How To Call an API That Uses Structures from eVB/VBCE6 (241530)

MORE INFORMATION

Microsoft provides programming examples for illustration only, without warranty either expressed or implied. This includes, but is not limited to, the implied warranties of merchantability or fitness for a particular purpose. This article assumes that you are familiar with the programming language that is being demonstrated and with the tools that are used to create and to debug procedures. Microsoft support engineers can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific requirements.
WARNING: This article assumes advanced knowledge of user-defined data types and their use with Win32 API calls in Microsoft Visual Basic. This method is not supported by Microsoft.

A UDT is a data structure that can store multiple related variables of different types. UDTs are very useful when you want to pass several related pieces of information in one parameter of a function call. Unfortunately, because UDTs are not directly supported in either eVB or VBCE, this limits you to which API calls you can use from your eVB/VBCE programs.

As a workaround to this limitation, there is a way you can manipulate the data and coax the contents of your would-be UDTs into a String variable that can be passed into an API call in the same format as a UDT would pass them. This example shows how to convert Long Integer data types into a string format compatible with how data is stored in memory.

NOTE: Be careful with the Declaration statement in eVB/VBCE. The UDT should always be specified as a String passed ByVal. This is in contrast to what you find in the "API Text Viewer" viewing the Winceapi.txt file.

1. Create a new Windows CE project in either Visual Basic or eMbedded Visual Basic. Form1 is created by default.
2. On the Project menu, select Add Module to add a module. Module1 is created by default.
3. Paste the the following code into Module1:

   Option Explicit
   '
   ' Note: lpRect is passed as ByVal.
   Declare Function GetWindowRect Lib "Coredll" ( _
       ByVal hwnd As Long, ByVal lpRect As String) As Long
   
   Public Function MemStringToLong(StringIn As String) As Long
       On Error Resume Next
       Dim hWorkVal As String
       '
       ' Convert the String back to Long Integer.
       ' Converting back to Big Endian format.
       hWorkVal = Hex(AscB(MidB(StringIn, 2, 1)))
       If Not Err Then
           hWorkVal = hWorkVal & Hex(AscB(MidB(StringIn, 1, 1)))
       End If
       '
       ' Return Long Integer value.
       MemStringToLong = CLng("&H" & hWorkVal)
   End Function
   
   Public Sub getRECT(ByVal sRect As String, _
                      ByRef rLeft As Long, ByRef rTop As Long, _
                      ByRef rRight As Long, ByRef rBottom As Long)
       '
       ' Convert memory-formatted String back to Long Integer.
       rLeft = MemStringToLong(MidB(sRect, 1, 4))
       rTop = MemStringToLong(MidB(sRect, 5, 4))
       rRight = MemStringToLong(MidB(sRect, 9, 4))
       rBottom = MemStringToLong(MidB(sRect, 13, 4))
   End Sub
   
   Public Function LongToMemoryString(ByVal lInputValue As Long) As String
   
       Dim hWorkVal As String
       '
       ' Convert to HEX value.
       hWorkVal = Hex(lInputValue)
       '
       ' Check to see if it is not zero.
       If hWorkVal <> "0" Then
           '
           ' Convert to memory storage format (Little Endian).
           ' For example, 0000A411 would convert to 11A40000.
           ' Use ChrB to rebuild Bytes.
           LongToMemoryString = ChrB(CInt("&H" & Mid(hWorkVal, 3, 2)))
           LongToMemoryString = LongToMemoryString & _
                                ChrB(CInt("&H" & Mid(hWorkVal, 1, 2)))
           LongToMemoryString = LongToMemoryString & ChrB(CInt("&H00"))
           LongToMemoryString = LongToMemoryString & ChrB(CInt("&H00"))
       Else
           ' Just return zeros.
           ' Use ChrB to build Bytes.
           LongToMemoryString = ChrB(CInt("&H00"))
           LongToMemoryString = LongToMemoryString & ChrB(CInt("&H00"))
           LongToMemoryString = LongToMemoryString & ChrB(CInt("&H00"))
           LongToMemoryString = LongToMemoryString & ChrB(CInt("&H00"))
       End If
   End Function
   
   Public Function RECT(rLeft As Long, rTop As Long, _
                        rRight As Long, rBottom As Long) As String
       '
       ' Convert inbound Long Integers to a memory storage String format.
       RECT = LongToMemoryString(rLeft) & _
              LongToMemoryString(rTop) & _
              LongToMemoryString(rRight) & _
              LongToMemoryString(rBottom)
   End Function
   					

4. Add a Command button to Form1.
5. Paste the following code into Form1:

   Option Explicit
   Private Sub Form_Load()
       Command1.Move 120, 120, 2000, 600
       Command1.Caption = "Get Form1 RECT Info."
   End Sub
   
   Private Sub Command1_Click()
       Dim lRet As Long
       Dim sMessage  As String
       Dim sRect As String
       Dim rLeft As Long
       Dim rTop As Long
       Dim rRight  As Long
       Dim rBottom  As Long
       '
       ' Initialize RECT memory string.
       ' Convert initial Rect values to String to pass into GetWindowRect API.
       sRect = RECT(rLeft, rTop, rRight, rBottom)
       '
       ' Call GetWindowRect.
       lRet = GetWindowRect(Me.hwnd, sRect)
       '
       ' Test return value.
       If lRet > 0 Then
           '
           ' Convert String back to Long Integer.
           getRECT sRect, rLeft, rTop, rRight, rBottom
           '
           ' Display results.
           sMessage = "RECT is defined as follows:" & vbCrLf
           sMessage = sMessage & "Left: " & rLeft & vbCrLf
           sMessage = sMessage & "Top: " & rTop & vbCrLf
           sMessage = sMessage & "Right: " & rRight & vbCrLf
           sMessage = sMessage & "Bottom: " & rBottom
           
           MsgBox sMessage, vbOKOnly + vbInformation, "Form Settings"
       Else
           MsgBox "Call to GetWindowRect Failed.", _
               vbOKOnly + vbCritical, "Failure"
       End If
   End Sub
   					

6. Run the application.
7. Click on the Command button and note the results.