Add-in stops working with no apparent error

[DavidStefan]

I am developing a simple add-in that will trigger an action after user selects features on the map. The action will essentially make sure a dockable window is open and in that window display information about the selected features.

I have implemented a simple function that loops over selected features using IEnumFeature and collects their OIDs. To make sure the OIDs are present for all features in the selection, I set the IEnumFeatureSetup.AllFields = true.

Now to the problem. After I've selected a number features several times (regardless of what features or how many I select), the SelectionChanged event stops from being fired. It appears as if the add-in has crashed, but no error message is displayed. This happens both in debug and production. If I try to restart the add-in by de-selecting it in the Customise->Extensions menu, I can no longer select it back. However, ArcMap works fine otherwise. To make the add-in work again I must restart ArcMap.

I have done some naive memory usage inspection using Task Manager. Memory usage raises as I keep re-selecting features. Is this a memory leak? I am releasing the IEnum reference so this should, at least in theory, not be a problem.

The features are pulled from a file geodatabase. They have 5 short text fields each and there is around 1300 of them. The more features I select at once, the sooner the add-in stops working (e.g. if I select all of the features in the map at once, the SelectionChanged event won't be fired again).

I am developing in Arcobjects 10.1 SDK with Visual Studio 10 for C# Express using .NET 3.5. I cannot upgrade nor can I use .NET 4.0 due to client constraints. The add-in is being tested in ArcMap 10.1.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;
using System.IO;
using System.Collections;
using ESRI.ArcGIS.Framework;
using ESRI.ArcGIS.esriSystem;
using ESRI.ArcGIS.Carto;
using ESRI.ArcGIS.Desktop.AddIns;
using ESRI.ArcGIS.Geodatabase;
using ESRI.ArcGIS.Geometry;
using System.Runtime.InteropServices;

namespace DfT.Maritime.DeepPort {

    public class DeepPortExtension: ESRI.ArcGIS.Desktop.AddIns.Extension {

        private static IActiveViewEvents_Event ViewEvents {
            get { return ArcMap.Document.ActiveView as IActiveViewEvents_Event; }
        }

        public DeepPortExtension() {
        }


        protected override void OnStartup() {
        }

        protected override bool OnSetState(ExtensionState state) {
            Init();
            return base.OnSetState(state);
        }

        private void Init() {
            if (ArcMap.Document.ActiveView.IsActive()) {
                InitAddin();
            } else {
                ArcMap.Events.OpenDocument += delegate() {
                    InitAddin();
                };
                ArcMap.Events.NewDocument += delegate() {
                    InitAddin();
                };
            }
        }

        private void InitAddin() {
            // Bind selection changed handler
            ViewEvents.SelectionChanged += SelectionChanged_Handler;
        }

        private void SelectionChanged_Handler() {

            ISelection selection = ArcMap.Document.FocusMap.FeatureSelection;
            IEnumFeature featuresEnum = (IEnumFeature)selection;
            IEnumFeatureSetup featureSetup = (IEnumFeatureSetup)selection;

            featureSetup.AllFields = true;

            string message = "";
            IFeature feature;

            while ((feature = featuresEnum.Next()) != null) {
                int fldID = feature.Fields.FindField("locode");
                message += feature.get_Value(fldID).ToString() + ",";
            }


            System.Windows.Forms.MessageBox.Show(message);


            Marshal.ReleaseComObject(featuresEnum);
            Marshal.ReleaseComObject(featureSetup);
            Marshal.ReleaseComObject(selection);
        }
    }
}

Please help, this has been bugging me for the good part of the last week!

[KenBuja]

Have you tried using Try..Catch in the SelectionChanged_Handler function to see if there is anything unusual happening?

[DavidStefan]

Yes. I have tried to wrap the whole block of code in a try..catch to no avail. Catching Exception doesn't seem to help, unfortunately, as none seems to be thrown...

[KenBuja]

How about using FinalReleaseComObject or the Garbage Collector, as shown in this discussion?

[DavidStefan]

All right, this bug is very odd. I have tried to force garbage collection but the only effect was that the SelectionChanged_Handler would get called only once upon selecting features. After that, the add-in would stop working as it does without the forced collection. I am no expert in using forced garbage collection, but this approach doesn't seem to be the answer to my problem and appears to be frowned upon by many C# programmes (that doesn't mean it's wrong, I know).

I have altered the code to localize the problem. It seems that the issue is caused by the growing message string, i.e. the step message += feature.HasOID.ToString() + "\n"; (line 15.)

The following code breaks as described previously:

private void SelectionChanged_Handler() {

    string message = "";

    try {
        ISelection selection = ArcMap.Document.FocusMap.FeatureSelection;
        IEnumFeature featuresEnum = (IEnumFeature)selection;
        IEnumFeatureSetup featureSetup = (IEnumFeatureSetup)selection;

        featureSetup.AllFields = true;

        IFeature feature;

        while ((feature = featuresEnum.Next()) != null) {
            message += feature.HasOID + ",";
            Marshal.FinalReleaseComObject(feature);
        }

        Marshal.FinalReleaseComObject(featuresEnum);
        Marshal.FinalReleaseComObject(featureSetup);

    } catch (Exception e) {
        System.Windows.Forms.MessageBox.Show(e.Message);
    }

    System.Windows.Forms.MessageBox.Show(message);
}

No exception is caught, no error displayed, the add-in simply stops showing the message as expected and cannot be re-initialized in the Customize menu.

However, the altered code below seems to run OK:

private void SelectionChanged_Handler() {

    List<string> message = new List<string>();

    try {
        ISelection selection = ArcMap.Document.FocusMap.FeatureSelection;
        IEnumFeature featuresEnum = (IEnumFeature)selection;
        IEnumFeatureSetup featureSetup = (IEnumFeatureSetup)selection;

        featureSetup.AllFields = true;

        IFeature feature;

        while ((feature = featuresEnum.Next()) != null) {
            message.Add(feature.HasOID.ToString());
            Marshal.FinalReleaseComObject(feature);
        }

        Marshal.FinalReleaseComObject(featuresEnum);
        Marshal.FinalReleaseComObject(featureSetup);

    } catch (Exception e) {
        System.Windows.Forms.MessageBox.Show(e.Message);
    }

    System.Windows.Forms.MessageBox.Show(String.Join(",", message.ToArray()));
    message.Clear();
}

Can someone explain what's going on behind the scene in these two cases?

[DavidStefan · Accepted solution]

For what it's worth, here's how I solved the problem. Is seems it was the repeated call to ArcMap.Document.FocusMap.FeatureSelection that was breaking my add-in. If I instead store the focus map in a member variable IMap map and retrieve the feature selection by calling map.FeatureSelection is works just fine.

using System;  
using System.Collections.Generic;  
using System.Diagnostics;  
using System.Text;  
using System.IO;  
using System.Collections;  
using ESRI.ArcGIS.Framework;  
using ESRI.ArcGIS.esriSystem;  
using ESRI.ArcGIS.Carto;  
using ESRI.ArcGIS.Desktop.AddIns;  
using ESRI.ArcGIS.Geodatabase;  
using ESRI.ArcGIS.Geometry;  
using System.Runtime.InteropServices;  
  
namespace DfT.Maritime.DeepPort {  
  
    public class DeepPortExtension: ESRI.ArcGIS.Desktop.AddIns.Extension {  
  
        IMap map;

        private static IActiveViewEvents_Event ViewEvents {  
            get { return ArcMap.Document.ActiveView as IActiveViewEvents_Event; }  
        }  
  
        public DeepPortExtension() {  
        }  
  
        protected override void OnStartup() {  
        }  
  
        protected override bool OnSetState(ExtensionState state) {  
            Init();  
            return base.OnSetState(state);  
        }  
  
        private void Init() {  
            if (ArcMap.Document.ActiveView.IsActive()) {  
                InitAddin();  
            } else {  
                ArcMap.Events.OpenDocument += delegate() {  
                    InitAddin();  
                };  
                ArcMap.Events.NewDocument += delegate() {  
                    InitAddin();  
                };  
            }  
        }  
  
        private void InitAddin() {  
            // Bind selection changed handler  
            ViewEvents.SelectionChanged += SelectionChanged_Handler;

            //
            // Store focus map in a member variable!!!
            //  
            map = ArcMap.Document.FocusMap;
        }  
  
        private void SelectionChanged_Handler() {  
  
            //
            // Retrieve selected features from member variable!!!
            //
            ISelection selection = map.FeatureSelection;  
            IEnumFeature featuresEnum = (IEnumFeature)selection;  
            IEnumFeatureSetup featureSetup = (IEnumFeatureSetup)selection;  
  
            featureSetup.AllFields = true;  
  
            string message = "";  
            IFeature feature;  
  
            while ((feature = featuresEnum.Next()) != null) {  
                int fldID = feature.Fields.FindField("locode");  
                message += feature.get_Value(fldID).ToString() + ",";  
            }  
  
            System.Windows.Forms.MessageBox.Show(message);  
  
            Marshal.ReleaseComObject(featuresEnum);  
            Marshal.ReleaseComObject(featureSetup);  
            Marshal.ReleaseComObject(selection);  
        }  
    }  
}