Transaction from an external application outside API not allowed

[lrsmns]

Hi i'm trying to place a family from a plug in i created; however, i'm getting a warning saying "Placement is not permitted in an already modifiable document. The active transaction must be closed first". Then as i continue further, it's returning "Starting a transaction from external application running outside of the API is not allowed"

using Autodesk.Revit.DB;
using Autodesk.Revit.UI;
using System.Collections.ObjectModel;
using System.Linq;
using System.Collections.Generic;


namespace Plugin.Revit
{
    public class FamilyLibraryModel
    {
        public UIApplication uiapp {  get; }
        public Document doc { get; }
        public UIDocument uidoc { get; }

        public FamilyLibraryModel(UIApplication uiApp)
        {
            uiapp = uiApp; 
            doc = uiApp.ActiveUIDocument.Document;
            uidoc = uiApp.ActiveUIDocument;
        }

       public void Place(IEnumerable<RevitFamilyWrapper> selected)
        {
            
            var selectedFamilySymbol = selected.Select(x=>x.familySymbol).First();
            using (var trans = new Transaction((doc), "Place Family"))
            {
                trans.Start();
                uidoc.PromptForFamilyInstancePlacement(selectedFamilySymbol);
                trans.Commit();
            }
        }      
    }
}

i'm not quite sure where i'm doing wrong, would appreciate any insights!

[Moustafa_K · Accepted solution]

the function PromptForFamilyInstancePlacement has its own transaction. you should not use it when any transaction is open.

just remove trans.Start and commit, and should work for you.

[jeremy_tammik]

Just looking at yur code snippet, it looks to me as if you have not implemented an external command or subscribed to any Revit events whatsoever. In that case, you will never have a valid Revit API context, and never be able to successfully execute any Revit API calls at all. Please work thorough the getting started material to see how to set up an external command and understand other basic aspects of the Revit API before doing anything else:

https://thebuildingcoder.typepad.com/blog/about-the-author.html#2

Maybe I am mistaken, not seeing the rest of your code...