asp.net 根据内容改变按钮可用不可用

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

<asp:TextBox 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>