Dealing with a memory leak when cropping images in vb.net - vb.net

Question

I keep getting a system out of memory error when I break an image up into sub images. There's nothing too fancy going on, just splitting an image into a bunch of smaller images. Here's my code:
Dim counterrr As Integer = 0
Dim sorucedir As String
Dim tardir As String
sorucedir = "C:\somedir\"
tardir = "C:\otherdir\"
Dim di2 As New DirectoryInfo(sorucedir)
Dim fiArr2 As FileInfo() = di2.GetFiles()
Dim fri2 As FileInfo
Dim hh As Integer
Dim ww As Integer
hh = 995 'height of source images
ww = 1080 'width of source images
Dim sizestepX As Integer = 180
Dim stepsizeY As Integer = 239
For i = 0 To ww - 1 Step sizestepX
For j = 0 To hh - 1 Step stepsizeY
For Each fri2 In fiArr2
Dim BM2 As Bitmap
BM2 = Image.FromFile(fri2.FullName)
''Threading.Thread.Sleep(3000)
'bm2 = Image.FromFile(fri2.FullName)
Dim BM3 As Bitmap
Dim rect As New Rectangle(i, j, sizestepX, stepsizeY)
BM3 = BM2.Clone(rect, Imaging.PixelFormat.DontCare)
counterrr += 1
Dim ss As String
ss = tardir & counterrr & ".png"
BM3.Save(ss, Imaging.ImageFormat.Png)
BM3 = Nothing
Next
Next
Next
The error comes up after creating as few as 6 images (occurs when creating the 7th image). The line where the error is thrown is this:
BM3 = BM2.Clone(rect, Imaging.PixelFormat.DontCare)
How can I modify my code so they objects are correctly disposed (?) of so they don't leak memory.

Answer 1

The Bitmap class, along with many of the other GDI+ classes implement IDisposable because they hold onto underlying GDI object handles which need to be released. Each process is limited by the OS to a maximum number of GDI objects, usually 10,000. You can view the total number of active GDI objects per process in the Windows task manager. To fix your problem, you need to either manually call the Dispose method on each Bitamp object you create, before it goes out of scope, or you need to wrap each Bitmap variable declaration in a Using block (so it will call Dispose for you). For instance:
Using BM2 As Bitmap = Image.FromFile(fri2.FullName)
Dim rect As New Rectangle(i, j, sizestepX, stepsizeY)
Using BM3 As Bitmap = BM2.Clone(rect, Imaging.PixelFormat.DontCare)
counterrr += 1
Dim ss As String
ss = tardir & counterrr & ".png"
BM3.Save(ss, Imaging.ImageFormat.Png)
End Using
End Using
Note: there's no point in setting the BM3 variable to Nothing at the end, like you're doing. That has no bearing on anything.