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See c# code below.

using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
using ESRI.ArcGIS.Geometry;
using System.Windows.Forms;
using ESRI.ArcGIS.Carto;
using ESRI.ArcGIS.Geodatabase;
using ESRI.ArcGIS.Framework;

namespace HansenAddin
{
    public class PickGeoTransButton : ESRI.ArcGIS.Desktop.AddIns.Button
    {
        public PickGeoTransButton()
        {
        }

        protected override void OnClick()
        {
            try
            {
                // let user choose a transformation
                var sr1 = ArcMap.Document.FocusMap.SpatialReference;
                var featClass = ((IFeatureLayer)ArcMap.Document.FocusMap.get_Layer(0)).FeatureClass;
                var sr2 = ((IGeoDataset)featClass).SpatialReference;

                IGeoTransformation geoTrans;
                esriTransformDirection direction;
                ChooseGeotrans(sr1, sr2, ArcMap.Application.hWnd, out geoTrans, out direction);
                if (geoTrans != null)
                {
                    MessageBox.Show(String.Format("{0} \n{1} \n{2} \n{3}", geoTrans.Name, sr1.Name, sr2.Name, direction));
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }

        public static void ChooseGeotrans(ISpatialReference sr1, ISpatialReference sr2, int hWnd,
            out IGeoTransformation geoTrans, out esriTransformDirection direction)
        {
            geoTrans = null;
            direction = esriTransformDirection.esriTransformForward;

            var list = GetTransformations(sr1, sr2);
            if (list.Count == 0)
            {
                MessageBox.Show(String.Format("No geotransforms to go between {0} and {1}", sr1.Name, sr2.Name));
                return;
            }
            IListDialog dlg = new ListDialogClass();
            foreach (IGeoTransformation gt in list)
                dlg.AddString(gt.Name);
            if (dlg.DoModal("Choose a Geotransformation", 0, hWnd))
            {
                geoTrans = list[dlg.Choice];
                direction = GetDir(geoTrans, sr1, sr2);
            }
        }

        public static List<IGeoTransformation> GetTransformations(ISpatialReference sr1, ISpatialReference sr2)
        {
            int factCode1 = GetGCSFactoryCode(sr1);
            int factCode2 = GetGCSFactoryCode(sr2);
            
            var outList = new List<IGeoTransformation>();
            // Use activator to instantiate arcobjects singletons ...
            var type = Type.GetTypeFromProgID("esriGeometry.SpatialReferenceEnvironment");
            var srf = Activator.CreateInstance(type) as ISpatialReferenceFactory2;

            var gtSet = srf.CreatePredefinedGeographicTransformations();
            gtSet.Reset();
            for (int i = 0; i < gtSet.Count; i++)
            {
                ISpatialReference fromSR;
                ISpatialReference toSR;
                var geoTrans = (IGeoTransformation)gtSet.Next();
                geoTrans.GetSpatialReferences(out fromSR, out toSR);
                if ((fromSR.FactoryCode == factCode1 && toSR.FactoryCode == factCode2) ||
                    (fromSR.FactoryCode == factCode2 && toSR.FactoryCode == factCode1))
                {
                    outList.Add(geoTrans);
                }
            }
            return outList;
        }
        private static esriTransformDirection GetDir(IGeoTransformation geoTrans, ISpatialReference sr1, ISpatialReference sr2)
        {
            int code1 = GetGCSFactoryCode(sr1);
            int code2 = GetGCSFactoryCode(sr2);
            ISpatialReference fromSR;
            ISpatialReference toSR;
            geoTrans.GetSpatialReferences(out fromSR, out toSR);
            if (fromSR.FactoryCode == code1 && toSR.FactoryCode == code2)
                return esriTransformDirection.esriTransformForward;
            else if (fromSR.FactoryCode == code2 && toSR.FactoryCode == code1)
                return esriTransformDirection.esriTransformReverse;
            else
                throw new Exception(String.Format("{0} does not support going between {1} and {2}",
                    geoTrans.Name, sr1.Name, sr2.Name));
        }

        private static int GetGCSFactoryCode(ISpatialReference sr)
        {
            if (sr is IProjectedCoordinateSystem)
                return ((IProjectedCoordinateSystem)sr).GeographicCoordinateSystem.FactoryCode;
            else if (sr is IGeographicCoordinateSystem)
                return ((IGeographicCoordinateSystem)sr).FactoryCode;
            else
                throw new Exception("unsupported spatialref type");
        }
        protected override void OnUpdate()
        {

        }

    }
}