Need some help understanding this advanced code which uses threading, pointers, etc..

[akhileshbc]

Hi guys..

I had recently(2 months before ) found some code for locking the screen. Here's the link to that code submission in our CodeBank : http://www.vbforums.com/showthread.php?t=639579

I wish to get a clear idea of what it actually does. As it is a bit advanced code for me, I need your help.

Here's the main part of the code:

Public Function LockSystemAndShow(ByVal myDForm As Form) As Boolean
        myForm = myDForm
 
        'Dim owner As Control = TryCast(Form1.Button1, Control)
        Dim scr As Screen
        'If owner Is Nothing Then
        scr = Screen.PrimaryScreen
        'Else
        'scr = Screen.FromControl(owner)
        'End If
        Dim background As Bitmap = New Bitmap(scr.Bounds.Width, scr.Bounds.Height)
        Using g As Graphics = Graphics.FromImage(background)
 
            g.CopyFromScreen(0, 0, 0, 0, scr.Bounds.Size)
            Using br As Brush = New SolidBrush(Color.FromArgb(192, Color.Black))
                g.FillRectangle(br, scr.Bounds)
            End Using
 
            'If owner IsNot Nothing Then
            ' Dim form As Form = owner.FindForm()
            ' g.CopyFromScreen(form.Location, form.Location, form.Size)
            ' Using br As Brush = New SolidBrush(Color.FromArgb(128, Color.Black))
            ' g.FillRectangle(br, New Rectangle(Form.Location, Form.Size))
            'End Using
            'End If
 
        End Using
 
        Dim originalThread As IntPtr
        Dim originalInput As IntPtr
        Dim newDesktop As IntPtr
 
        originalThread = GetThreadDesktop(Thread.CurrentThread.ManagedThreadId)
        originalInput = OpenInputDesktop(0, False, DESKTOP_SWITCHDESKTOP)
 
        newDesktop = CreateDesktop("Desktop" & Guid.NewGuid().ToString(), Nothing, Nothing, 0, GENERIC_ALL, Nothing)
        SetThreadDesktop(newDesktop)
        SwitchDesktop(newDesktop)
 
        Dim newThread As Thread = New Thread(AddressOf NewThreadMethod)
        newThread.CurrentCulture = Thread.CurrentThread.CurrentCulture
        newThread.CurrentUICulture = Thread.CurrentThread.CurrentUICulture
        newThread.Start(New abcLockSystemParameters(newDesktop, background))
        newThread.Join()
 
        SwitchDesktop(originalInput)
        SetThreadDesktop(originalThread)
 
        CloseDesktop(newDesktop)
        CloseDesktop(originalInput)
 
        Return True 'Result
 
    End Function
 
    Private Sub NewThreadMethod(ByVal params As Object)
        Dim v As abcLockSystemParameters = DirectCast(params, abcLockSystemParameters)
        SetThreadDesktop(v.NewDesktop)
        Using f As Form = New BackgroundForm(v.Background)
            f.Show()
            myForm.ShowDialog()  '~~~~ Show the login screen here
            f.BackgroundImage = Nothing
            Application.DoEvents()
            Thread.Sleep(250)
        End Using
    End Sub

Looks like pretty easy one, isn't it ? But it's a tough one for me.

Can you please help me to understand this code ?

Thanks in advance...

[BackWoodsCoder]

maybe some more pointed questions will help you get answers. What exactly about the above code do you not understand or needs more explanation?

[akhileshbc]

Hi..

Thanks for the reply.

I'm having trouble understanding what it does in this part:

originalThread = GetThreadDesktop(Thread.CurrentThread.ManagedThreadId)
        originalInput = OpenInputDesktop(0, False, DESKTOP_SWITCHDESKTOP)
 
        newDesktop = CreateDesktop("Desktop" & Guid.NewGuid().ToString(), Nothing, Nothing, 0, GENERIC_ALL, Nothing)
        SetThreadDesktop(newDesktop)
        SwitchDesktop(newDesktop)
 
        Dim newThread As Thread = New Thread(AddressOf NewThreadMethod)
        newThread.CurrentCulture = Thread.CurrentThread.CurrentCulture
        newThread.CurrentUICulture = Thread.CurrentThread.CurrentUICulture
        newThread.Start(New abcLockSystemParameters(newDesktop, background))
        newThread.Join()
 
        SwitchDesktop(originalInput)
        SetThreadDesktop(originalThread)
 
        CloseDesktop(newDesktop)
        CloseDesktop(originalInput)

[Lord Orwell]

although you don't get easy access to it, windows actually supports multiple desktops similar to osx and linux. The only thing it's used for by the OS is to create UAC prompts. They take a screen shot of the main desktop and create a new one with that shaded in the background and then show you the new desktop. You can use the same generalities to do the same thing and that is what this code does. It locks you out of the main desktop by showing you a different one.

however, i don't think that's the entire code. I don't see closedesktop declared anywhere, for example.

[akhileshbc]

although you don't get easy access to it, windows actually supports multiple desktops similar to osx and linux. The only thing it's used for by the OS is to create UAC prompts. They take a screen shot of the main desktop and create a new one with that shaded in the background and then show you the new desktop. You can use the same generalities to do the same thing and that is what this code does. It locks you out of the main desktop by showing you a different one.

however, i don't think that's the entire code. I don't see closedesktop declared anywhere, for example.

Thanks
That means it's perfect to use as a lock screen, right ?

I haven't posted the entire code in the previous post. You can find the attachment in the other thread.

[Lord Orwell]

it should be fine, but if your code ever crashes you'll have to reboot.

[akhileshbc]

it should be fine, but if your code ever crashes you'll have to reboot.

Ok.. Thanks