Form.vb

Public Class Form
    Sub New()
        InitializeComponent()
    End Sub

    Protected Overrides Sub OnVisibleChanged(ByVal e As System.EventArgs)
        If Me.Visible Then Me.FadeIn() Else Me.FadeOut()
        MyBase.OnVisibleChanged(e)
    End Sub

    Protected Overrides Sub OnFormClosed(ByVal e As System.Windows.Forms.FormClosedEventArgs)
        Me.FadeOut()
        MyBase.OnFormClosed(e)
    End Sub

    Protected Sub FadeIn()
        Dim msec As Integer = 200
        Dim animateFlags As Win32.AnimateWindowFlags = _
            Win32.AnimateWindowFlags.AW_ACTIVATE Or Win32.AnimateWindowFlags.AW_BLEND
        Win32.AnimateWindow(Me.Handle, msec, animateFlags)
    End Sub

    Protected Sub FadeOut()
        Dim msec As Integer = 200
        Dim animateFlags As Win32.AnimateWindowFlags = _
            Win32.AnimateWindowFlags.AW_HIDE Or Win32.AnimateWindowFlags.AW_BLEND
        Win32.AnimateWindow(Me.Handle, msec, animateFlags)
    End Sub
End Class

gistfile3.txt

Win32.vb

Imports System.Runtime.InteropServices

Partial Public Class Win32
    ''' <remarks>http://www.pinvoke.net/default.aspx/Enums/AnimateWindowFlags.html</remarks>
    <Flags()> _
    Public Enum AnimateWindowFlags
        AW_HOR_POSITIVE = &H1
        AW_HOR_NEGATIVE = &H2
        AW_VER_POSITIVE = &H4
        AW_VER_NEGATIVE = &H8
        AW_CENTER = &H10
        AW_HIDE = &H10000
        AW_ACTIVATE = &H20000
        AW_SLIDE = &H40000
        AW_BLEND = &H80000
    End Enum

    ''' <remarks>http://pinvoke.net/default.aspx/user32/AnimateWindow.html</remarks>
    <DllImport("user32.dll")> _
    Public Shared Function AnimateWindow(ByVal hwnd As IntPtr, ByVal time As Integer, ByVal flags As AnimateWindowFlags) As Boolean
    End Function
End Class