Retrieve a ComboBox counts and items by using SendMessage API

Question

I want to get a count and list of ComboBox control which is not mine so that I cannot modify the code.

For example, controlling the target app can be done by using SendMessage API.

But, how can I retrieve a whole list of the target control by hooking?

Answer 1

You can find a list of ComboBox control messages here:

• MSDN - ComboBox Control Messages
• PInvoke - CB_ Constants

To get items count you need to use CB_GETCOUNT message and to get text of an item you can use CB_GETLBTEXT message.

Example

Here I created a ComboBoxHelper class which you can create its instance by passing Handle of the ComboBox and use its properties:

• SelectedIndex as Integer: Returns selected index, returns -1 if no item is selected.
• Selectedtext as String: Returns text of selected item, returns String.Empty if no item is selected.
• ItemsCount as Integer: returns count of items.
• Items(index) as String: returns text of specified item (the item at specified index)
• Items as List(of String): returns the list of items of combo box. If there is no items, it returns an empty list.

Public Class ComboBoxHelper
    Private hWnd As IntPtr
    Const CB_GETCURSEL As Integer = &H147
    Const CB_SETCURSEL As Integer = &H14E
    Const CB_GETCOUNT As Integer = &H146
    Const CB_GETLBTEXT As Integer = &H148
    Const CB_GETLBTEXTLEN As Integer = &H149
    <System.Runtime.InteropServices.DllImport("user32.dll")> _
    Shared Function SendMessage(ByVal hWnd As IntPtr, ByVal Msg As Integer, _
        ByVal wParam As Integer, ByRef lParam As Integer) As IntPtr
    End Function
    <System.Runtime.InteropServices.DllImport("user32.dll")> _
    Shared Function SendMessage(ByVal hWnd As IntPtr, ByVal Msg As Integer, _
        ByVal wParam As Integer, ByVal lParam As System.Text.StringBuilder) As IntPtr
    End Function
    Public Sub New(handle As IntPtr)
        hWnd = handle
    End Sub
    Public Property SelectedIndex As Integer
        Get
            Return SendMessage(hWnd, CB_GETCURSEL, 0, 0).ToInt32()
        End Get
        Set(ByVal value As Integer)
            SendMessage(hWnd, CB_SETCURSEL, value, 0).ToInt32()
        End Set
    End Property
    Public ReadOnly Property ItemsCount As Integer
        Get
            Return SendMessage(hWnd, CB_GETCOUNT, 0, 0).ToInt32()
        End Get
    End Property
    Public ReadOnly Property SelectedText As String
        Get
            Dim index = Me.SelectedIndex
            If (Me.SelectedIndex = -1) Then
                Return String.Empty
            End If
            Return Me.Items(index)
        End Get
    End Property
    Public ReadOnly Property Items() As List(Of String)
        Get
            If (ItemsCount > 0) Then
                Return Enumerable.Range(0, ItemsCount) _
                                 .Select(Function(index) Items(index)).ToList()
            Else
                Return New List(Of String)
            End If
        End Get
    End Property
    Public ReadOnly Property Items(index As Integer) As String
        Get
            If (index < 0 OrElse index >= ItemsCount) Then
                Throw New ArgumentOutOfRangeException("index")
            End If
            Dim length = SendMessage(hWnd, CB_GETLBTEXTLEN, index, 0).ToInt32()
            Dim text As New System.Text.StringBuilder(length)
            SendMessage(hWnd, CB_GETLBTEXT, index, text)
            Return text.ToString()
        End Get
    End Property
End Class

Here is an example of usage of the class:

Dim combo As New ComboBoxHelper(hWnd) 'You have hWnd
MessageBox.Show(combo.ItemsCount.ToString())
MessageBox.Show(combo.SelectedIndex.ToString())
MessageBox.Show(combo.SelectedText.ToString())
combo.Items.ForEach(Function(item) MessageBox.Show(item))