Threading with WPF Images (System.InvalidOperationException)

Question

I'm using a thread to get an image from a website and shoot it back to the parent form (WPF) to display. I ran into an issue and have managed to debug it to this example:

public void Watch()
{
  while (true)
  {
    Bitmap bmp = new Bitmap(1, 1);
    BitmapImage bmpImg = new BitmapImage();

    this.SetImage(bmp, bmpImg);
  }
}

public delegate void SetImageCallback(Bitmap bmp, BitmapImage bmpImg);

private void SetImage(Bitmap bmp, BitmapImage bmpImg)
{
  if (!this.imgVideo.Dispatcher.CheckAccess())
  {
    SetImageCallback del = new SetImageCallback(SetImage);
    this.Dispatcher.Invoke(del, bmp, bmpImg);
  }
  else
  {
    Bitmap bitmap = bmp;
    BitmapImage bitmapImage = bmpImg;
  }
}

Keep in mind that Watch() runs on its own thread. If I use the bitmap object (which I can use with PictureBox in Window Forms) everything works great. That is, debugging this code, when I get to the line

Bitmap bitmap = bmp;

And inspect the variable bmp, everything is great and works as expected. HOWEVER, when I get to the next line

BitmapImage bitmapImage = bmpImg;

And inpsect the variable bmpImage, I get a ton of System.InvalidOperationException's. When this is in practice and gets assigned to a WPF Image object, it says that "The calling thread cannot access this object because a different thread owns it." Why am I running into this issue with WPF BitmapImages (which are required to set an ImageSource) but NOT in Windows Forms Bitmap objects (which can be used to set a PictureBox)? How do I fix this in WPF?

Read this: msdn.microsoft.com/en-us/library/ms750509.aspx

Hans Passant
– Hans Passant

2011-05-04 01:28:11 +00:00
Commented May 4, 2011 at 1:28 — Hans Passant
– Hans Passant, Commented May 4, 2011 at 1:28

Rick Sladkey · Accepted Answer · 2011-05-04 02:25:30Z

4

Most objects in WPF are of this category: they cannot be shared between different threads. However certain low-level resources such as brushes and bitmaps are derived from a special class called Freezable that if frozen can be shared between different threads. Of course once an object is frozen is can no longer be modified in any way. To freeze a freezable object simply call Freeze and this will prevent cross-thread exceptions.

answered May 4, 2011 at 2:25

Rick Sladkey

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2 Comments

jack1818 Over a year ago

Ok, so I started tried with both a BitmapImage and an ImageBrush (with source assigned to a BitmapImage) and I keep getting a "This Freezable cannot be frozen" error. Why am I getting this?

jack1818 Over a year ago

Does this have to do with me getting the source of the image from a URL? Looks like I'm running into this problem but cannot raise the DownloadCompleted event. Also, if I check the IsDownlading property, it is always true. What should I do?

Oxyprogrammer · Accepted Answer · 2011-05-04 11:31:48Z

1

Instead of

if (!this.imgVideo.Dispatcher.CheckAccess())
  {
    SetImageCallback del = new SetImageCallback(SetImage);
    this.Dispatcher.Invoke(del, bmp, bmpImg);
  }

try using :

if (!App.Current.Dispatcher.CheckAccess())
                App.Current.Dispatcher.Invoke(DispatcherPriority.Normal, new Action<CustomObject>(SetImage),CustomeObjectInstance );

Here Cutom object will be a wrapper class wrapping

Bitmap bmp, BitmapImage bmpImg

Obviously, your SetImage signature will change to

SetImage(CutomObject custObj)

I have not tested the code but this may solve the issue. Let us know if this works so that some poor soul can be benefitted from this post. All the best! Sid

answered May 4, 2011 at 11:31

Oxyprogrammer

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