DirectX Game Screenshot

Software: Visual Basic 2008
My problem:
I make a program for taking screenshots from the game, this is the current code that I use:

Code:

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Dim bounds As Rectangle
            Dim screenshot As System.Drawing.Bitmap
            Dim graph As Graphics
            bounds = Screen.PrimaryScreen.Bounds
            screenshot = New System.Drawing.Bitmap(bounds.Width, bounds.Height, System.Drawing.Imaging.PixelFormat.Format32bppArgb)
            graph = Graphics.FromImage(screenshot)
            graph.CopyFromScreen(bounds.X, bounds.Y, 0, 0, bounds.Size, CopyPixelOperation.SourceCopy)
            PictureBox1.Image = screenshot

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and it works great when i need to take a desktop screenshot, but when i try take a game screenshot it does not work! I read on the internet and it says that I need to take screenshots from DirectX. The problem is that I never worked with DirectX and I need your help, any help is welcome if it is related to my problem, I would appreciate if you have some kind of example.





Thank you!