Let's look at some examples, starting with simple drag
and drop operation. Create a Visual Basic.net windows application
and design a form with control & Drag Drop event procedure
as follows
To enable drag & drop for text
1. Place two textboxes and set Allowdrop
property of a second textbox to true.
2. Add the following code
Private MouseIsDown As Boolean = False 'variable declaration
Private Sub TextBox1_MouseDown(ByVal sender As Object, ByVal e As _
System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseDown
'Set a flag to show that the mouse is down.
MouseIsDown = True
End Sub
Private Sub TextBox1_MouseMove(ByVal sender As Object, ByVal e As _
System.Windows.Forms.MouseEventArgs) Handles TextBox1.MouseMove
If MouseIsDown Then
' Initiate dragging.
TextBox1.DoDragDrop(TextBox1.Text,DragDropEffects.Copy)
End If
MouseIsDown = False
End Sub
Private Sub TextBox2_DragEnter(ByVal sender As Object, ByVal e As _
System.Windows.Forms.DragEventArgs) Handles TextBox2.DragEnter
'Check the format of the data being dropped.
If (e.Data.GetDataPresent(DataFormats.Text)) Then
' Display the copy cursor.
e.Effect = DragDropEffects.Copy
Else
' Display the no-drop cursor.
e.Effect = DragDropEffects.None
End If
End Sub
Private Sub TextBox2_DragDrop(ByVal sender As Object, ByVal e As _
System.Windows.Forms.DragEventArgs) Handles TextBox2.DragDrop
'Paste the text.
TextBox2.Text = e.Data.GetData(DataFormats.Text)
End Sub
In the above example, the MouseDown event is used
to set a flag showing that the mouse is down, and then the
DoDragDrop method is called in the MouseMove
event. Although you could initiate the drag in the MouseDown
event, doing so would create undesirable behavior: Every
time a user clicks the control, the no-drag cursor would
be displayed.
The DoDragDrop method takes two parameters:
Data parameter, which in this case takes the Text
property of the TextBox
allowedEffects parameter, which in this case only
allows copying
Also in the MouseMove event the MouseIsDown
flag is set to False. Although unnecessary in this example,
if you had multiple controls that support dragging you could
get a run-time exception.
In the DragEnter event, the GetDataPresent
method checks the format of the data being dragged. In this
case it is text, so the Effect property is set to
Copy, which in turn displays the copy cursor.
In the DragDrop event, the GetData method is
used to retrieve the text from the DataObject and assign
it to the target TextBox.
The next section provides an example of dragging a different
type of data and providing support for both cutting and copying.
To enable drag and drop for a picture
Add two picturebox control to a form
Add the following code.
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As _
System.EventArgs) Handles MyBase.Load
' Enable dropping.
PictureBox2.AllowDrop = True
End Sub
Private Sub PictureBox1_MouseDown(ByVal sender As Object, ByVal e As _
System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseDown
If Not PictureBox1.Image Is Nothing Then
' Set a flag to show that the mouse is down.
m_MouseIsDown = True
End If
End Sub
Private Sub PictureBox1_MouseMove(ByVal sender As Object, ByVal e As _
System.Windows.Forms.MouseEventArgs) Handles PictureBox1.MouseMove
If m_MouseIsDown Then
' Initiate dragging and allow either copy or move.
PictureBox1.DoDragDrop(PictureBox1.Image, DragDropEffects.Copy Or _
DragDropEffects.Move)
End If
m_MouseIsDown = False
End Sub
Private Sub PictureBox2_DragEnter(ByVal sender As Object, ByVal e As _
System.Windows.Forms.DragEventArgs) Handles PictureBox2.DragEnter
If e.Data.GetDataPresent(DataFormats.Bitmap) Then
'Check for the CTRL key.
If e.KeyState = 9 Then
e.Effect = DragDropEffects.Copy
Else
e.Effect = DragDropEffects.Move
End If
Else
e.Effect = DragDropEffects.None
End if
End sub
Private Sub PictureBox2_DragDrop(ByVal sender As Object, ByVal e As _
System.Windows.Forms.DragEventArgs) Handles PictureBox2.DragDrop
' Assign the image to the PictureBox.
PictureBox2.Image = e.Data.GetData(DataFormats.Bitmap)
' If the CTRL key is not pressed, delete the source picture.
If Not e.KeyState = 8 Then
PictureBox1.Image = Nothing
End If
End Sub
In the above example, note that the AllowDrop property
for the second PictureBox control is set in the Form1_Load
event. This is necessary because the AllowDrop property
is not available at design time.
In the MouseDown event, the code first checks to make
sure that there is an image assigned to the PictureBox;
otherwise, after you moved the picture, subsequent clicks would
raise an exception.
Also note that in both the DragEnter and DragDrop
events the code checks to see if the CTRL key is pressed to
determine whether to copy or move the picture. Why are the values
different? In the DragEnter event, the left mouse button
is down, resulting in a value of 8 for the CTRL key plus 1 for
the left mouse button. For a list of KeyState enumerations,
see
DragEventArgs.KeyState Property.
Both examples so far have dealt with dragging between two
controls on the same form; they would also work for dragging
items between controls on different forms within an application.
The next example demonstrates accepting items dropped from another
application — in this case, files that are dragged from
Windows Explorer.
Screen shots for the above example is as shown below
Fig1: Screenshot for example1 control before being dragged
to a target
Fig2: Screenshot for example1 control after being dragged
to a target
