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>