TextBox选中后默认值消失

  <asp:TextBox runat="server" ID="txtEyeException" style="color:silver"  value="请输入眼情况" onfocus="if(value=='请输入眼情况') {value='';getFocus(this);}" onblur="if(value=='') {value='请输入眼情况';getFocus(this);}" CssClass="text"></asp:TextBox>
                         <script>
                             function getFocus(obj) {
                                 if (obj.style.color == 'silver') {
                                     obj.value = '';
                                     obj.style.color = 'black';
                                 } else {
                                     obj.style.color = 'silver'
                                 }
                             }
                            </script>