How to set dropdown edit type for the .NET TreeGrid column?

[ASPX]
<ej:treegrid ID="Treegrid" runat="server">
     <Columns>
         <ej:treegridcolumn HeaderText="Resource" Field="Resource" EditType="Dropdown"/>
         //…
     </Columns>
</ej:treegrid>

[ASPX.CS]
protected void Page_Load(object sender, EventArgs e)
        {
            //…
            ResourceList resource = new ResourceList();
            var data = resource.GetResources();
            var index1 = this.Treegrid.Columns.FindIndex(col => col.Field == "Resource");
            this.Treegrid.Columns.ElementAt(index1).DropDownData = data;
 
        }
public class Resource
        {
            public int id { get; set; }
            public string text { get; set; }
            public string value { get; set; }    
        }
 
        public class ResourceList
        {
            public List<Resource> GetResources()
            {
                List<Resource> ResourceCollection = new List<Resource>();
                ResourceCollection.Add(new Resource() { id = 1, text = "Project Manager", value = "Project Manager" });
                ResourceCollection.Add(new Resource() { id = 2, text = "Software Analyst", value = "Software Analyst" });
                 //…
                return ResourceCollection;
            }
        }