Why will this run in 32bit, but not 64bit? (x86 vs x64 in compiler options)

Question

I'm using the following code to iterate through and find windows containing specific strings in their title bars. I have two projects containing this identical information, yet for some reason - it works when targeted to x64, but FindWindowLike returns 0 always on x86. I need this to run in x86.

I am developing this on Windows 7 x64, but it needs to run on my machine as well as XPx32. What I can't fathom is why it compiles/runs on x64 but not x32 (considering I'm targetting user*32*.dll)

Here comes the code:

    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> Private Shared Function FindWindow(ByVal lpClassName As String, ByVal lpWindowName As String) As IntPtr
    End Function

    <DllImport("user32.dll", EntryPoint:="FindWindow", SetLastError:=True, CharSet:=CharSet.Auto)> Private Shared Function FindWindowByClass(ByVal lpClassName As String, ByVal zero As IntPtr) As IntPtr
    End Function

    <DllImport("user32.dll", EntryPoint:="FindWindow", SetLastError:=True, CharSet:=CharSet.Auto)> Private Shared Function FindWindowByCaption(ByVal zero As IntPtr, ByVal lpWindowName As String) As IntPtr
    End Function

    <DllImport("user32.dll", CharSet:=CharSet.Auto, ExactSpelling:=True)> Public Shared Function IsWindow(ByVal hWnd As IntPtr) As Boolean
    End Function

    Private Declare Function GetDC Lib "user32.dll" (ByVal hwnd As IntPtr) As IntPtr
    Private Declare Function ReleaseDC Lib "user32.dll" (ByVal hwnd As IntPtr, ByVal hdc As IntPtr) As IntPtr
    Public Declare Function GetDesktopWindow Lib "user32" () As Long
    Public Declare Function GetWindow Lib "user32" (ByVal hwnd As Long, ByVal wCmd As Long) As Long
    Public Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As Long, ByVal lpString As String, ByVal cch As Long) As Long
    Public Declare Function GetClassName Lib "user32" Alias "GetClassNameA" (ByVal hwnd As Long, ByVal lpClassName As String, ByVal nMaxCount As Long) As Long
    Public Const GW_HWNDFIRST = 0
    Public Const GW_HWNDLAST = 1
    Public Const GW_HWNDNEXT = 2
    Public Const GW_HWNDPREV = 3
    Public Const GW_OWNER = 4
    Public Const GW_CHILD = 5
    Public listWindows As New List(Of String)

Private Function FindWindows() As Long
        Dim hWndStart As Long = 0
        Dim WindowText As String = "*message*"
        Dim Classname As String = "*"
        Dim hwnd As Long
        Dim sWindowText As String
        Dim sClassname As String
        Dim r As Long

        'Hold the level of recursion and
        'hold the number of matching windows
        Static level As Integer

        'Initialize if necessary. This is only executed 
        'when level = 0 and hWndStart = 0, normally 
        'only on the first call to the routine.
        If level = 0 Then
            If hWndStart = 0 Then hWndStart = GetDesktopWindow()
        End If

        'Increase recursion counter      
        level = level + 1

        'Get first child window
        hwnd = GetWindow(hWndStart, GW_CHILD)

        Do Until hwnd = 0

            'Search children by recursion
            Call FindWindows()

            'Get the window text and class name
            sWindowText = Space$(255)
            r = GetWindowText(hwnd, sWindowText, 255)
            sWindowText = Microsoft.VisualBasic.Left(sWindowText, r)
            sClassname = Space$(255)
            r = GetClassName(hwnd, sClassname, 255)
            sClassname = Microsoft.VisualBasic.Left(sClassname, r)

            'Check if window found matches the search parameters
            If (sWindowText Like WindowText) And (sClassname Like Classname) Then
                If listWindows.Contains(hwnd & "||||" & sClassname & "||||" & sWindowText) = False Then
                    listWindows.Add(hwnd & "||||" & sClassname & "||||" & sWindowText)
                End If
                FindWindows = hwnd

                'uncommenting the next line causes the routine to
                'only return the first matching window.
                'Exit Do

            End If

            'Get next child window
            hwnd = GetWindow(hwnd, GW_HWNDNEXT)

        Loop

        'Reduce the recursion counter
        level = level - 1
End Function

Answer 1

Your function definitions are wrong. In C LONG is 32 bits wide, but in C# and VB.NET long is 64 bits wide on both 32-bit and 64-bit systems. Also, your window handle parameters should be IntPtrs.

Private Declare Function GetDC Lib "user32.dll" (ByVal hwnd As IntPtr) As IntPtr
Private Declare Function ReleaseDC Lib "user32.dll" (ByVal hwnd As IntPtr, ByVal hdc As IntPtr) As IntPtr
Private Declare Function GetDesktopWindow Lib "user32" () As IntPtr
Private Declare Function GetWindow Lib "user32" (ByVal hwnd As IntPtr, ByVal wCmd As Integer) As IntPtr
Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As IntPtr, ByVal lpString As String, ByVal cch As Integer) As Integer
Private Declare Function GetClassName Lib "user32" Alias "GetClassNameA" (ByVal hwnd As IntPtr, ByVal lpClassName As String, ByVal nMaxCount As Integer) As Integer

Note that you shouldn't be passing Strings to GetWindowText and GetClassName. Try StringBuilder.

Answer 2

Another piece of advice is to wrap up the calls that target 32 & 64 bit systems. Here's an example for GetWindowLong:

Public Shared Function GetWindowLong(ByVal hWnd As HandleRef, ByVal nIndex As Integer) As IntPtr
    If (IntPtr.Size = 4) Then
        Return NativeMethods.GetWindowLong32(hWnd, nIndex)
    End If
    Return NativeMethods.GetWindowLongPtr64(hWnd, nIndex)
End Function

As nobugz says, you can have a look at pinvoke.net. I prefer to use reflector and see, exactly, how the framework handles it.