hi,
there, i need to know why is it i get this error when i run the web site.A page can have only one server-side Form tag

This can happen if you are using master pages and you have a <form runat="server"> element in your master page and in your web content page you have another <form runat="server"> . Here is an example.

Master Page:

<%@ Master Language="C#" AutoEventWireup="true" CodeBehind="Site1.master.cs" Inherits="daniweb.web.Site1" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <link rel="stylesheet" href="Stylesheet1.css" type="text/css" />
    <title>Master Page</title>
    <asp:ContentPlaceHolder ID="head" runat="server">
    </asp:ContentPlaceHolder>
</head>
<body>
    <form id="form1" runat="server">
    <center> 
            <asp:ContentPlaceHolder id="ContentPlaceHolder1" runat="server">
            </asp:ContentPlaceHolder>

Note in the above example the content place holder is inside the <form> element. Now on your child page:

<%@ Page Title="" Language="C#" MasterPageFile="~/Site1.Master" AutoEventWireup="true" CodeBehind="Page1.aspx.cs" Inherits="daniweb.web.Page1" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
	<form id="hello" runat="server">
		<asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" OnClientClick="javascript:self.close(); " />
	</form>
</asp:Content>

Resulting in: [HttpException (0x80004005): A page can have only one server-side Form tag.] Here is an example of two embedded forms causing that error without using master pages:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="daniweb.web.Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
			<form id="form2" runat="server">
				<p>hello</p>
			</form>
    </div>
    </form>
</body>
</html>

A page can have only one server-side Form tag

As the error message itself suggests, a web page can have only one run at server form tag.

Please check your markup, you must have more than one form tags written there.

Comments
worthless post
This article has been dead for over six months. Start a new discussion instead.