ASP.NET Tutorial/Cookie/Introduction

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Create a persistent cookie.

<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
    void Page_Load()
    {
        int counter = 0;
        if (Request.Cookies["counter"] != null)
            counter = Int32.Parse(Request.Cookies["counter"].Value);
        counter++;
        Response.Cookies["counter"].Value = counter.ToString();
        Response.Cookies["counter"].Expires = DateTime.Now.AddYears(2);
        lblCounter.Text = counter.ToString();
    }
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
    <title>Set Persistent Cookie</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    You have visited this page
    <asp:Label
        id="lblCounter"
        Runat="server" />
    times!
    </div>
    </form>
</body>
</html>


Creating a new cookie.

Be warned that cookie names are case sensitive. 

<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
    protected void btnAdd_Click(object sender, EventArgs e)
    {
        Response.Cookies["message"].Value = txtCookieValue.Text;
    }
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
    <title>Set Cookie</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
    <asp:Label
        id="lblCookieValue"
        Text="Cookie Value:"
        AssociatedControlID="txtCookieValue"
        Runat="server" />
    <asp:TextBox
        id="txtCookieValue"
        Runat="server" />
    <asp:Button
        id="btnAdd"
        Text="Add Value"
        OnClick="btnAdd_Click"
        Runat="server" />
    </div>
    </form>
</body>
</html>


Using Browser Cookies

When a web server creates a cookie, an additional HTTP header is sent to the browser when a page is served to the browser. 
The HTTP header looks like this:
Set-Cookie: message=Hello

After a cookie has been created on a browser, whenever the browser requests a page from the same application in the future, the browser sends a header that looks like this:
Cookie: message=Hello
You can store only string values when using a cookie.
Two types of cookies: session cookies and persistent cookies. 
A session cookie exists only in memory. 
A persistent cookie can last for months or even years. 
A persistent cookie is stored permanently by the user"s browser on the user"s computer.
Источник — «http://nfex.ru/index.php?title=ASP.NET_Tutorial/Cookie/Introduction&oldid=2920»