namespace Rekursive
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            //treeView1.Nodes.Add("Test");
            iloader MyLoad = new TreeViewLoad();
            MyLoad.loader("test", treeView1, 1);
        }
    }

    interface iloader
    {
        void loader(string nodeName, TreeView myTre, int id);
    }

    class TreeViewLoad : iloader
    {
       public void TreeLoader(TreeView tre)
        {
           // Here I want to call the loader
        }

        public void loader(string nodeName, TreeView myTre, int id)
        {
            myTre.Nodes.Add(nodeName + id.ToString());
            if (id < 10)
            {
                id++;
                loader(nodeName, myTre, id);
            }
        }
    }
}

TreeViewLoad myLoader = new TreeViewLoad();
// now you can access loader and TreeLoader.

ILoader myLoad = new TreeViewLoad();

((TreeViewLoad)myLoad).TreeLoader(...);