Understanding DoDragDrop

[robertb_NZ]

Summary
DoDragDrop unexpectedly reset the cursor position. I have a solution but it feels like a kludge. What don't I understand? Is there a better way?

Detail
I am developing an editor with a RichTextBox. Logic contains essentially this code supporting Drag&Drop

Private Sub rtbReport_MouseDown(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles rtbReport.MouseDown

MouseIsDown = True​

End Sub​

Private Sub rtbReport_MouseMove(sender As Object, e As System.Windows.Forms.MouseEventArgs) Handles rtbReport.MouseMove

If MouseIsDown Then

[SNIP - locate edit element (LinkedListNode) at this position]
Dim RI As ReptItem = RINode.Value
If RI.Type = ReptItem.RIType.Field OrElse RI.Type = ReptItem.RIType.Text Then

DoDragDrop(Me.DragNode, DragDropEffects.Move)​

End If
MouseIsDown = False​

End If​

End Sub​

For typing I expect users will normally click where they want to start typing. This raises the MouseMove event, which executes the above code. This should have been no problem as DoDragDrop effectively does nothing if the cursor is not moved more than DragSize, but I found that unless I had a checkpoint within the MouseMove event the cursor position was being reset to the start of the RTB

I have "solved" the problem with code like this: -

Debug.Print("#2. rtbReport.SelectionStart:" & rtbReport.SelectionStart)
Dim SaveSelectionstart = rtbReport.SelectionStart
DoDragDrop(Me.DragNode, Effect)
Debug.Print("#3. rtbReport.SelectionStart:" & rtbReport.SelectionStart)
rtbReport.SelectionStart = SaveSelectionstart
Debug.Print("#4. rtbReport.SelectionStart:" & rtbReport.SelectionStart)
​

This produces the correct result, with debug output like this: -

#2. rtbReport.SelectionStart:447
#3. rtbReport.SelectionStart:0
#4. rtbReport.SelectionStart:447​

Although I could simply remove the diagnostics and move on, this solution smells: it doesn't seem right that DoDragDrop has this unexpected side effect, and I worry that there is something going on that I don't understand that could bite me later. Can anybody tell me if there is a proper way to solve this problem?

 

[jmcilhinney]

For future reference, please use appropriate formatting tags for code snippets, i.e.

[xcode=vb]your code here[/xcode]

 

[robertb_NZ]

Thanks, I'll do that in future.

 

[JohnH]

You can use SystemInformation.DragSize to implement as treshold for mouse to move before initiating a dragdrop operation, example here: Control.DoDragDrop Method (Object, DragDropEffects) (System.Windows.Forms)