I created a sample UserControl that exposes a Collection
Serialization works in this sample, the single problem I found is that I
want to have an edit updated with number of items in the collection. For
that I had to call Control.Refresh() when CollectionEditor.SetItems() is
called. Can you please use this sample and tell if it is close to what you
need?
Tip1:  I have to restart VStudio 2005 quite often (almost each time I modify
control)
Tip 2: I use dbmon or DbgView (from www.sysinternals.com) to see the output
of System.Diagonostics.Trace.WriteLine even if is in VStudio Designer

Here follows the source code
TestUserControl.cs
using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Drawing;

using System.Data;

using System.Text;

using System.Windows.Forms;

using System.Collections.ObjectModel;

using System.Drawing.Design;

using System.ComponentModel.Design;

namespace TestCollectionProperty

{

public class TestData

{

int intValue;

String strValue = string.Empty;

public int IntValue

{

get { return intValue; }

set { intValue = value; }

}

public String StrValue

{

get { return strValue; }

set { strValue = value; }

}

}

public class ValuesEditor : CollectionEditor

{

public ValuesEditor()

{ }

protected override object SetItems(object editValue, object[] value)

{

System.Diagnostics.Trace.WriteLine("SetItems1");

System.Diagnostics.Trace.WriteLine("SetItems2: " + editValue);

object retValue = base.SetItems(editValue, value);

Control control = this.Context.Instance as Control;

System.Diagnostics.Trace.WriteLine("SetItems3: " + control);

if (control != null)

{

control.Refresh();

}

return retValue;

}

}

public partial class TestUserControl : UserControl

{

private List<TestData> values = new List<TestData>();

public TestUserControl()

{

InitializeComponent();

}

[Browsable(true)]

[DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]

[RefreshProperties(RefreshProperties.All)]

[Editor(typeof(ValuesEditor), typeof(UITypeEditor))]

public List<TestData> Values

{

get { return values; }

}

protected override void OnPaint(PaintEventArgs e)

{

this.textBox.Text = values.Count.ToString();

base.OnPaint(e);

}

}

}

TestUserCode.Design.cs

namespace TestCollectionProperty

{

partial class TestUserControl

{

/// <summary>

/// Required designer variable.

/// </summary>

private System.ComponentModel.IContainer components = null;

/// <summary>

/// Clean up any resources being used.

/// </summary>

/// <param name="disposing">true if managed resources should be disposed;
otherwise, false.</param>

protected override void Dispose(bool disposing)

{

if (disposing && (components != null))

{

components.Dispose();

}

base.Dispose(disposing);

}

#region Component Designer generated code

/// <summary>

/// Required method for Designer support - do not modify

/// the contents of this method with the code editor.

/// </summary>

private void InitializeComponent()

{

this.textBox = new System.Windows.Forms.TextBox();

this.SuspendLayout();

//

// textBox

//

this.textBox.Location = new System.Drawing.Point(24, 50);

this.textBox.Name = "textBox";

this.textBox.Size = new System.Drawing.Size(100, 20);

this.textBox.TabIndex = 0;

//

// TestUserControl

//

this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);

this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;

this.Controls.Add(this.textBox);

this.Name = "TestUserControl";

this.ResumeLayout(false);

this.PerformLayout();

}

#endregion

private System.Windows.Forms.TextBox textBox;

}

}