i want to display the first node xml attribute on form?

I learn C# already, so I have this xml file:

<tree>
<grand name ="tom"  id="1" sex="m" status="d" child="2" father=""  /> 
<grand name="adam"  id="11" sex="m" status="d" child="1" father="1"   /> 
<grand name="john"  id="111" sex="m" status="d" child="1"  father="11"   />  
<grand name="pierre"  id="1111" sex="m" status="d" child="3"  father="111"   />  
<grand name="jan"   id="11111" sex="f" status="d" child=""  father="1111"  />  
<grand name="marc"    id="11112" sex="m" status="d" child=""  father="1111"  />  
</tree>

i try this code to display attribute of the first node only on load the form, but I get nothing:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Xml;
using System.IO;
using System.Xml.Linq;

namespace olwantree
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
   private void Form1_Load(object sender, EventArgs e)
        {
            XmlDocument XDoc = new XmlDocument();
            XDoc.Load("F:\\tree.xml");
            XmlNode att = XDoc.SelectSingleNode("//grand/@name");
            string nam = att.ToString();
            label1.Text = att;
        }      
    } 
}

thank you very much