Please try with the below code snippet.
protected void Page_Load(object sender, System.EventArgs e)
{
List<TestModels> models = new List<TestModels>();
models.Add(new TestModels() { ID = 1, ParentID = null, Text = "a1" });
models.Add(new TestModels() { ID = 2, ParentID = 1, Text = "a2" });
models.Add(new TestModels() { ID = 3, ParentID = 2, Text = "a3" });
models.Add(new TestModels() { ID = 4, ParentID = null, Text = "a4" });
models.Add(new TestModels() { ID = 5, ParentID = 4, Text = "a5" });
RadTreeView1.DataTextField = "Text";
RadTreeView1.DataFieldID = "ID";
RadTreeView1.DataFieldParentID = "ParentID";
RadTreeView1.DataSource = models;
RadTreeView1.DataBind();
}
Class
public class TestModels
{
public int ID { get; set; }
public int? ParentID { get; set; }
public string Text { get; set; }
}
Please Set NULL in ParentID in place-of 0.
Because It consider 0 as value. RadTreeView is not able to found any ID field which have 0 value.