单一登陆就是一个帐号只能在一个地方登陆。后登陆的帐号会把前面登陆的帐号踢走。注意，只是介绍思路，代码很乱。

login.aspx.cs

    protected void Button1_Click(object sender, EventArgs e)
    {
        Session["name"] = TextBox1.Text;
        if (Cache["userlist"] == null)
        {
            System.Collections.Generic.Dictionary userlist = new System.Collections.Generic.Dictionary();
            userlist.Add(Session["name"].ToString(), Session.SessionID);
            Cache["userlist"] = userlist;
        }
        else
        {
            System.Collections.Generic.Dictionary userlist = Cache["userlist"] as System.Collections.Generic.Dictionary;
            userlist[Session["name"].ToString()] = Session.SessionID;
            Cache["userlist"] = userlist;
        }
        Response.Redirect("default.aspx");
    }
default.aspx.cs

    protected void Page_Load(object sender, EventArgs e)
    {
        if (Session["name"] == null)
            Response.Write("");
        else if((Cache["userlist"] as System.Collections.Generic.Dictionary)[Session["name"].ToString()]!= Session.SessionID)
            Response.Write("");

    }