How to Prevent Events to Fire More Than Once

[silvio71]

[inline:RemoveHandler.jpg=How to Prevent Events to Fire More Than Once]
If you are new to VB.NET most likely you encounter a problem with events like TextChanged or ValueChanged events.

In VB 6.0, change event is not fired when changing a value programmatically. However, in the .NET version this has been changed.

In order to avoid this problem you need to call a RemoveHandler Statement.

The following code is an example of this.

1. Public
   
   Class
   
   Form1
   
2. Const
   
   curAmount As
   
   Double
   
   =
   
   100
   
3. 
   
4. Private
   
   Sub
   
   ComputeTotal(
   
   )
   
5. txtTotal.
   
   Text
   
   =
   
   curAmount *
   
   txtDays.
   
   Text
   
6. End
   
   Sub
   
7. 
   
8. Private
   
   Sub
   
   dtpFrom_ValueChanged(
   
   ByVal
   
   sender As
   
   System.
   
   Object
   
   , ByVal
   
   e As
   
   System.
   
   EventArgs
   
   )
   
   Handles
   
   dtpFrom.
   
   ValueChanged
   
9. ComputeTotal(
   
   )
   
10. End
    
    Sub
    
11. 
    
12. Private
    
    Sub
    
    dtpTo_ValueChanged(
    
    ByVal
    
    sender As
    
    System.
    
    Object
    
    , ByVal
    
    e As
    
    System.
    
    EventArgs
    
    )
    
    Handles
    
    dtpTo.
    
    ValueChanged
    
13. ComputeTotal(
    
    )
    
14. End
    
    Sub
    
15. 
    
16. Private
    
    Sub
    
    Form1_Load(
    
    ByVal
    
    sender As
    
    System.
    
    Object
    
    , ByVal
    
    e As
    
    System.
    
    EventArgs
    
    )
    
    Handles
    
    MyBase
    
    .
    
    Load
    
17. Dim
    
    dtpStartDate As
    
    Date
    
18. 
    
19. 'Remove/comment the line below to simulate the error
    
20. RemoveHandler
    
    dtpTo.
    
    ValueChanged
    
    , AddressOf
    
    dtpTo_ValueChanged
    
21. 
    
22. dtpStartDate =
    
    dtpFrom.
    
    Value
    
23. 
    
24. dtpTo.
    
    Value
    
    =
    
    System.
    
    DateTime
    
    .
    
    FromOADate
    
    (
    
    dtpStartDate.
    
    ToOADate
    
    +
    
    1
    
    )
    
25. 
    
26. AddHandler
    
    dtpTo.
    
    ValueChanged
    
    , AddressOf
    
    dtpTo_ValueChanged
    
27. 
    
28. txtDays.
    
    Text
    
    =
    
    dtpTo.
    
    Value
    
    .
    
    Subtract
    
    (
    
    Format
    
    (
    
    dtpFrom.
    
    Value
    
    , "Short Date"
    
    )
    
    )
    
    .
    
    Days
    
    .
    
    ToString
    
29. 
    
30. ComputeTotal(
    
    )
    
31. End
    
    Sub
    
32. End
    
    Class
    

Remove the line: RemoveHandler

dtpTo.

ValueChanged

, AddressOf

dtpTo_ValueChanged

to produce the error.