Untitled

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="WebApplication2.WebForm1" %>

<!DOCTYPE html>
<script runat="server">

    protected void TextBox1_TextChanged(object sender, EventArgs e)
    {
        if (TextBox1.Text.Length == 0)
        { Button1.Enabled = false; }
        else
        {
            Button1.Enabled = true;
        }

    }
</script>


<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
    <title></title>
</head>
<body style="height: 517px">
    <form id="form1" runat="server">
        <asp:TextBox ID="TextBox1" runat="server" Height="26px"  Width="458px" OnTextChanged =" TextBox1_TextChanged" ></asp:TextBox>
        <p>
            <asp:TextBox ID="TextBox2" runat="server" Height="26px" Width="457px"  ></asp:TextBox>
        </p>
        <p>
            <asp:Button ID="Button1" runat="server" Height="32px" Text="Вычислить" Width="91px" OnClick="Button1_Click" Enabled ="false"  />
        </p>
        <asp:Button ID="Button2" runat="server" Height="29px" Text="Отмена" Width="89px" OnClick="Button2_Click" />


    </form>
</body>
</html>