Question

When I click the Delete Selected button, the wrong picture is being deleted. Most of the times it deleted the picture on the right of a selected picture. Other times, for example when everything is selected, it only deletes a single picture.

I don’t know what’s going on. Here’s a picture to illustrate what happens:

Here is the code for both user controls I’m using: HorizontalPictureScroller and SelectablePicture.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Collections.ObjectModel;
using System.Runtime.Serialization;
using System.ComponentModel.Design.Serialization;

namespace WinformsPlayground
{    
    public partial class HorizontalPictureScroller : UserControl
    {
        public HorizontalPictureScroller()
        {
            InitializeComponent();
            Pictures = new ObservableCollection<SelectablePicture>();
            Pictures.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(Pictures_CollectionChanged);
        }       

        #region "Properties"
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        public ObservableCollection<SelectablePicture> Pictures { get; set; }
        private int PositionControlX = 0;
        #endregion

        #region "Methods"
        private void RedrawPictures()
        {
            PositionControlX = 0;

            foreach (var picture in Pictures)
            {
                picture.Location = new Point(PositionControlX + panelPicturesWrapper.AutoScrollPosition.X, 0);
                PositionControlX += 130;
                panelPicturesWrapper.Controls.Add(picture);
            }
        }

        public void AddPicture(SelectablePicture picture)
        {
            Pictures.Add(picture);
        }

        public void RemovePicture(SelectablePicture picture)
        {
            Pictures.Remove(picture);
        }

        public void MovePictureLeft(int index)
        {
            SelectablePicture tmpPicture = Pictures[index];
            Pictures[index] = Pictures[index - 1];
            Pictures[index - 1] = tmpPicture;
        }

        public void MovePictureRight(int index)
        {
            SelectablePicture tmpPicture = Pictures[index];
            Pictures[index] = Pictures[index + 1];
            Pictures[index + 1] = tmpPicture;
        }

        public void SelectAllImages()
        {
            foreach (var picture in panelPicturesWrapper.Controls)
            {
                ((SelectablePicture)picture).SelectPicture();
            }
        }

        public void DeselectAllImages()
        {
            foreach (var picture in panelPicturesWrapper.Controls)
            {
                ((SelectablePicture)picture).DeselectPicture();
            }
        }

        public void DeleteSelectedImages()
        {
            foreach (var picture in panelPicturesWrapper.Controls)
            {
                if (((SelectablePicture)picture).IsSelected())
                {
                    this.RemovePicture((SelectablePicture)picture);
                }
            }
        }
        #endregion

        #region "Events"
        void Pictures_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
            RedrawPictures();
        }
        #endregion

    }
}



using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Runtime.Serialization;

namespace WinformsPlayground
{
    public partial class SelectablePicture : UserControl
    {
        public SelectablePicture()
        {
            InitializeComponent();
            panel1.BackgroundImageLayout = ImageLayout.Zoom;
        }

        public SelectablePicture(Image image)
        {
            InitializeComponent();
            panel1.BackgroundImage = image;
            panel1.BackgroundImageLayout = ImageLayout.Zoom;
        }

        #region "Properties"
        public Image Image()
        {
            return panel1.BackgroundImage;
        }

        public bool IsSelected()
        {
            return chkSelected.Checked;
        }
        #endregion

        #region "Methods"
        public void ToggleCheckBox()
        {
            chkSelected.Checked = chkSelected.Checked ? false : true;
        }

        public void VisuallySelect()
        {
            this.BackColor = Color.FromArgb(51, 153, 255);
        }

        public void VisuallyDeselect()
        {
            //If none of the controls inside the usercontrol have focus, set this control to white.
            if (!this.Focused && !this.panel1.Focused && !this.chkSelected.Focused)
            {
                this.BackColor = Color.White;
            }
        }        

        public void SelectPicture()
        {
            this.chkSelected.Checked = true;
        }

        public void DeselectPicture()
        {
            this.chkSelected.Checked = false;
        }
        #endregion

        #region "Events"
        private void panel1_Click(object sender, EventArgs e)
        {
            VisuallySelect();
            ToggleCheckBox();
            panel1.Focus();
        }

        private void chkSelected_Click(object sender, EventArgs e)
        {
            VisuallySelect();
            ToggleCheckBox();
            chkSelected.Focus();
        }

        private void SelectablePicture_Click(object sender, EventArgs e)
        {
            VisuallySelect();
            ToggleCheckBox();
            this.Focus();
        }

        private void panel1_Leave(object sender, EventArgs e)
        {
            VisuallyDeselect();
        }

        private void chkSelected_Leave(object sender, EventArgs e)
        {
            VisuallyDeselect();
        }

        private void SelectablePicture_Leave(object sender, EventArgs e)
        {
            VisuallyDeselect();
        }
        #endregion        
    }
}

The code is simple enough. I’ve used breakpoints and I can confirm that the HorizontalPictureScroller.DeleteSelectedImages() method is correctly iterating through selected pictures. I’m not sure what else I could do to see what’s going on. I’m stumped!

Any ideas? Maybe the .Remove() method that comes with the ObservableCollection type doesn’t work as I thought it did.

Answer 1

Every time the Pictures collection changes, you’re calling RedrawPictures. That adds all of the pictures again, but doesn’t clear anything… so you’ll be trying to add a control to panelPicturesWrapper.Controls when it already contains that control. That sounds like a bad idea to me.

Further, you’re iterating over the Controls collection while changing it, which is generally a bad idea.

I suggest you change your removal code to something like this:

public void DeleteSelectedImages()
{
    var picturesToRemove = panelPicturesWrapper.Controls
                                               .Cast<SelectablePicture>();
                                               .Where(p => p.IsSelected())
                                               .ToList();

    foreach (var picture in picturesToRemove)
    {
        RemovePicture(picture);
    }
}