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Editorial Team
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Asked: 2026-06-12T16:38:59+00:00

I am testing my WPF application connecting to Azure Blob Storage to download a

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I am testing my WPF application connecting to Azure Blob Storage to download a bunch of images using TPL (tasks).
It is expected that in Live environment, there will be highly transient connection to the internet at deployed locations.

I have set Retry Policy and time-out in BlobRequestOptions as below:

//Note the values here are for test purposes only
//CloudRetryPolicy is a custom method returning adequate Retry Policy
// i.e. retry 3 times, wait 2 seconds between retries
blobClient.RetryPolicy = CloudRetryPolicy(3, new TimeSpan(0, 0, 2));

BlobRequestOptions bro = new BlobRequestOptions() { Timeout = TimeSpan.FromSeconds(20) };
blob.DownloadToFile(LocalPath, bro);

The above statements are in a background task that work as expected and I have appropriate exception handling in background task and the continuation task.

In order to test exception handling and my recovery code, I am simulating internet disconnection by pulling out the network cable. I have hooked up a method to System.Net.NetworkChange.NetworkAvailabilityChanged event on UI thread and I can detect connection/disconnection as expected and update UI accordingly.

My problem is: If I pull the network cable while a file is being downloaded (via blob.DownloadToFile), the background thread just hangs. It does not timeout, does not crash, does not throw exception, nothing!!! As I write, I have been waiting ~30 mins and no response/processing has happened in relation to background task.

If I pull the network cable, before download starts, execution is as expected. i.e. I can see retries happening, exceptions raised and passed ahead and so on.

Has anyone experienced similar behaviour? Any tips/suggestions to overcome this behaviour/problem?

By the way, I am aware that I can cancel the download task on detection of network connectivity loss, but I do not want to do this as network connectivity can get restored within the time-out duration and the download process can continue from where it was interrupted. I have tested this auto resumption and works nicely.

Below is a rough indication of my code structure (not syntactically correct, just a flow indication)

btnClick()
{
  declare background_task
  attach continuewith_task to background task
  start background task
}

background_task()
{
  try
  {
    ... connection setup ... 
    blob.DownloadToFile(LocalPath, bro);
  }
  catch(exception ex)
  {
     ... exception handling ....
     // in case of connectivity loss while download is in progress
     // this block is not getting executed
     // debugger just sits idle without a current statement
  }
}

continuewith_task()
{
  check if antecedent task is faulted
  {
     ... do recovery work ...
     // this is working as expected if connectivity is lost
     // before download starts
     // this task does not get called if connectivity is lost
     // while file transfer is taking place
  }
  else
  {
     .. further processing ...
  }
}
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1 Answer

  1. Editorial Team

    Your problem is mainly caused because Azure storage client libraries uses file streaming classes underneath and that why the API hang is not directly related with Windows Azure Blob client library. Calling file streaming API directly over network you can see the exact same behavior when network cable is suddenly removed, however removing network gracefully will return different behavior.

    If you search on internet you will find streaming classes does not detect the network loss and that’s why in your code you can check the network disconnect event and then stop the background streaming thread.

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