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Customizable ComboBox Drop-Down

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Introduction

This article presents an extension of the .NET ComboBox which provides custom drop-downs. The control can automatically add a resizer grip at the bottom of a drop-down.
Screenshot Examples
Design

Custom drop-down functionality can be achieved using the .NET ToolStripDropDown and ToolStripControlHost classes. These two classes are great because they prevent application forms from becoming inactive during drop-down. The custom popup functionality required by this control has been encapsulated within the class PopupControl (a new addition since the original article posting).

In addition to facilitating this control, the PopupControl class can be used to provide drop-down support in your own controls. The PopupControl also integrates the drop-down resize support (now without the need for a nested resizable container, see below for details).

Previously the drop-down control was resizable by nesting a resizable container. This was an okay solution (because it worked) but it caused some redraw errors during resize operations. The resize functionality has now been significantly improved, and it is now also possible to define which sides of the drop-down are resizable (if any). Resizing can now also be done by dragging the resizable edges of the drop-down. The new resize functionality is achieved by intercepting Win32 hit testing messages, and responding appropriately based upon the set PopupResizeMode property.

The following UML class diagram provides an overview of the presented classes:
UML Overview
Using the Code

As with most controls, this control can be created dynamically at runtime, or by using the Visual C# IDE designer's drag and drop features. During runtime, the property DropDownControl must be assigned to an instance of another control. The assigned control must not be contained elsewhere as this will cause problems.

Most drop-down controls appear better when their borders are removed.
namespace CustomComboDemo
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        } 
        private void Form1_Load(object sender, EventArgs e)
        {
            // Dynamically created controls. 
            // Create grid view control.
            DataGridView gridView = new DataGridView();
            gridView.BorderStyle = BorderStyle.None;
            gridView.Columns.Add("Column1", "Column 1");
            gridView.Columns.Add("Column2", "Column 2");
            gridView.Columns.Add("Column3", "Column 3");
            gridView.Columns.Add("Column4", "Column 4");
            gridView.Columns.Add("Column5", "Column 5");
            this.customComboBox1.DropDownControl = gridView; 
            // Create user control.
            UserControl1 userControl = new UserControl1();
            userControl.BorderStyle = BorderStyle.None;
            this.customComboBox2.DropDownControl = userControl; 
            // Create rich textbox control.
            RichTextBox richTextBox = new RichTextBox();
            richTextBox.BorderStyle = BorderStyle.None;
            this.customComboBox3.DropDownControl = richTextBox;
        }
    }
}

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Original Article

This is an article which I originally wrote and posted at the CodeProject website: http://www.codeproject.com/KB/combobox/CustomDDComboBox.aspx.
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