MembershipUser user = Membership.GetUser();
    string id = user.ProviderUserKey.ToString();
       SqlConnection connection = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["Connection"].ConnectionString);
        try
        {
            using (connection)
            {
                using (SqlCommand con_get = new SqlCommand("SELECT firstname FROM x WHERE userId='" + id + "'", connection)) //Executes SQL command
                {
                    try
                    {
                        connection.Open(); //Opens the database connection
                        using (SqlDataReader reader = con_get.ExecuteReader())
                        {
                            if (reader != null)
                            {
                                while (reader.Read())
                                {
                                    Label label1 = (Label)sender;
                                    label1.Text = reader["x"].ToString();
                                }
                            }
                        }
                    }
                    catch
                    {

                    }

                    finally
                    {
                        connection.Close(); //Closes the database connection
                    }
                }
            }
        }
        catch
        {

        }

protected void Page_Load(object sender, EventArgs e)
{
   if(!Page.IsPostBack)
   {
      var welcomeLabel = Page.Master.FindControl("lblWelcome") as Label;
      SetName(welcomeLabel);
   }
}

protected void SetName(Label welcomeLabel)
{
    //this is the type of thing you'd probably wanna do in the Global.asax on session start
    if (Session["userName"] == null || !Login()) return; //session variable is empty and the login attempt failed, give up
    var usersName = Session["userName"].ToString();
    welcomeLabel.Text = usersName;
}
protected bool Login()
{
    const string query = "SELECT Name FROM Users WHERE Password = @Password AND UserName = @UserName";
    using (var conn = new SqlConnection(connectionString))
    {
        using (var comm = new SqlCommand(query, conn))
        {
            comm.CommandType = CommandType.Text;
            comm.Parameters.Add(new SqlParameter("@Password", password)); //or get this from a control or wherever
            comm.Parameters.Add(new SqlParameter("@UserName", userName)); //or get this from a control or wherever
            conn.Open();
            var name = (string)comm.ExecuteScalar();
            if (!string.IsNullOrEmpty(name))
            {
                Session["userName"] = name;
                return true;
            }
            //"Login information is wrong or user doesn't exist.
            return false;
        }
    }
}