You will want to use HandledMouseEventArgs primarily when developing a control that needs to handle mouse wheel events itself, and prevent them from being seen by the control's immediate parent container. HandledMouseEventArgs is not passed directly by MouseEventHandler when it handles the MouseWheel event. Rather, MouseEventHandler uses a MouseEventArgs, which you must cast to a HandledMouseEventArgs in order to cancel an occurrence of MouseWheel.

The following code example demonstrates how to mark the mouse wheel event as handled in a custom control.

Public Class MouseWheelControl
    Sub New()
        ' Add initialization code for the control here. 
    End Sub

    Protected Sub MouseWheelControl_MouseWheel(ByVal sender As Object, ByVal e As MouseEventArgs) Handles Me.MouseWheel
        Dim Hme As HandledMouseEventArgs = e
        Hme.Handled = True
        ' Perform custom mouse wheel action here. 
    End Sub
End Class

.NET Framework

.NET Framework Client Profile

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

Reference