How to get the index of the combo box item which is currently under the cursor.
The provided code is written in C# for the Windows Forms library of the .NET Framework and uses native function calls and structs.
This repo provides an example Windows Forms project.
In order to register the mouse on the drop-down list of a combo box we will need two native WinAPI functions as well as three native structs.
List of native components:
POINT structureRECT structureCOMBOXBOXINFO structureGetComboBoxInfo functionLBItemFromPt function
To get a list box item from a point we have to use the function LBItemFromPt. The function requires the following parameters:
HWND hLB (Handle to list box)POINT pt (POINT struct that contains the screen coordinates to check)BOOL bAutoScroll (Scroll flag. Ignore this if you are only interested in getting the items index)
The function returns the item index if the point is over a list item, or -1 otherwise.
To get informations such as the edit box handle or drop-down list handle we have to use the function GetComboBoxInfo. The function requires the following parameters:
HWND hwndCombo (Handle to the combo box)PCOMBOBOXINFO pcbi (Pointer to a COMBOBOXINFO structure)
The function returns a boolean value. The COMBOBOXINFO structure will be populated and contains all informations needed.
I recommand just copying the structs from pinvoke. There is no need to wrap your head around these.
Step by step implementation of a combo box class that implements the native functions and structures above. The code was written in C# with Visual Studio 2017 using the .Net 4.5.2 framework. You will need the following namespaces:
- System.Runtime.InteropServices
- System.Windows.Forms
- System.Drawing
- System
Create a new class that derives from the System.Windows.Forms.ComboBox class. I will call it MouseOverItemComboBox.
namespace X.Y.Z
{
class MouseOverItemComboBox : ComboBox
{
}
}Add the structs above to the same namespace.
Create a custom EventArgs class that suits your needs.
using System;
namespace X.Y.Z
{
public class MouseOverEventArgs : EventArgs
{
public int Index { get; }
public MouseOverEventArgs(int index)
{
Index = index;
}
}
}Implement MouseOver event and native functions.
class MouseOverItemComboBox : ComboBox
{
// Event delegate
public delegate void OnMouseOverItem(object sender, MouseOverEventArgs e);
// Event which the parent can subscribe to.
public event OnMouseOverItem MouseOverItem;
// The possible combo box button states.
enum ButtonState {
STATE_SYSTEM_NONE = 0, // Button exists and is not pressed.
STATE_SYSTEM_INVISIBLE = 0x00008000, // There is no button.
STATE_SYSTEM_PRESSED = 0x00000008, // Button is pressed.
}
/* Native COMBOBOXINFO struct implementation.
** Contains combo box status information. */
[StructLayout(LayoutKind.Sequential)]
struct COMBOBOXINFO
{
public int cbSize; // Size in bytes of struct.
public RECT rcItem; // RECT that specifies the coordinates of the edit box.
public RECT rcButton; // RECT that specifies the coordinates of the drop-down button.
public ButtonState stateButton; // Drop-down button state.
public IntPtr hwndCombo; // Handle to the combo box.
public IntPtr hwndEdit; // Handle to the edit box.
public IntPtr hwndList; // Handle to the drop-down list.
}
/* Sent to parent window of a list box before the system draws the list box.
** Can set text and background color of the list box by using the specified
** device context handle. */
const int WM_CTLCOLORLISTBOX = 0x0134;
// Native function that retreives information about the specified combo box.
// https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getcomboboxinfo
[DllImport("user32.dll", SetLastError = true)]
static extern bool GetComboBoxInfo(IntPtr hWnd, [In][Out] ref COMBOBOXINFO pcbi);
// Native function that retreives the index of the item at the specified point in a list box.
// https://docs.microsoft.com/en-us/windows/win32/api/commctrl/nf-commctrl-lbitemfrompt
[DllImport("Comctl32.dll", SetLastError = true)]
static extern int LBItemFromPt(IntPtr hLB, POINT pt, bool bAutoScroll);Override the WndProc method of the parent and implement a helper method that will do the actual job of getting the list box item index.
// Helper method which will invoke the MouseOverItem event.
private void HandleMouseOverItem()
{
// cbSize must be set before calling GetComboBoxInfo.
COMBOBOXINFO pcbi = new COMBOBOXINFO();
pcbi.cbSize = Marshal.SizeOf(pcbi);
// Get combo box information.
GetComboBoxInfo(Handle, ref pcbi);
// Check for invalid pointer... just in case.
if (pcbi.hwndList == IntPtr.Zero)
return;
// Current position of cursor.
POINT pt = Cursor.Position;
// LBItemFromPt will return the Index of the Item on success.
int retVal = LBItemFromPt(pcbi.hwndList, pt, false);
if (retVal == -1)
return;
// Invoke the event.
MouseOverItem?.Invoke(this, new MouseOverEventArgs(retVal));
}
protected override void WndProc(ref Message m)
{
switch (m.Msg)
{
/* This message is sent by the list box of the combo box.
** It is sent before the system draws the list box.
** Whenever the cursor enters a list item this message will be
** sent to the parent i.e. the combox box.
** NOTE that this message is always sent twice.
** First time for drawing the default item background.
** Second time for drawing the highlighted item background. */
case WM_CTLCOLORLISTBOX:
{
// Let the helper method do the rest.
HandleMouseOverItem();
base.WndProc(ref m);
break;
}
default:
{
base.WndProc(ref m);
break;
}
}
}You do not need much code to get this to work nor is it hard. This code is not limited to combo boxes. Any control that utilizes a list box supports this. If you cannot get a hold of the list box handle it won't work though.
For the sake of completeness, the whole implementation:
using System.Runtime.InteropServices;
using System.Windows.Forms;
using System.Drawing;
using System;
class MouseOverItemComboBox : ComboBox
{
// Event delegate
public delegate void OnMouseOverItem(object sender, MouseOverEventArgs e);
// Event which the parent can subscribe to.
public event OnMouseOverItem MouseOverItem;
// The possible combo box button states.
enum ButtonState {
STATE_SYSTEM_NONE = 0, // Button exists and is not pressed.
STATE_SYSTEM_INVISIBLE = 0x00008000, // There is no button.
STATE_SYSTEM_PRESSED = 0x00000008, // Button is pressed.
}
/* Native COMBOBOXINFO struct implementation.
** Contains combo box status information. */
[StructLayout(LayoutKind.Sequential)]
struct COMBOBOXINFO
{
public int cbSize; // Size in bytes of struct.
public RECT rcItem; // RECT that specifies the coordinates of the edit box.
public RECT rcButton; // RECT that specifies the coordinates of the drop-down button.
public ButtonState stateButton; // Drop-down button state.
public IntPtr hwndCombo; // Handle to the combo box.
public IntPtr hwndEdit; // Handle to the edit box.
public IntPtr hwndList; // Handle to the drop-down list.
}
/* Sent to parent window of a list box before the system draws the list box.
** Can set text and background color of the list box by using the specified
** device context handle. */
const int WM_CTLCOLORLISTBOX = 0x0134;
// Native function that retreives information about the specified combo box.
// https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getcomboboxinfo
[DllImport("user32.dll", SetLastError = true)]
static extern bool GetComboBoxInfo(IntPtr hWnd, [In][Out] ref COMBOBOXINFO pcbi);
// Native function that retreives the index of the item at the specified point in a list box.
// https://docs.microsoft.com/en-us/windows/win32/api/commctrl/nf-commctrl-lbitemfrompt
[DllImport("Comctl32.dll", SetLastError = true)]
static extern int LBItemFromPt(IntPtr hLB, POINT pt, bool bAutoScroll);
// Helper method which will invoke the MouseOverItem event.
private void HandleMouseOverItem()
{
// cbSize must be set before calling GetComboBoxInfo.
COMBOBOXINFO pcbi = new COMBOBOXINFO();
pcbi.cbSize = Marshal.SizeOf(pcbi);
// Get combo box information.
GetComboBoxInfo(Handle, ref pcbi);
// Check for invalid pointer... just in case.
if (pcbi.hwndList == IntPtr.Zero)
return;
// Current position of cursor.
POINT pt = Cursor.Position;
// LBItemFromPt will return the Index of the Item on success.
int retVal = LBItemFromPt(pcbi.hwndList, pt, false);
if (retVal == -1)
return;
// Invoke the event.
MouseOverItem?.Invoke(this, new MouseOverEventArgs(retVal));
}
protected override void WndProc(ref Message m)
{
switch (m.Msg)
{
/* This message is sent by the list box of the combo box.
** It is sent before the system draws the list box.
** Whenever the cursor enters a list item this message will be
** sent to the parent i.e. the combox box.
** NOTE that this message is always sent twice.
** First time for drawing the default item background.
** Second time for drawing the highlighted item background. */
case WM_CTLCOLORLISTBOX:
{
// Let the helper method do the rest.
HandleMouseOverItem();
base.WndProc(ref m);
break;
}
default:
{
base.WndProc(ref m);
break;
}
}
}
}
public class MouseOverEventArgs : EventArgs
{
public int Index { get; }
public MouseOverEventArgs(int index)
{
Index = index;
}
}
/// <summary>
/// Native RECT struct implementation
/// </summary>
[StructLayout(LayoutKind.Sequential)]
public struct RECT
{
public int Left, Top, Right, Bottom;
public RECT(int left, int top, int right, int bottom)
{
Left = left;
Top = top;
Right = right;
Bottom = bottom;
}
public RECT(Rectangle r) : this(r.Left, r.Top, r.Right, r.Bottom) { }
public int X
{
get { return Left; }
set { Right -= (Left - value); Left = value; }
}
public int Y
{
get { return Top; }
set { Bottom -= (Top - value); Top = value; }
}
public int Height
{
get { return Bottom - Top; }
set { Bottom = value + Top; }
}
public int Width
{
get { return Right - Left; }
set { Right = value + Left; }
}
public Point Location
{
get { return new Point(Left, Top); }
set { X = value.X; Y = value.Y; }
}
public Size Size
{
get { return new Size(Width, Height); }
set { Width = value.Width; Height = value.Height; }
}
public static implicit operator Rectangle(RECT r)
{
return new Rectangle(r.Left, r.Top, r.Width, r.Height);
}
public static implicit operator RECT(Rectangle r)
{
return new RECT(r);
}
public static bool operator ==(RECT r1, RECT r2)
{
return r1.Equals(r2);
}
public static bool operator !=(RECT r1, RECT r2)
{
return !r1.Equals(r2);
}
public bool Equals(RECT r)
{
return r.Left == Left && r.Top == Top && r.Right == Right && r.Bottom == Bottom;
}
public override bool Equals(object obj)
{
if (obj is RECT)
return Equals((RECT)obj);
else if (obj is Rectangle)
return Equals(new RECT((Rectangle)obj));
return false;
}
public override int GetHashCode()
{
return ((Rectangle)this).GetHashCode();
}
public override string ToString()
{
return string.Format(System.Globalization.CultureInfo.CurrentCulture, "{{Left={0},Top={1},Right={2},Bottom={3}}}", Left, Top, Right, Bottom);
}
}
/// <summary>
/// Native POINT struct implementation.
/// </summary>
public struct POINT
{
public int X;
public int Y;
public POINT(Point p)
{
X = p.X;
Y = p.Y;
}
public POINT(int x, int y)
{
X = x;
Y = y;
}
public static implicit operator Point(POINT p)
{
return new Point(p.X, p.Y);
}
public static implicit operator POINT(Point p)
{
return new POINT(p.X, p.Y);
}
public static bool operator ==(POINT p1, POINT p2)
{
return (p1.X == p2.X && p1.Y == p2.Y);
}
public static bool operator !=(POINT p1, POINT p2)
{
return (p1.X == p2.X && p1.Y == p2.Y);
}
public bool Equals(POINT p)
{
return X == p.X && Y == p.Y;
}
public override bool Equals(object obj)
{
if (obj is POINT)
return Equals((POINT)obj);
else if (obj is Point)
return Equals(new POINT((Point)obj));
return false;
}
public override int GetHashCode()
{
return ((POINT)this).GetHashCode();
}
public override string ToString()
{
return string.Format(System.Globalization.CultureInfo.CurrentCulture, "{{X={0},Y={1}}}", X, Y);
}
}