Visual Studio fires an error when I drag my user control onto the design view

Question

I have two user controls, one that is a simple picture holder with a checkbox in it. And another which acts a container than has a collection of the previous control.

So a HorizontalPictureScroller can have many SelectablePicture controls. I'll paste the small code for each of the controls:

First, HorizontalPictureScroller:

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;

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

        #region "Properties"
        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;
        }
        #endregion

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

Now, the SelectablePicture control:

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;

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

        public SelectablePicture(Image image)
        {
            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;
            }
        }        
        #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        
    }
}

Here's a screenshot of the error I get when trying to drag the HorizontalPictureScroller onto my Winform design view (Sorry I can't paste the text here):

My user controls are very simple and I can't see what's going wrong in the code.

Maybe it's a blatant mistake on my part. :P Thanks a lot for your time.

Answer 1

It seems that the UserControl SelectablePicture needs to be serialized, because it is used in a Collection. So you were near from the solution with the attribut [Serializable()]. But you'd better implement the ISerializable interface in your UserControl SelectablePicture

public partial class SelectablePicture : UserControl, ISerializable
{
    #region ISerializable Membres

    public void GetObjectData(SerializationInfo info, StreamingContext context)
    {
        ...
    }

    #endregion
}

This is the first step to solve your problem. With that you souldn't have the error message when you drag-drop your UserControl. But now you have to manage the serialization.

If anyone knows why serialization is needed, I'm interested.

Answer 2

The exception is being thrown because you are using the SerializableAttribute, but UserControl does not.

From the documentation for SerializableAttribute:

The common language runtime throws SerializationException if any type in the graph of objects being serialized does not have the SerializableAttribute attribute applied.