Tip 86: Allowing a Visual Basic Application to Accept Drag-and-Drop

* VB-CODE (4)

Abstract
Many Windows=AE-based applications can accept, or process, a file that
has been dragged from File Manager. This article explains how you can
add this feature to your own Visual Basic=AE application.

Using MSGBLAST.VBX to Accept Drag-and-Drop Files
Using File Manager, you can drag a file to another application and,
when you release the mouse button (drop the file), the target
application can process the file any way it wants to.

In order for a program to be able to accept drag-and-drop files,
however, the program must have a method of recognizing when a file has
been sent to it. In Visual Basic=AE, this can be done by using the
Message Blaster custom control and three Windows=AE application
programming interface (API) functions: DragAcceptFiles, DragQueryFile,
and DragFinish.
The DragAcceptFiles function tells Windows that a specific window
(that is, your Visual Basic application's form) can accept files
dropped from File Manager. The Declare statement for this function is:

Private Declare Sub DragAcceptFiles Lib "shell" (ByVal hWnd
 As Integer, ByVal bool As Integer)

(Note that this Declare statement must be typed as a single line of
code.)
The DragAcceptFiles function takes only two arguments: the handle of
the window that will accept the dropped files, and an integer value
that specifies if the file can be accepted or ignored. If the Boolean
argument is set to True, the window can accept dropped files; if it is
set to zero, the window can no longer accept dropped files.

You can retrieve the name of the file that was dropped on the target
window by calling the DragQueryFile function. This function's
declaration statement is:

Private Declare Function DragQueryFile Lib "shell" (ByVal wParam
 As Integer, ByVal Index As Integer, ByVal lpszFile
 As Any, ByVal BufferSize As Integer) As Integer

(Note that this Declare statement must be typed as a single line of
code.)
DragQueryFile requires four arguments, as follows:

wParam      An integer value that contains the internal data
          structure's handle. This is provided by the WM_DROPFILES
          message.
Index       An integer value containing the number of the individual
          file to be retrieved. If this value is -1, the number of
          files listed in the wParam structure will be returned.
lpszFile    A string buffer that contains the name of the dropped file.
BufferSize  An integer value containing the maximum number of
          characters in lpszFile.

After calling the DragQueryFile function, an integer value reports the
status of the function. This value contains the number of characters
copied to the lpszFile string or the number of files available if
Index was set to zero.
The third function needed to work with drag-and-drop files is the
DragFinish function. This function simply requires that the internal
data structure's handle be passed to it. DragFinish frees all
structures used when transferring the file to the target application.

The final step is to process the WM_DROPFILES message. This message is
sent by Windows each time it needs to send a drag-and-drop request to
a program. In your Visual Basic program you need only use the Message
Blaster custom control to intercept the WM_DROPFILES message before
Windows actually processes it itself. In the example program below,
we use the Message Blaster control to retrieve the name of the dropped
file and store that name in the List Box control.

Example Program
The example program below shows how to allow your Visual Basic
application to accept drag-and-drop files from File Manager. To use
this demonstration program, first execute the Windows Explorer or
File Manager application. Then run the DEMO.EXE program. When you drag
a file from File Manager to DEMO.EXE's window and release the mouse
button, the filename will be displayed in the List Box control.

1. Create a new project in Visual Basic. Form1 is created by default.
2. Add the following code to the General Declarations section of
  Form1 (note that each Declare statement must be typed as a single
  line of code):

Option Explicit

Private Declare Sub DragAcceptFiles Lib "shell" (ByVal hWnd
 As Integer, ByVal bool As Integer)
Private Declare Function DragQueryFile Lib "shell" (ByVal wParam
 As Integer, ByVal Index As Integer, ByVal lpszFile
 As Any, ByVal BufferSize As Integer) As Integer
Private Declare Sub DragFinish Lib "shell" (ByVal hDrop As Integer)

Const WM_DROPFILES =3D &H233

3. Add the following code to the Form_Load event for Form1:

Private Sub Form_Load()
  msgblaster1.MsgList(0) =3D WM_DROPFILES
  msgblaster1.hWndTarget =3D Me.hWnd
  msgblaster1.MsgPassage(0) =3D 1
  DragAcceptFiles Me.hWnd, True
End Sub

4. Add a Message Blaster custom control to Form1. MsgBlaster1 is
  created by default.
5. Add the following code to the MsgBlaster1_Message event for
  MsgBlaster1:

Private Sub MsgBlaster1_Message(MsgVal As Integer, wParam As Integer,
 lParam As Long, ReturnVal As Long)
  Dim hFilesInfo As Integer
  Dim szFileName As String
  hFilesInfo =3D wParam
  wTotalFiles =3D DragQueryFile(hFilesInfo, &HFFFF, ByVal 0&, 0)
  For wIndex =3D 0 To wTotalFiles
      szFileName =3D Space$(50)
      Retv% =3D DragQueryFile(hFilesInfo, wIndex, szFileName, 50)
      list1.AddItem szFileName
  Next wIndex
  DragFinish (hFilesInfo)
End Sub

6. Compile the program. From Visual Basic's File menu, select Make
  EXE File to create the executable file called DEMO.EXE.

Additional References
"DragAcceptFiles." (Product Documentation, SDKs, Windows 3.1 SDK,
 Programmer's Reference, Volume 2, Functions)
"Drop Everything: How to Make Your Application Accept and Source
 Drag-and-Drop Files." (Books and Periodicals, Microsoft Systems
 Journal)
"Message Blaster: Processing Messages in Visual Basic." (Technical
 Articles, Visual Basic Articles)
"Using Drag-Drop in an Edit Control or a Combo Box." (Knowledge
 Base and Bug Lists, Windows SDK KBase, Related Information)
"Using MSGBLAST.VBX Control to Process Windows Messages from VB."
 (Knowledge Base and Bug Lists, Visual Basic for Windows KBase,
 Related Information)