Mouse event wheel occurs last - vb.net

Question

something strange is happening in my application and I can't figure out what it is. I am trying to simulate Mouse click and scroll programmatically.
I can successfully perform Mouse click and scroll programmatically, but the problem is that the mouse wheel occurs last.
This is the code:
<DllImport("user32.dll", EntryPoint:="mouse_event")>
Private Shared Sub mouse_event(ByVal dwFlags As UInteger, ByVal dx As
Integer, ByVal dy As Integer, ByVal dwData As Integer, ByVal dwExtraInfo
As UInteger)
End Sub
Flags:
Const MOUSEEVENTF_LEFTDOWN As UInteger = &H2 '0x0002'
Const MOUSEEVENTF_LEFTUP As UInteger = &H4 '0x0004'
Const MOUSEEVENTF_WHEEL As UInteger = &H800
Const MOUSEEVENTF_XDOWN As UInteger = &H80
Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
scroll_to_end()
Thread.Sleep(300)
click_perf()
MessageBox.Show("S")
End Sub
Private Sub scroll_to_end()
'Focus panel1'
c_point = New Point(Panel1.Location.X + 1, Panel1.Location.Y + 1)
Cursor.Position = Me.PointToScreen(c_point)
mouse_event(MOUSEEVENTF_LEFTDOWN, 0, 0, 0, 0)
mouse_event(MOUSEEVENTF_LEFTUP, 0, 0, 0, 0)
'Wait'
Thread.Sleep(150)
'Scroll to the end of the page'
Dim scroll_down As Integer = -(10 * 120) 'Perform 10 scrolls down'
mouse_event(MOUSEEVENTF_WHEEL, 0, 0, scroll_down, 0)
'This is the fix:'
Application.DoEvents()
End Sub
Private Sub click_perf()
'Move mouse diagonally 200px'
c_point.X += 200
c_point.Y += 200
Cursor.Position = Me.PointToScreen(c_point)
'Perform click'
mouse_event(MOUSEEVENTF_LEFTDOWN, c_point.X, c_point.Y, 0, 0)
mouse_event(MOUSEEVENTF_LEFTUP, 0, 0, 0, 0)
End Sub
When I click button3, I want to focus panel1 then scroll to the end and move mouse 200px diagonally downwards and then simulate click.
What actually happens: Mouse moves to panel1, simulates mouse button down and mouse button up. Then, it moves the mouse diagonally 200px and the messagebox shows up. The scroll didn't even occur and now when I close the messagebox panel1 probably loses the focus and doesn't scroll.
If I remove messagebox.show(""): Mouse moves to panel1 and focuses it just like above. It scrolls to the end, but it performs click before it was scrolled down.
EDIT:
To fix this issue we need to put Application.DoEvents() After WHEEL MOUSE EVENT.

Answer 1

Put this after each UI operation to force it to happen in order
Application.DoEvents()
UI events are handled in a message loop, which looks at a queue of messages and handles them. There is no guarantee that they will be handled in the order they are raised if they are raised around the same time in your code because your code is faster than the loop. Application.DoEvents() will halt your code execution and force the loop to handle all pending messages, thus making them happen in order.
It's generally considered poor practice to use this method, however, on account of its widespread misuse. I think your case for it is a good one and maybe the only I've ever seen.
I'd be interested to see if someone can provide a preferred alternative...