Why System.Drawing.Graphics.GetHdc() increases memory usage

Question

Public Shared Function Render(ByRef pdfDoc As PDFLibNet.PDFWrapper) As System.Drawing.Bitmap
    Try
        If pdfDoc IsNot Nothing Then
            Dim PW As Integer = pdfDoc.PageWidth
            Dim PH As Integer = pdfDoc.PageHeight
            Dim backbuffer As System.Drawing.Bitmap = New Bitmap(PW, PH)
            pdfDoc.ClientBounds = New Rectangle(0, 0, pdfDoc.PageWidth, pdfDoc.PageHeight)
            Dim g As Graphics = Graphics.FromImage(backbuffer)
            Using g
                Dim hdc As IntPtr = g.GetHdc()
                pdfDoc.DrawPageHDC(hdc)
                g.ReleaseHdc()
            End Using
            g.Dispose()
            Return backbuffer
        End If
    Catch ex As Exception
        Throw ex
        Return Nothing
    End Try
    Return Nothing
End Function

This function is drawing the image into the hdc of variable "backbuffer" of type bitmap, that return from function. But I dissatisfied by two big increases of memory (my image is big) in this function:
at line

Dim backbuffer As System.Drawing.Bitmap = New Bitmap(PW, PH)

and at line

Dim hdc As IntPtr = g.GetHdc()

So, I have a questions:
why second line increases a memory for the size that equivalent size of my image
and why if I draw into the hdc, I get the drawn bitmap, that I return (maybe first there is boxing, and second there is unboxing) ?

And if I draw into the hdc (optimized copy of my image for the concrete drawing surface), how my drawing became back to the original "backbuffer" Bitmap from that optimized copy?

But what did you expect? HDC is actually a bitmap optimized for concrete drawing surface. And it will have a size equal to the size of that surface. — David Goshadze
Thanks! And if I draw into the hdc, how my drawing became back to the original "backbuffer" Bitmap from that concrete drawing surface? — Alexandr Lyumitskij
Pay attention to compiler warnings (Unreachable code, etc.). — Mohammad Dehghan

06needhamt 06needhamt · Accepted Answer · 2013-03-07T18:22:34

I think it is because as you create new instances of the bitmap class and then g.hdc() forces it to create new bitmap file equivalent to the size you have stated of your page, so these memory usage spikes are not abnormal and you should not worry about them.

Also make sure you dispose the back-buffer variable at the end of the function to make sure all memory used by that object is released.

Why System.Drawing.Graphics.GetHdc() increases memory usage

1 Answers