Application Programming Interface Compatibility
The combination of VBA and type libraries gives you lots of functionality to create Microsoft Office applications. However, sometimes you must communicate directly with the computer’s operating system and other components such as when you manage memory or processes, when working with the user interface such as windows and controls, or when modifying the Windows registry. In these scenarios, your best option is to use one of the external functions that are embedded in dynamic linked library (DLL) files. You do this in VBA by making API calls using Declare statements.
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Microsoft provides a Win32API.txt file which contains 1,500 Declare statements and a tool to cut and paste the Declare statement that that you want into your code. However, these statements are for 32-bit systems and must be converted to 64-bit by using the information discussed later in this article. Existing Declare statements will not compile in 64-bit VBA until they have been marked as safe for 64-bit by using the PtrSafe attribute. You can find samples of this type of conversion at Excel MVP Jan Karel Pieterse’s Web site at: http://www.jkp-ads.com/articles/apideclarations.asp.
The Office Code Compatibility Inspector user’s guide is a useful tool to inspect the syntax of API Declare statements for the PtrSafe attribute, if needed, and the appropriate return type. |
Declare statements resemble one of the following, depending whether you are calling a subroutine (which has no return value) or a function (which does have a return value).
Public/Private Declare Sub SubName Lib "LibName" Alias "AliasName" (argument list)
Public/Private Declare Function FunctionName Lib "Libname" alias "aliasname" (argument list) As Type
The SubName function or FunctionName function is replaced by the actual name of the procedure in the DLL file and represents the name that is used when the procedure is called from VBA code. You can also specify an AliasName argument for the name of the procedure, if desired. The name of the DLL file that contains the procedure being called follows the Lib keyword. And finally, the argument list contains the parameters and the data types that must be passed to the procedure.
The following Declare statement opens a subkey in the Windows registry and replaces its value.
Declare Function RegOpenKeyA Lib "advapi32.dll" (ByVal Key As Long, ByVal SubKey As String, NewKey As Long) As Long
The Windows.h (window handle) entry for the RegOpenKeyA function is as follows:
LONG RegOpenKeyA ( HKEY hKey, LPCSTR lpSubKey, HKEY *phkResult );
In Microsoft Visual C and Microsoft Visual C++, the previous example compiles correctly for both 32-bit and 64-bit. This is because HKEY is defined as a pointer, whose size reflects the memory size of the platform that the code is compiled in.
In previous versions of VBA, there was no specific pointer data type so the Long data type was used. And because the Long data type is always 32-bits, this breaks when used on a system with 64-bit memory because the upper 32-bits may be truncated or may overwrite other memory addresses. Either of these situations can result in unpredictable behavior or system crashes.
To resolve this, VBA now contains a true pointer data type: LongPtr. This new data type enables you to write the original Declare statement correctly as:
Declare PtrSafe Function RegOpenKeyA Lib “advapire32.dll” (ByVal hKey as LongPtr, ByVal lpSubKey As String, phkResult As LongPtr) As Long
This data type and the new PtrSafe attribute enable you to use this Declare statement on either 32-bit or 64-bit systems. The PtrSafe attribute indicates to the VBA compiler that the Declare statement is targeted for the 64-bit version of Office 2010. Without this attribute, using the Declare statement in a 64-bit system will result in a compile-time error. Note that the PtrSafe attribute is optional on the 32-bit version of Office 2010. This enables existing Declare statements to work as they always have.
The following table provides more information on the new qualifier and data type already discussed as well as another data type, two conversion operators, and three functions.
Type |
Item |
Description |
Qualifier |
PtrSafe |
Indicates that the Declare statement is compatible with 64-bits. This attribute is mandatory on 64-bit systems. |
Data Type |
LongPtr |
A variable data type which is a 4-bytes data type on 32-bit versions and an 8-byte data type on 64-bit versions of Office 2010. This is the recommended way of declaring a pointer or a handle for new code but also for legacy code if it has to run in the 64-bit version of Office 2010. It is only supported in the VBA 7 runtime on 32-bit and 64-bit. Note that you can assign numeric values to it but not numeric types. |
Data Type |
LongLong |
This is an 8-byte data type which is available only in 64-bit versions of Office 2010. You can assign numeric values but not numeric types (to avoid truncation). |
Conversion Operator |
CLngPtr |
Converts a simple expression to a LongPtr data type. |
Conversion Operator |
CLngLng |
Converts a simple expression to a LongLong data type. |
Function |
VarPtr |
Variant converter. Returns a LongPtr on 64-bit versions, and a Long on 32-bit versions (4 bytes). |
Function |
ObjPtr |
Object converter. Returns a LongPtr on 64-bit versions, and a Long on 32-bit versions (4 bytes). |
Function |
StrPtr |
String converter. Returns a LongPtr on 64-bit versions, and a Long on 32-bit versions (4 bytes). |
The follow example shows how to use some of these items in a Declare statement.
Declare PtrSafe Function RegOpenKeyA Lib "advapi32.dll" (ByVal Key As LongPtr, ByVal SubKey As String, NewKey As LongPtr) As Long
Note that Declare statements without the PtrSafe attribute are assumed not to be compatible with the 64-bit version of Office 2010.
As stated earlier, there are two new conditional compilation constants: VBA7 and Win64. To ensure backward compatibility with previous versions of Microsoft Office, you use the VBA7 constant (this is the more typical case) to prevent 64-bit code from being used in the earlier version of Microsoft Office. For code that is different between the 32-bit version and the 64-bit version, such as calling a math API which uses LongLong for its 64-bit version and Long for its 32-bit version, you use the Win64 constant. The following code demonstrates the use of these two constants.
#if Win64 then
Declare PtrSafe Function MyMathFunc Lib "User32" (ByVal N As LongLong) As LongLong
#else
Declare Function MyMathFunc Lib "User32" (ByVal N As Long) As Long
#end if
#if VBA7 then
Declare PtrSafe Sub MessageBeep Lib "User32" (ByVal N AS Long)
#else
Declare Sub MessageBeep Lib "User32" (ByVal N AS Long)
#end if
To summarize, if you write 64-bit code and intend to use it in previous versions of Microsoft Office, you will want to use the VBA7 conditional compilation constant. However, if you write 32-bit code in Office 2010, that code works as is in previous versions of Microsoft Office without the need for the compilation constant. If you want to ensure that you are using 32-bit statements for 32-bit versions and 64-bit statements for 64-bit versions, your best option is to use the Win64 conditional compilation constant.
Using Conditional Compilation Attributes
The following code is an example of legacy VBA code that needs to be updated. Notice the data types in the legacy code that are updated to use LongPtr because they refer to handles or pointers
Legacy VBA Code
Declare Function SHBrowseForFolder Lib "shell32.dll" _
Alias "SHBrowseForFolderA" (lpBrowseInfo As BROWSEINFO) As Long
Public Type BROWSEINFO
hOwner As Long
pidlRoot As Long
pszDisplayName As String
lpszTitle As String
ulFlags As Long
lpfn As Long
lParam As Long
iImage As Long
End Type
New VBA Code
#if VBA7 then ' VBA7
Declare PtrSafe Function SHBrowseForFolder Lib "shell32.dll" _
Alias "SHBrowseForFolderA" (lpBrowseInfo As BROWSEINFO) As Long
Public Type BROWSEINFO
hOwner As LongPtr
pidlRoot As Long
pszDisplayName As String
lpszTitle As String
ulFlags As Long
lpfn As LongPtr
lParam As LongPtr
iImage As Long
End Type
#else ' Downlevel when using previous version of VBA7
Declare Function SHBrowseForFolder Lib "shell32.dll" _
Alias "SHBrowseForFolderA" (lpBrowseInfo As BROWSEINFO) As Long
Public Type BROWSEINFO
hOwner As Long
pidlRoot As Long
pszDisplayName As String
lpszTitle As String
ulFlags As Long
lpfn As Long
lParam As Long
iImage As Long
End Type
#end if
Sub TestSHBrowseForFolder ()
Dim bInfo As BROWSEINFO
Dim pidList As Long
bInfo.pidlRoot = 0&
bInfo.ulFlags = &H1
pidList = SHBrowseForFolder(bInfo)
End Sub
Frequently Asked Questions
The following are frequently asked questions that relate to the 32-bit and 64-bit versions of Microsoft Office.
- When should I use the 64-bit version of Microsoft Office?
- This is more a matter of which host application (Excel, Word, and so forth) you are using. For example, Excel is able to handle much larger worksheets with the 64-bit version of Microsoft Office.
- Can I install 64-bit and 32-bit versions of Microsoft Office side-by-side?
- No.
- When should I convert Long parameters to LongPtr?
- You need to check the Windows API documentation on the Microsoft Developers Network for the function you want to call. Handles and pointers need to be converted to LongPtr. As an example, the documentation for RegOpenKeyA provides the following signature:
No code example is currently available or this language may not be supported.
The parameters are defined as:
Parameter |
Description |
hKey [in] |
A handle to an open registry key. |
lpSubKey [in, optional] |
The name of the registry subkey to be opened. |
ulOptions |
This parameter is reserved and must be zero. |
samDesired [in] |
A mask that specifies the desired access rights to the key. |
phkResult [out] |
A pointer to a variable that receives a handle to the opened key. | In Win32API_PtrSafe.txt, the Declare statement is defined as:
Declare PtrSafe Function RegOpenKeyEx Lib "advapi32.dll" Alias "RegOpenKeyExA" (ByVal hKey As LongPtr, ByVal lpSubKey As String, ByVal ulOptions As Long, ByVal samDesired As Long, phkResult As LongPtr) As Long
- Should I convert pointers and handles in structures?
- Yes. See the MSG type in Win32API_PtrSafe.txt:
Type MSG
hwnd As LongPtr
message As Long
wParam As LongPtr
lParam As LongPtr
time As Long
pt As POINTAPI
End TypeF
- When should I use strptr, varpt, and objptr?
- You should use these functions to retrieve pointers to strings, variables and objects, respectively. On the 64-bit version of Microsoft Office, these functions will return a 64-bit LongPtr, which can be passed to Declare statements. The use of these functions has not changed from previous versions of VBA. The only difference is that they now return a LongPtr.
Conclusion
The addition of a 64-bit version of Office 2010 enables you to move more data around for increased capability. When writing 32-bit code, you can use the 64-bit version of Microsoft Office without any changes. However, when you write 64-bit code, you should ensure that your code contains specific keywords and conditional compilation constants to ensure that the code is backward compatible with earlier version of Microsoft Office, and that the correct code is being executed if you mix 32-bit and 64-bit code.
Additional Resources
For more information about Declare statements, see the following resources:
출처 : http://msdn.microsoft.com/en-us/library/aa671659.aspx
Anatomy of a Declare Statement
Here is an example of the Declare statement for the GetTempPath function, which returns the path to the Microsoft® Windows® temporary folder:
Private Declare Function GetTempPath Lib "kernel32" _
Alias "GetTempPathA" (ByVal nBufferLength As Long, _
ByVal lpBuffer As String) As Long
The Declare keyword alerts VBA that you want to include the definition for a DLL function in your project. A Declare statement in a standard module can be public or private, depending on whether you want the API function to be available only to a single module or to the entire project. In a class module, a Declare statement must be private.
The name of the function that follows the Function keyword is the name you use to call the function from VBA. This name can be identical to the name of the API function itself, or you can use the Alias keyword within the Declare statement to indicate that you intend to call the function by a different name (an alias) in VBA.
In the previous example, the name of the API function in the DLL is GetTempPathA, and the name by which you would call it from VBA is GetTempPath.
Note The actual name of the DLL function appears after the Alias keyword.
Note GetTempPath is the name the Win32API.txt file uses to alias the function, but you could change this to be any name you wanted.
Here are a few reasons why you might want to use an alias within a Declare statement:
- Some API function names begin with an underscore character (_), which is not legal in VBA. To call the function from VBA, you must use an alias name.
- Because an alias makes it possible for you to name a DLL function anything you want to, you can make the function name conform to your own naming standards within VBA.
- Because API functions are case-sensitive and VBA functions are not, you can use an alias to change the case of a function name.
- Some DLL functions have arguments that can take different data types. The VBA Declare statements for these functions define these arguments as type Any. Calling a DLL function with arguments declared as Any can be perilous, because VBA does not perform any data type checking for you. If you want to avoid the hazards of passing arguments as Any, you can declare multiple versions of the same DLL function, each with a different name and a different data type.
- The Windows API contains two versions of all functions that take string arguments: an ANSI version and a Unicode version. The ANSI version has an "A" suffix, as shown in the previous example, while the Unicode version has a "W" suffix. Although VBA uses Unicode internally, it converts all strings to ANSI strings before calling a function in a DLL, so you usually will use the ANSI version when calling a Windows API function from VBA. The API Viewer add-in automatically aliases all functions that take string arguments, so you can call the function without including the "A" suffix.
The Lib keyword specifies which DLL contains the function. Note that the name of the DLL is contained in a string within the Declare statement. If the DLL specified after the Lib keyword is not found on the user's system, a call to the function will fail with run-time error number 48, "Error in loading DLL." Because you can handle this error in your VBA code, you can write robust code that deals with the error gracefully.
Note This is not an issue if you are calling a function in one of the basic Windows DLLs, because those DLLs must be present for your application to load.
The following table describes the most commonly used DLLs in the Windows API:
DLL |
Contains |
Kernel32.dll |
Low-level operating system functions, such as those for memory management and resource handling. |
User32.dll |
Windows management functions, such as those for message handling, timers, menus, and communications. |
GDI32.dll |
The Graphics Device Interface (GDI) library, which contains functions for device output, such as those for drawing, display context, and font management. |
Most DLLs, including those in the Windows API, are written in C or C++. Passing arguments to a DLL function therefore requires some understanding of the arguments and data types expected by a C or C++ function, which differ in several ways from those expected by a VBA function.
In addition, many arguments to DLL functions are passed by value. By default, arguments in VBA are passed by reference, so it is imperative you include the ByVal keyword in the function definition when the DLL function requires that an argument be passed by value. Omitting the ByVal keyword in a function definition might cause an invalid page fault in your application in some cases. In other cases, the VBA run-time error number 49, "Bad DLL calling convention," might occur.
Passing an argument by reference passes the memory location of that argument to the procedure being called. If the procedure modifies that argument's value, it modifies the only copy of that argument, so when execution returns to the calling procedure, the argument contains the modified value.
Passing an argument to a DLL function by value, on the other hand, passes a copy of the argument; the function operates on a copy of the argument instead. This prevents that function from modifying the contents of the actual argument. When execution returns to the calling procedure, the argument contains the same value it did before the other procedure was called.
Because passing by reference makes it possible for an argument to be modified in memory, if you incorrectly pass an argument by reference, the DLL function might overwrite memory that it should not, causing an error or otherwise unexpected behavior. Windows maintains many values that should not be overwritten. For example, Windows assigns to every window a unique 32-bit identifier called a handle. Handles are passed to API functions by value, because if Windows were to modify a window's handle, it would no longer be able to track that window.
Note Although the ByVal keyword appears in front of some arguments of type String, strings always are passed to Windows API functions by reference.
See Also
API Basics | What Is an API? | Why Use VBA to Call the Windows API? | API Resources | Accessing Functions in a DLL | Constants and User-Defined Types | Understanding Handles | Calling DLL Functions | Passing Arguments by Value or by Reference | Getting the Most Out of Visual Basic for Applications
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