<script language="javascript">
&nbsp;       function YANDNCheck() {              
            if (document.getElementById("txtName") != null) {
                if (document.getElementById("txtName").value == "") {
                    alert('名称不能为空！');
                    document.getElementById("txtName").focus();
                    return false;
                }
            }          
            return true;
        }
    </script>

<script language="javascript">
        function YANDNCheck() {              
            if (document.getElementById("源代码中当前对应textbox的id") != null) {
                if (document.getElementById("源代码中当前对应textbox的id").value == "") {
                    alert('名称不能为空！');
                    document.getElementById("源代码中当前对应textbox的id").focus();
                    return false;
                }
            }          
            return true;
        }
    </script>

<table>
<tr>
<td>名称：</td>
<td><asp:TextBox id="txtName" />
<td><asp:Button onClientClick="return YANDNCheck();"/>
</tr>
</table>

<script language="javascript">
        function YANDNCheck() {              
            if (document.getElementById("txtName") != null) {
                if (document.getElementById("txtName").value == "") {
                    alert('名称不能为空！');
                    document.getElementById("txtName").focus();
                    return false;
                }
            }          
            return true;
        }
    </script>

<table>
<tr>
<td>名称：</td>
<td><ext:TextBox id="txtName" />
<td><ext:Button onClientClick="return YANDNCheck();"/>
</tr>
</table>