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Using Drag and Drop with files and folders

In this article I will carry on from last week when I showed you how to search you computer for a specified program. This week I will show you how to allow the user to be able to drag a file or folder onto a list box or textbox and it will display the relative path and file name if you dragged a file. It will also tell you how many files you last dropped to the control on the form.

This is very useful, when you want a number of files, say to check the integrity of the specified file. It also can be used for many other functions, including the path of where certain files are. This can be accomplished by using four Windows API's. Which are:

  • DragAcceptFile
  • DragFinish
  • DragQueryFile
  • PeekMessage
Sample Project

1. Start a new standard EXE project in Visual Basic. Form one is created by default

2. Add a normal module to the project, by clicking on the Project menu and choosing insert module.

3. You will need to declare the following types, by typing the following in to the beginning of the module

    x As Long
    y As Long
End Type      

Type MSG
    hWnd As Long
    message As Long
    wParam As Long
    lParam As Long
    time As Long
    pt As POINTAPI
End Type      

4. You now need to add the following Windows API calls to the module. Make sure that each API is on one line

Declare Sub DragAcceptFiles Lib "shell32.dll" (ByVal hWnd _
As Long, ByVal fAccept As Long)      

Declare Sub DragFinish Lib "shell32.dll" (ByVal hDrop _
As Long)      

Declare Function DragQueryFile Lib "shell32.dll" Alias _
"DragQueryFileA" (ByVal hDrop As Long, ByVal UINT As _
Long, ByVal lpStr As String, ByVal ch As Long) As Long      

Declare Function PeekMessage Lib "user32" Alias _
"PeekMessageA" (lpMsg As MSG, ByVal hWnd As Long, _
ByVal wMsgFilterMin As Long, ByVal wMsgFilterMax As _
Long, ByVal wRemoveMsg As Long) As Long      

Public Const PM_NOREMOVE = &H0
Public Const PM_NOYIELD = &H2
Public Const PM_REMOVE = &H1
Public Const WM_DROPFILES = &H233      

5. You will now need to add the following procedures to the module.

' This is the sub main procedure, that is called
' for any of the forms are loaded.      

Sub Main()
' In order for this to function properly you should 
' place of of your program execution code in 
' the Sub Main(), Make sure you change the project 
' startup to sub Main


' This must be the last line! Nothing gets called 
' after this


End Sub      

Public Sub WatchForFiles()

' This subroutine watchs for all of your WM_DROPFILES 
' messages
' Dim Variables

Dim FileDropMessage As MSG ' Msg Type
Dim fileDropped As Boolean ' True if Files where dropped
Dim hDrop As Long ' Pointer to the dropped file structure
Dim filename As String * 128 ' the dropped filename
Dim numOfDroppedFiles As Long ' the amount of dropped files
Dim curFile As Long ' the current file number

' loop to keep checking for files
' NOTE : Do any code you want to execute before this set      


' check for Dropped file messages
fileDropped = PeekMessage(FileDropMessage, 0, _

If fileDropped Then
    ' Get the pointer to the dropped file structure
    hDrop = FileDropMessage.wParam
    ' Get the toal number of files
    numOfDroppedFiles = DragQueryFile(hDrop, True, _
    filename, 127)
    For curFile = 1 To numOfDroppedFiles
    ' Get the file name
    ret% = DragQueryFile(hDrop, curFile - 1, filename, 127)
    ' at this pointer you can do what you want with the 
    ' filename the filename will be a full qalified path
    Form1.lblNumDropped = LTrim$(Str$(numOfDroppedFiles))
    Form1.List1.AddItem filename
    Next curFile
    ' We are now done with the structure, tell windows 
    ' to discard it
    DragFinish (hDrop)
End If



End Sub

6. Open up form one. Draw the following

  • 3 command buttons

  • 1 list box

  • 2 labels - one called "lblNumDropped"

7. Now open up the code window for form1 and type the following

Private Sub Command1_Click()
' You can turn the form's / controls ability
' to accept the files by passing the hWnd as
' the first parameter and True/False as the
' Second      

If Command1.Caption = "&Accept Files" Then
    ' allow the application to accept files
    DragAcceptFiles Form1.hWnd, True
    Command1.Caption = "&Do Not Accept"
    DragAcceptFiles Form1.hWnd, False
    Command1.Caption = "&Accept Files"
End If      

End Sub

Private Sub Command2_Click()
' Clears the contents of the list box
End Sub      

Private Sub Command3_Click()
' End the program
End Sub

Private Sub Form_Unload(Cancel As Integer)
End Sub      

Private Sub List1_Click()
Dim x, y, z
Dim AnyString, MyStr
z = UCase$(Right(List1.Text, 3)) ' Define string.
If z = "EXE" Then
    y = List1.Text
    x = Shell(y, vbNormalFocus)
    y = "Explorer " & List1.Text
    x = Shell(y, vbNormalFocus)
End If
End Sub

Private Sub Form_Load()
Command1.caption = "&Accept Files"
Command2.caption = "Clear"
Command3.caption = "End"
Label1.caption = "The number of files that were " & _
"last dropped on the list box"
End Sub      

8. Go into the Project Properties dialog box and change the Startup Object to Sub Main.

9. Now run the project. Click on the Accept files command button. Now drag some files and folder on the list box. They will be disaplyed in the list box. If you click on a file or folder in the list box the folder or file will be opened.



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