Hacking the Combo Box to give it horizontal scrolling

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Runtime.InteropServices;


namespace TB.ExtComboBox{
	public class Form : System.Windows.Forms.Form {

		private System.ComponentModel.IContainer components;
		private COMBOBOXINFO cbi;
		private System.Windows.Forms.Button btnCancel;
		private System.Windows.Forms.Label label1;
		private int pixelWidth = -1;
		private RECT cboListRect;
		private SCROLLINFO si;
		private SubClass scList = null;
		private int xMaxScroll = 0;
		private int xCurrentScroll = 0;
		private int xMinScroll = 0;
		private int xNewSize = 0;
		private System.Windows.Forms.Label label3;
		private System.Windows.Forms.ComboBox cboBoxEnhanced;
		private System.Windows.Forms.ComboBox cboBoxStandard;
		private System.Windows.Forms.Label lblDropDownStyle;
		private System.Windows.Forms.Label lblStyle;
		private System.Windows.Forms.RadioButton radioBtnDropDown;
		private System.Windows.Forms.RadioButton radioBtnDropDownList;
		private bool gotCBI = false;

		#region Win32 Imports
		[DllImport("user32")] public static extern bool GetComboBoxInfo(IntPtr hwndCombo, ref COMBOBOXINFO info);
		[DllImport("user32")] public static extern int SendMessage(IntPtr hwnd, int wMsg, int wParam, IntPtr lParam);
		[DllImport("user32")] public static extern int GetWindowLong(IntPtr hwnd, int nIndex);
		[DllImport("user32")] public static extern int SetWindowLong(IntPtr hwnd, int nIndex, int dwNewLong);
		[DllImport("user32")] public static extern int GetClientRect(IntPtr hwnd, ref RECT lpRect);
		[DllImport("user32")] public static extern int SetScrollInfo(IntPtr hwnd, int n, ref SCROLLINFO lpcScrollInfo, bool redraw);
		#endregion

		#region Win32 Constants
		public const int WS_HSCROLL = 0x100000;
		public const int GWL_STYLE = (-16);
		public const int LB_SETHORIZONTALEXTENT = 0x194;
		public const int WM_SIZE = 0x5;
		public const int WM_HSCROLL = 0x114;
		#endregion

		#region Scroll Bar Messages - Private Constant Ints
		private const int SB_LINEUP = 0;
		private const int SB_LINELEFT = 0;
		private const int SB_LINEDOWN = 1;
		private const int SB_LINERIGHT = 1;
		private const int SB_PAGEUP = 2;
		private const int SB_PAGELEFT = 2;
		private const int SB_PAGEDOWN = 3;
		private const int SB_PAGERIGHT = 3;
		private const int SB_THUMBPOSITION = 4;
		private const int SB_THUMBTRACK = 5;
		private const int SB_TOP = 6;
		private const int SB_LEFT = 6;
		private const int SB_BOTTOM = 7;
		private const int SB_RIGHT = 7;
		private const int SB_ENDSCROLL = 8;
		private const int SB_HORZ = 0;
		private const int SB_VERT = 1;
		private const int SB_CTL = 2;
		private const int SB_BOTH = 3;
		private const int SBM_ENABLE_ARROWS = 0x00E4; /*not in win3.1 */
		private const int SBM_SETSCROLLINFO = 0x00E9;
		private const int SBM_GETSCROLLINFO = 0x00EA;
		private const int SIF_RANGE = 0x0001;
		private const int SIF_PAGE = 0x0002;
		private const int SIF_POS = 0x0004;
		private const int SIF_DISABLENOSCROLL = 0x0008;
		private const int SIF_TRACKPOS = 0x0010;
		private const int SIF_ALL = (SIF_RANGE | SIF_PAGE | SIF_POS | SIF_TRACKPOS);
		#endregion
		//
		#region Form Constructor
		public Form() {
			InitializeComponent();
			InitComboStrings(this.cboBoxEnhanced);
			InitComboStrings(this.cboBoxStandard);
			//
			this.gotCBI = this.InitComboBoxInfo(this.cboBoxEnhanced);
			if (this.gotCBI){
				this.cboListRect = new RECT();
				this.si = new SCROLLINFO();
				this.scList = new SubClass(this.cbi.hwndList, false);
				this.scList.SubClassedWndProc += new SubClass.SubClassWndProcEventHandler(scList_SubClassedWndProc);
			}
		}
		#endregion

		#region Dispose
		protected override void Dispose(bool disposing){
			if(disposing){
				if(components != null){
					components.Dispose();
				}
				if (this.scList != null){
					this.scList.ReleaseHandle();
				}
			}
			base.Dispose(disposing);
		}
		#endregion

		#region InitComboBoxInfo Method - Private
		private bool InitComboBoxInfo(System.Windows.Forms.ComboBox cbo){
			this.cbi = new COMBOBOXINFO();
			this.cbi.cbSize = Marshal.SizeOf(this.cbi);
			if (!GetComboBoxInfo(cbo.Handle, ref this.cbi)){
				return false;
			}
			return true;
		}
		#endregion

		#region Windows Form 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.cboBoxEnhanced = new System.Windows.Forms.ComboBox();
			this.btnCancel = new System.Windows.Forms.Button();
			this.label1 = new System.Windows.Forms.Label();
			this.label3 = new System.Windows.Forms.Label();
			this.cboBoxStandard = new System.Windows.Forms.ComboBox();
			this.lblDropDownStyle = new System.Windows.Forms.Label();
			this.lblStyle = new System.Windows.Forms.Label();
			this.radioBtnDropDown = new System.Windows.Forms.RadioButton();
			this.radioBtnDropDownList = new System.Windows.Forms.RadioButton();
			this.SuspendLayout();
			// 
			// cboBoxEnhanced
			// 
			this.cboBoxEnhanced.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList;
			this.cboBoxEnhanced.DropDownWidth = 150;
			this.cboBoxEnhanced.Location = new System.Drawing.Point(96, 32);
			this.cboBoxEnhanced.Name = "cboBoxEnhanced";
			this.cboBoxEnhanced.Size = new System.Drawing.Size(112, 21);
			this.cboBoxEnhanced.TabIndex = 3;
			this.cboBoxEnhanced.DropDown += new System.EventHandler(this.cboBoxEnhanced_DropDown);
			this.cboBoxEnhanced.DropDownStyleChanged += new System.EventHandler(this.cboBoxEnhanced_DropDownStyleChanged);
			// 
			// btnCancel
			// 
			this.btnCancel.Anchor = ((System.Windows.Forms.AnchorStyles)((System.Windows.Forms.AnchorStyles.Bottom | System.Windows.Forms.AnchorStyles.Right)));
			this.btnCancel.Cursor = System.Windows.Forms.Cursors.Hand;
			this.btnCancel.DialogResult = System.Windows.Forms.DialogResult.Cancel;
			this.btnCancel.FlatStyle = System.Windows.Forms.FlatStyle.Popup;
			this.btnCancel.Location = new System.Drawing.Point(130, 104);
			this.btnCancel.Name = "btnCancel";
			this.btnCancel.TabIndex = 4;
			this.btnCancel.Text = "Cancel";
			this.btnCancel.Click += new System.EventHandler(this.btnCancel_Click);
			// 
			// label1
			// 
			this.label1.AutoSize = true;
			this.label1.Location = new System.Drawing.Point(0, 33);
			this.label1.Name = "label1";
			this.label1.Size = new System.Drawing.Size(98, 16);
			this.label1.TabIndex = 2;
			this.label1.Text = "Enhanced Combo:";
			this.label1.TextAlign = System.Drawing.ContentAlignment.MiddleLeft;
			// 
			// label3
			// 
			this.label3.AutoSize = true;
			this.label3.Location = new System.Drawing.Point(0, 8);
			this.label3.Name = "label3";
			this.label3.Size = new System.Drawing.Size(93, 16);
			this.label3.TabIndex = 0;
			this.label3.Text = "Standard Combo:";
			this.label3.TextAlign = System.Drawing.ContentAlignment.MiddleLeft;
			// 
			// cboBoxStandard
			// 
			this.cboBoxStandard.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList;
			this.cboBoxStandard.Location = new System.Drawing.Point(96, 8);
			this.cboBoxStandard.Name = "cboBoxStandard";
			this.cboBoxStandard.Size = new System.Drawing.Size(112, 21);
			this.cboBoxStandard.TabIndex = 1;
			this.cboBoxStandard.DropDown += new System.EventHandler(this.cboBoxStandard_DropDown);
			// 
			// lblDropDownStyle
			// 
			this.lblDropDownStyle.AutoSize = true;
			this.lblDropDownStyle.Location = new System.Drawing.Point(0, 64);
			this.lblDropDownStyle.Name = "lblDropDownStyle";
			this.lblDropDownStyle.Size = new System.Drawing.Size(89, 16);
			this.lblDropDownStyle.TabIndex = 5;
			this.lblDropDownStyle.Text = "DropDown Style:";
			this.lblDropDownStyle.TextAlign = System.Drawing.ContentAlignment.MiddleLeft;
			// 
			// lblStyle
			// 
			this.lblStyle.AutoSize = true;
			this.lblStyle.Location = new System.Drawing.Point(88, 64);
			this.lblStyle.Name = "lblStyle";
			this.lblStyle.Size = new System.Drawing.Size(29, 16);
			this.lblStyle.TabIndex = 6;
			this.lblStyle.Text = "Style";
			this.lblStyle.TextAlign = System.Drawing.ContentAlignment.MiddleLeft;
			// 
			// radioBtnDropDown
			// 
			this.radioBtnDropDown.Checked = true;
			this.radioBtnDropDown.Location = new System.Drawing.Point(0, 80);
			this.radioBtnDropDown.Name = "radioBtnDropDown";
			this.radioBtnDropDown.Size = new System.Drawing.Size(80, 18);
			this.radioBtnDropDown.TabIndex = 7;
			this.radioBtnDropDown.TabStop = true;
			this.radioBtnDropDown.Text = "DropDown";
			this.radioBtnDropDown.Click += new System.EventHandler(this.radioBtnDropDown_Click);
			// 
			// radioBtnDropDownList
			// 
			this.radioBtnDropDownList.Location = new System.Drawing.Point(96, 80);
			this.radioBtnDropDownList.Name = "radioBtnDropDownList";
			this.radioBtnDropDownList.Size = new System.Drawing.Size(104, 18);
			this.radioBtnDropDownList.TabIndex = 8;
			this.radioBtnDropDownList.Text = "DropDownList";
			this.radioBtnDropDownList.Click += new System.EventHandler(this.radioBtnDropDown_Click);
			// 
			// Form
			// 
			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
			this.CancelButton = this.btnCancel;
			this.ClientSize = new System.Drawing.Size(210, 130);
			this.Controls.Add(this.radioBtnDropDownList);
			this.Controls.Add(this.radioBtnDropDown);
			this.Controls.Add(this.lblStyle);
			this.Controls.Add(this.lblDropDownStyle);
			this.Controls.Add(this.cboBoxStandard);
			this.Controls.Add(this.label3);
			this.Controls.Add(this.btnCancel);
			this.Controls.Add(this.cboBoxEnhanced);
			this.Controls.Add(this.label1);
			this.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedToolWindow;
			this.Name = "Form";
			this.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen;
			this.Text = "Combo Demo";
			this.Load += new System.EventHandler(this.Form_Load);
			this.ResumeLayout(false);

		}
		#endregion

		#region BootStrapper
		[STAThread]
		static void Main() {
			Application.Run(new Form());
		}
		#endregion

		#region btnCancel Click Event Handler
		private void btnCancel_Click(object sender, System.EventArgs e) {
			this.Close();
		}
		#endregion

		#region Initialise the Combo Box with strings
		private void InitComboStrings(ComboBox cbo){
			cbo.Items.AddRange(new string[]{
											   "Ottawa, Ontario", "St. John's, Newfoundland and Labrador",
											   "Halifax, Nova Scotia", "Charlottetown, Prince Edward Island",
											   "Fredericton, New Brunswick", "Qu�bec, Qu�bec", "Toronto, Ontario",
											   "Winnipeg, Manitoba", "Regina, Saskatchewan", "Edmonton, Alberta", "Victoria, British Colombia",
											   "Whitehorse, Yukon", "Yellowknife, Northwest Territories", "Iqaluit, Nunavut", 
											   "lllllllllllllll, lllllllllllllllllllllllll",
											   "WWWWWWWWWWWWWWW, WWWWWWWWWWWWWWWWWWWWWWWW"});
		}
		#endregion

		#region GetLargestTextExtent - Obtain largest string in pixels
		private void GetLargestTextExtent(System.Windows.Forms.ComboBox cbo, ref int largestWidth){
			int maxLen = -1;
			if (cbo.Items.Count >= 1){
				using (Graphics g = cbo.CreateGraphics()){
					int vertScrollBarWidth = 0;
					if (cbo.Items.Count > cbo.MaxDropDownItems){
						vertScrollBarWidth = SystemInformation.VerticalScrollBarWidth;
					}
					for (int nLoopCnt = 0; nLoopCnt < cbo.Items.Count; nLoopCnt++){
						int newWidth = (int) g.MeasureString(cbo.Items[nLoopCnt].ToString(), cbo.Font).Width + vertScrollBarWidth; 
						if (newWidth > maxLen) {
							maxLen = newWidth;
						}
					}
				}
			}
			largestWidth = maxLen;
		}
		#endregion

		#region cboBoxEnhanced_DropDown Event Handler
		private void cboBoxEnhanced_DropDown(object sender, System.EventArgs e) {
			this.GetLargestTextExtent(this.cboBoxEnhanced, ref this.pixelWidth);
			if ((this.pixelWidth != -1) && (this.pixelWidth > this.cboBoxEnhanced.DropDownWidth) && this.gotCBI){
				// Adjust our dropdown list box to include the horizontal scroll!
				int listStyle = GetWindowLong(this.cbi.hwndList, GWL_STYLE);
				listStyle |= WS_HSCROLL;
				listStyle = SetWindowLong(this.cbi.hwndList, GWL_STYLE, listStyle);
				// Switch on the Subclassing....
				this.scList.SubClassed = true;
				// Set the horizontal extent for the listbox!
				SendMessage(this.cbi.hwndList, LB_SETHORIZONTALEXTENT, this.pixelWidth, IntPtr.Zero);
			}
		}
		#endregion

		#region scList_SubClassedWndProc Event Handler
		private void scList_SubClassedWndProc(ref Message m) {
			switch (m.Msg){
				case WM_SIZE:
					GetClientRect(this.cbi.hwndList, ref this.cboListRect);
					this.xNewSize = this.scList.LoWord(m.LParam.ToInt32());
					this.xMaxScroll = Math.Max(this.pixelWidth - this.xNewSize, 0);
					this.xCurrentScroll = Math.Min(this.xCurrentScroll, this.xMaxScroll);
					this.si.cbSize = Marshal.SizeOf(this.si);
					this.si.nMax = this.xMaxScroll;
					this.si.nMin = this.xMinScroll;
					this.si.nPos = this.xCurrentScroll;
					this.si.nPage = this.xNewSize;
					this.si.fMask = SIF_RANGE | SIF_PAGE | SIF_POS;
					SetScrollInfo(this.cbi.hwndList, SB_HORZ, ref this.si, false);
					break;
				case WM_HSCROLL:
					int xDelta = 0;
					int xNewPos = 0;
					int modulo = (this.xNewSize > this.pixelWidth) ? (this.xNewSize % this.pixelWidth) : (this.pixelWidth % this.xNewSize);
					switch (this.scList.LoWord(m.WParam.ToInt32())){
						case SB_PAGEUP:
							xNewPos = this.xCurrentScroll - modulo;
							break;
						case SB_PAGEDOWN:
							xNewPos = this.xCurrentScroll + modulo;
							break;
						case SB_LINEUP:
							xNewPos = this.xCurrentScroll - 1;
							break;
						case SB_LINEDOWN:
							xNewPos = this.xCurrentScroll + 1;
							break;
						case SB_THUMBPOSITION:
							xNewPos = this.scList.HiWord(m.WParam.ToInt32());
							break;
						default:
							xNewPos = this.xCurrentScroll;
							break;
					}
					xNewPos = Math.Max(0, xNewPos);
					xNewPos = Math.Min(xMaxScroll, xNewPos);
					if (xNewPos == this.xCurrentScroll) break;
					xDelta = xNewPos - this.xCurrentScroll;
					this.xCurrentScroll = xNewPos;
					this.si.cbSize = Marshal.SizeOf(this.si);
					this.si.fMask = SIF_POS;
					this.si.nPos = this.xCurrentScroll;
					SetScrollInfo(this.cbi.hwndList, SB_HORZ, ref this.si, true);
					break;
			}
		}
		#endregion

		#region cboBoxStandard_DropDown Event Handler
		private void cboBoxStandard_DropDown(object sender, System.EventArgs e) {
			int pw = -1;
			this.GetLargestTextExtent(this.cboBoxStandard, ref pw);
			this.cboBoxStandard.DropDownWidth = pw;
		}
		#endregion

		// Proof to myself that this darn thing works in a singular code base such as this.
		// Irrespective of dropdown styles it works for both DropDown + DropDownList!
		#region cboBoxEnhanced_DropDownStyleChanged Event Handler
		private void cboBoxEnhanced_DropDownStyleChanged(object sender, System.EventArgs e) {
			this.lblStyle.Text = this.cboBoxEnhanced.DropDownStyle.ToString();
		}
		#endregion

		#region radioBtnDropDown_Click Event Handler
		private void radioBtnDropDown_Click(object sender, System.EventArgs e) {
			if (radioBtnDropDown.Checked){
				this.cboBoxEnhanced.DropDownStyle = ComboBoxStyle.DropDown;
			}else{
				this.cboBoxEnhanced.DropDownStyle = ComboBoxStyle.DropDownList;
			}
			// We need to call this again since we've changed the dropdown style.
			this.gotCBI = this.InitComboBoxInfo(this.cboBoxEnhanced);
			if (this.gotCBI){
				// No need to call new on scroll class etc..
				this.scList = new SubClass(this.cbi.hwndList, false);
				this.scList.SubClassedWndProc += new SubClass.SubClassWndProcEventHandler(scList_SubClassedWndProc);
			}
		}
		#endregion

		private void Form_Load(object sender, System.EventArgs e) {
			radioBtnDropDown.PerformClick();
		}
	}

	#region RECT struct
	[StructLayout(LayoutKind.Sequential)]
	public struct RECT {
		public int Left;
		public int Top;
		public int Right;
		public int Bottom;
	}
	#endregion

	#region COMBOBOXINFO Struct
	[StructLayout(LayoutKind.Sequential)]
	public struct COMBOBOXINFO {
		public int cbSize;
		public RECT rcItem;
		public RECT rcButton;
		public IntPtr stateButton;
		public IntPtr hwndCombo;
		public IntPtr hwndEdit;
		public IntPtr hwndList;
	}
	#endregion

	#region SCROLLINFO struct
	[StructLayout(LayoutKind.Sequential)]
	public struct SCROLLINFO {
		public int cbSize;
		public int fMask;
		public int nMin;
		public int nMax;
		public int nPage;
		public int nPos;
		public int nTrackPos;
	}
	#endregion

	#region SubClass Classing Handler Class
	public class SubClass : System.Windows.Forms.NativeWindow{
		public delegate void SubClassWndProcEventHandler(ref System.Windows.Forms.Message m);
		public event SubClassWndProcEventHandler SubClassedWndProc;
		private bool IsSubClassed = false;

		public SubClass(IntPtr Handle, bool _SubClass){
			base.AssignHandle(Handle);
			this.IsSubClassed = _SubClass;
		}

		public bool SubClassed{
			get{ return this.IsSubClassed; }
			set{ this.IsSubClassed = value; }
		}

		protected override void WndProc(ref Message m) {
			if (this.IsSubClassed){
				OnSubClassedWndProc(ref m);
			}
			base.WndProc (ref m);
		}

		#region HiWord Message Cracker
		public int HiWord(int Number) {
			return ((Number >> 16) & 0xffff);
		}
		#endregion

		#region LoWord Message Cracker
		public int LoWord(int Number) {
			return (Number & 0xffff);
		}
		#endregion

		#region MakeLong Message Cracker
		public int MakeLong(int LoWord, int HiWord) { 
			return (HiWord << 16) | (LoWord & 0xffff); 
		} 
		#endregion
 
		#region MakeLParam Message Cracker
		public IntPtr MakeLParam(int LoWord, int HiWord) { 
			return (IntPtr) ((HiWord << 16) | (LoWord & 0xffff)); 
		} 
		#endregion

		private void OnSubClassedWndProc(ref Message m){
			if (SubClassedWndProc != null){
				this.SubClassedWndProc(ref m);
			}
		}
	}
	#endregion

}