Why is the MemoryStream data not released from memory

Question

Is there something that needs to be done with the following code to release the memory it uses?

  Dim objImage As MemoryStream
  Dim objwebClient As WebClient
  Dim sURL As String = Trim(m_StationInterface.PicLocation)

  objwebClient = New WebClient
  objImage = New MemoryStream(objwebClient.DownloadData(sURL))
  m_imgLiftingEye.Image = Image.FromStream(objImage)

The code is on a popup form that shouldn't ever get disposed. A new image is loaded onto the form every time it pops up. However, the process size for the application continues to grow each time it makes it through that code block.

I've tried objImage.Close() and .Flush(), objWebClient.Dispose(). The process size still grows by a good 4mb after every call. It's like the old image is kept in memory.

Answer 1

Image implements IDisposable, so you should Dispose the old image before replacing it with a new one.

Something like (bear with me, I haven't used VB in a while):

Dim objImage As MemoryStream
Dim objwebClient As WebClient
Dim sURL As String = Trim(m_StationInterface.PicLocation)

objwebClient = New WebClient
objImage = New MemoryStream(objwebClient.DownloadData(sURL))

If m_imgLiftingEye.Image Is Not Nothing Then
    m_imgLiftingEye.Image.Dispose()
End If
m_imgLiftingEye.Image = Image.FromStream(objImage)

Answer 2

Try this:

Function GetImage() As Image
    Using wc As New WebClient(), _
          ms As New MemoryStream(wc.DownloadData(m_StationInterface.PicLocation.Trim())

        GetImage = Image.FromStream(ms).Clone()
    End Using
End Function

Answer 3

MemoryStream implements the IDisposable interface, so you should call Dispose on that object when you are done using it:

objImage = New MemoryStream(objwebClient.DownloadData(sURL))
m_imgLiftingEye.Image = Image.FromStream(objImage)
objImage.Dispose()

I would guess your conclusion was right; the image (in the memory stream) does remain in the memory.

Update: as Marc pointed out Image.FromStream requires the stream to remain open for the lifetime of the image. To resolve this the MemoryStream variable should be declared in same scope as the image (as a field in the form). When loading the image, there should first be a check whether the MemoryStream already is open and if so, close and dispose it before using the variable for a new stream (let's assume that we call it m_imageStream). Since the image also implements IDisposable, the same is true for that one:

If Not m_imageStream Is Nothing Then
    m_imageStream.Dispose()
End If

If m_imgLiftingEye.Image Is Not Nothing Then
    m_imgLiftingEye.Image.Dispose()
End If

m_imageStream = New MemoryStream(objwebClient.DownloadData(sURL))
m_imgLiftingEye.Image = Image.FromStream(m_imageStream)

Answer 4

I know I already gave one answer, but I've been thinking since then...

You said that this form should never be disposed. In that case, when exactly is this image load happening? My previous answer assumed it was during the form Shown event. However, if it's during the form Load event, it should only happen once total.

That is, unless more than one instance of the form is being created. If that's the case, and the previous form isn't being reused, you're ending up with multiple copies of the same form loaded in memory, each with its own copy of the image.

Answer 5

You could try

set objImage = nothing
set objwebClient = nothing

Often, like with ADO, if you don't explicitly set it to nothing it doesn't get released properly.