ComboBox Dropdown Position

Tricky problem. I could not find a good fix for this, merely a workaround. Add a new class and paste the code shown below. Compile. Drop the new control from the top of the toolbox onto your form.

The workaround is not a very good one. The problem is that the dropdown window will ignore the attempt to move it until the drop-down animation is complete. The visual effect is not great, you’ll see the window drop down, then jump to the left. I don’t know an other way to slap it over the head, maybe somebody else does.

C# version:

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

public class MyComboBox : ComboBox {
  protected override void OnDropDown(EventArgs e) {
    // Is dropdown off the right side of the screen?
    Point pos = this.PointToScreen(this.Location);
    Screen scr = Screen.FromPoint(pos);
    if (scr.WorkingArea.Right < pos.X + this.DropDownWidth) {
       this.BeginInvoke(new Action(() => {
        // Retrieve handle to dropdown list
        COMBOBOXINFO info = new COMBOBOXINFO();
        info.cbSize = Marshal.SizeOf(info);
        SendMessageCb(this.Handle, 0x164, IntPtr.Zero, out info);
        // Move the dropdown window
        RECT rc;
        GetWindowRect(info.hwndList, out rc);
        int x = scr.WorkingArea.Right - (rc.Right - rc.Left);
        SetWindowPos(info.hwndList, IntPtr.Zero, x, rc.Top, 0, 0, 5);
      }));
    }
    base.OnDropDown(e);
  }

  // P/Invoke declarations
  private struct COMBOBOXINFO {
    public Int32 cbSize;
    public RECT rcItem, rcButton;
    public int buttonState;
    public IntPtr hwndCombo, hwndEdit, hwndList;
  }
  private struct RECT {
    public int Left, Top, Right, Bottom;
  }
  [DllImport("user32.dll", EntryPoint = "SendMessageW", CharSet = CharSet.Unicode)]
  private static extern IntPtr SendMessageCb(IntPtr hWnd, int msg, IntPtr wp, out COMBOBOXINFO lp);
  [DllImport("user32.dll")]
  private static extern bool SetWindowPos(IntPtr hWnd, IntPtr after, int x, int y, int cx, int cy, int flags);
  [DllImport("user32.dll")]
  private static extern bool GetWindowRect(IntPtr hWnd, out RECT rc);
}

VB.Net version:

Imports System
Imports System.Drawing
Imports System.Windows.Forms
Imports System.Runtime.InteropServices

Public Class MyComboBox
    Inherits ComboBox

    'P/Invoke declarations
    Private Structure COMBOBOXINFO
        Public cbSize As Int32
        Public rcItem As RECT
        Public rcButton As RECT
        Public buttonState As Integer
        Public hwndCombo As IntPtr
        Public hwndEdit As IntPtr
        Public hwndList As IntPtr
    End Structure

    Private Structure RECT
        Public Left As Integer
        Public Top As Integer
        Public Right As Integer
        Public Bottom As Integer
    End Structure


    <DllImport("user32.dll", EntryPoint:="SendMessageW", CharSet:=CharSet.Unicode)>
    Private Shared Function SendMessageCb(hWnd As IntPtr, msg As Integer, wp As IntPtr, ByRef lp As COMBOBOXINFO) As IntPtr
    End Function

    <DllImport("user32.dll")>
    Private Shared Function SetWindowPos(hWnd As IntPtr, after As IntPtr, x As Integer, y As Integer, cx As Integer, cy As Integer, flags As Integer) As Boolean
    End Function

    <DllImport("user32.dll")>
    Private Shared Function GetWindowRect(hWnd As IntPtr, ByRef rc As RECT) As Boolean
    End Function


    Protected Overrides Sub OnDropDown(e As EventArgs)
          ' Is dropdown off the right side of the screen?
          Dim pos As Point = Me.PointToScreen(Me.Location)
          Dim scr As Screen = Screen.FromPoint(pos)

          If (scr.WorkingArea.Right < pos.X + Me.DropDownWidth) Then
              Me.BeginInvoke(New Action(Sub()

                                            'Retrieve handle to dropdown list
                                            Dim info As COMBOBOXINFO = New COMBOBOXINFO()
                                            info.cbSize = Marshal.SizeOf(info)
                                            SendMessageCb(Me.Handle, &H164, IntPtr.Zero, info)
                                            'Move the dropdown window
                                            Dim rc As RECT
                                            GetWindowRect(info.hwndList, rc)
                                            Dim x As Integer = scr.WorkingArea.Right - (rc.Right - rc.Left)
                                            SetWindowPos(info.hwndList, IntPtr.Zero, x, rc.Top, 0, 0, 5)
                                        End Sub))
          End If

          MyBase.OnDropDown(e)

    End Sub



End Class