Cannot implicitly convert type 'System.Data.DataRelation' to 'DataRelation'

using System;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

public partial class DataRelation : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string connetionString = null;
        SqlConnection con;
        SqlDataAdapter da1;
        SqlDataAdapter da2;
        DataRelation relation;
        SqlCommand command = new SqlCommand();
        DataSet ds = new DataSet();
        connetionString = ConfigurationManager.ConnectionStrings["ConnectionStringDemo"].ConnectionString.ToString();
        con = new SqlConnection(connetionString);
        da1 = new SqlDataAdapter("select  top  (20)* from Production.Product", con);
        da2 = new SqlDataAdapter("select  top  (20) * from Sales.SalesOrderDetail", con);
        con.Open();
        da1.Fill(ds, "Production.Product");
        da2.Fill(ds, "Sales.SalesOrderDetail");
        con.Close();

        relation = ds.Relations.Add("CustOrders",
            ds.Tables["Production.Product"].Columns["ProductID"],
            ds.Tables["Sales.SalesOrderDetail"].Columns["ProductID"]);

        foreach (DataRow pRow in ds.Tables["Production.Product"].Rows)
        {
            Console.WriteLine(pRow["ProductID"]);
            foreach (DataRow cRow in pRow.GetChildRows(relation))
                Console.WriteLine("\t" + cRow["SalesOrderID"]);
        }
    }
}