How to Prevent Events to Fire More Than Once

Submitted by admin on Monday, January 4, 2010 - 13:53.

[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.

Public Class Form1
    Const curAmount As Double = 100
    
    Private Sub ComputeTotal()
        txtTotal.Text = curAmount * txtDays.Text
    End Sub
 
    Private Sub dtpFrom_ValueChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles dtpFrom.ValueChanged
        ComputeTotal()
    End Sub
 
    Private Sub dtpTo_ValueChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles dtpTo.ValueChanged
        ComputeTotal()
    End Sub
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim dtpStartDate As Date
 
        'Remove/comment the line below to simulate the error
        RemoveHandler dtpTo.ValueChanged, AddressOf dtpTo_ValueChanged
 
        dtpStartDate = dtpFrom.Value
 
        dtpTo.Value = System.DateTime.FromOADate(dtpStartDate.ToOADate + 1)
 
        AddHandler dtpTo.ValueChanged, AddressOf dtpTo_ValueChanged
 
        txtDays.Text = dtpTo.Value.Subtract(Format(dtpFrom.Value, "Short Date")).Days.ToString
 
        ComputeTotal()
    End Sub
End Class

Remove the line: RemoveHandler dtpTo.ValueChanged, AddressOf dtpTo_ValueChanged to produce the error.

Comments

Hello admin !

Do you have a "grading utility system" which the authorized user can upload a file, an excel file in particular..i have no idea how to do it, it is our project in our visual basic programming... pls help me

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