Convert DataTable (DataSet) to String in C# and VB.Net

 [WebMethod]
    public String AuthenticateUser(String username, String password) 
    {
        con.ConnectionString = ConfigurationManager.ConnectionStrings["cn"].ConnectionString;

        if (con.State == ConnectionState.Closed)
        {
            con.Open();
        }

  if (username.Equals("admin") && password.Equals("pass"))
        {
            adp = new SqlDataAdapter(@"select Username,UserType,MembershipID from Membership where MembershipID='4'", con);

            dt = new DataTable();
            adp.Fill(dt);

            if (dt.Rows.Count > 0)
            {
                return dt.ToString();

            }

            else { }




            
        }

        return dt.ToString();
    
    }

Sir this is my code in the Webservice page, i want to return select query results as a string through this webservice.

Please give suggestions on the above code where i need to make changes.

Also how will i write the code on the page where i am reading the returned value from this webservice?

Thanks