这段JavaScript代码为什么无法执行 - .NET技术 / C#

以下是一段JavsScript脚本，但运行时总会产生 “Microsoft JScript 运行时错误: 'null' 为空或不是对象”异常，大家帮看一下。 
<form id="form1" runat="server">
  <asp:TextBox ID="TextBox1" runat="server" Text =""></asp:TextBox>
  <script id="scr1" type ="text/javascript" >
  function getValue() {
  var x = document.getElementById("Text1");
  alert (x.getAttribute("Text"))
  }
  </script>

  <input id="Button1" type="button" value="button" onclick ="getValue()"

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<asp:TextBox ID="TextBox1" runat="server" Text =""> </asp:TextBox> 
  <script id="scr1" type ="text/javascript" > 
  function getValue() { 
  var x = document.getElementById("<%= TextBox1.ClientID %>"); 
  alert (x.getAttribute("value")) 
  } 
  </script> 

  <input id="Button1" type="button" value="button" onclick ="getValue()"

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document.getElementById("TextBox1"); 
或document.getEl