C# – ListView SelectedIndexChanged Event no Selected Items problem

The .NET Framework does not actually keep track of the selected index of the combo box's drop down list; this is handled internally by the Windows API. As a consequence of this, .NET is reliant on the Windows API to notify it when the selected index changes by means of a notification message sent to the combo box's window handle, so that it can in turn fire the SelectedIndexChanged event.

Unfortunately, it turns out that the particular notification message that .NET watches for (CBN_SELCHANGE to be exact) does NOT cover all possible scenarios in which the selected index could change. Specifically, CBN_SELCHANGE is only sent by the Windows API if the user clicks on, or selects using the arrow keys, an item in the drop down list. However, in a DropDown style combo box, the act of opening the combo box causes Windows to look at the text in the edit portion of the combo box, search through the list of items for a match, and if a match is found, automatically select the matching item (or the first matching item, if there are multiple matching items). This can change the selected index, but does NOT send a CBN_SELCHANGE notification message, so .NET misses the fact that it changed and does not fire the SelectedIndexChanged event.

Windows does all this in a DropDown style combo box because the user does not HAVE to pick something in the drop down list; they can type whatever they want. So each time you open the combo box it assumes that the user might have changed the text and tries to re-sync with what's in the list if it can.

In your case, when you open the combo box for the second time, it is re-syncing and selecting the first match for the text in the edit portion, which is "John Doe" #0, and changing the selected index to 0 without .NET being aware.

So it basically is a bug in the .NET Framework. Unfortunately, there is no perfect workaround -- you can't get Windows to not do the re-sync, and there is no event that fires right after the re-sync occurs in which you can get the new selected index. (The DropDown event actually fires right before the re-sync occurs, so it will not see the new index.) About the best you can do is handle the DropDownClosed event, assume that the index might have changed at that point, and act accordingly.

Here's what I created to hold the images that my app is currently displaying. Please note that the "Log" object in use here is my custom wrapper around the final Log class inside Android.

package com.wilson.android.library;

/*
 Licensed to the Apache Software Foundation (ASF) under one or more
contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied.  See the License for the
specific language governing permissions and limitations
under the License.
*/
import java.io.IOException;

public class DrawableManager {
    private final Map<String, Drawable> drawableMap;

    public DrawableManager() {
        drawableMap = new HashMap<String, Drawable>();
    }

    public Drawable fetchDrawable(String urlString) {
        if (drawableMap.containsKey(urlString)) {
            return drawableMap.get(urlString);
        }

        Log.d(this.getClass().getSimpleName(), "image url:" + urlString);
        try {
            InputStream is = fetch(urlString);
            Drawable drawable = Drawable.createFromStream(is, "src");


            if (drawable != null) {
                drawableMap.put(urlString, drawable);
                Log.d(this.getClass().getSimpleName(), "got a thumbnail drawable: " + drawable.getBounds() + ", "
                        + drawable.getIntrinsicHeight() + "," + drawable.getIntrinsicWidth() + ", "
                        + drawable.getMinimumHeight() + "," + drawable.getMinimumWidth());
            } else {
              Log.w(this.getClass().getSimpleName(), "could not get thumbnail");
            }

            return drawable;
        } catch (MalformedURLException e) {
            Log.e(this.getClass().getSimpleName(), "fetchDrawable failed", e);
            return null;
        } catch (IOException e) {
            Log.e(this.getClass().getSimpleName(), "fetchDrawable failed", e);
            return null;
        }
    }

    public void fetchDrawableOnThread(final String urlString, final ImageView imageView) {
        if (drawableMap.containsKey(urlString)) {
            imageView.setImageDrawable(drawableMap.get(urlString));
        }

        final Handler handler = new Handler() {
            @Override
            public void handleMessage(Message message) {
                imageView.setImageDrawable((Drawable) message.obj);
            }
        };

        Thread thread = new Thread() {
            @Override
            public void run() {
                //TODO : set imageView to a "pending" image
                Drawable drawable = fetchDrawable(urlString);
                Message message = handler.obtainMessage(1, drawable);
                handler.sendMessage(message);
            }
        };
        thread.start();
    }

    private InputStream fetch(String urlString) throws MalformedURLException, IOException {
        DefaultHttpClient httpClient = new DefaultHttpClient();
        HttpGet request = new HttpGet(urlString);
        HttpResponse response = httpClient.execute(request);
        return response.getEntity().getContent();
    }
}