SPField’s to the dropdown List programmatically:

By default, we can get the number of built-in fields or internal fields if you try to retrieve the fields in a list. My below post will help to add only the custom fields to the dropdown list by avoiding all the internal, built-in and hidden fields.

using (SPSite site = new SPSite(SPContext.Current.Site.ID)) {     SPWeb web = site.OpenWeb(SPContext.Current.Web.ID);     web.AllowUnsafeUpdates = true;     SPList list = web.Lists[new Guid(listId)];     var fields = from SPField f in list.Fields                         where !f.Hidden && f.AuthoringInfo == "" &&                                       f.Type != SPFieldType.Attachments &&                                       f.Type != SPFieldType.WorkflowStatus                         orderby f.Title                         select new { ListItem = new ListItem(f.Title, f.Id.ToString()),                                                Readonly = f.ReadOnlyField, FieldType = f.Type }; ddlDestfield.DataSource = from g in fields where !g.Readonly && (g.FieldType == SPFieldType.Note || g.FieldType == SPFieldType.Text)                                                select g.ListItem; ddlDestfield.DataBind(); ddlDestfield.Items.Insert(0, new ListItem("[Select]", "none")); }

Note: Set f.AuthoringInfo == null [In SP2007] and Set f.AuthoringInfo == "" [In SP2010]

Output: