asp.net 根据内容改变按钮可用不可用

<asp:Button ID="Button3" runat="server" Text="Button" Enabled="false" />

<asp:TextBox ID="TextBox1" runat="server" onkeyup="TextboxOnTextChange(this.value);"></asp:TextBox>

    <script language="javascript" type="text/javascript">
// <![CDATA[
        function TextboxOnTextChange(text) {
            var button = document.getElementById("MainContent_Button3");
            if (button == null) {
                alert("Button is null");
                return false;
            }
            if (text == "") {
                button.disabled = true;
             }
            else {
                button.disabled = false;
            }
            
        }
// ]]>
    </script>