server" ValidationGroup="add" Enabled="true" ControlToValidate="txt_username" OnServerValidate="cv_login_validate"></asp:CustomValidator>

.cs
protected void cv_login_validate(object sender, ServerValidateEventArgs e)
  {
............
........
if (temp > 0)
  {
  e.IsValid = false;
  }
  else
  {
  e.IsValid = true;
  }
  }

button code
<asp:Button ID="btn_save" runat="server" Font-Bold="true" Text="Save"
  onclick="btn_save_Click" ValidationGroup="add" style="height: 26px" CausesValidation="true"/>