Question

Is it safe to use a Using block for an asynchronous object? If the block is exited before async task is complete, will the object be disposed too soon? Will it be disposed properly when the task completes?

This code works, but I don’t know whether it’s abusing my memory.

For i = 1 To nPings
  Thread.Sleep(10)
  Using pPing As New System.Net.NetworkInformation.Ping
    AddHandler pPing.PingCompleted, AddressOf pingHandler
    pPing.SendAsync(ip, timeout)
  End Using
Next i

Answer 1

If the block is exited before async task is complete, will the object be disposed too soon?

Yes. As soon as the End Using line completes, the Ping object will be disposed.

Will it be disposed properly when the task completes?

In your example, the Ping object will always be disposed of. However, this may or may not happen after the async code completes as that behavior, the way you’ve coded it, is undetermined.

To make this behavior more deterministic, dispose of the Ping object inside your pingHandler code, such that:

Class PingAsyncState

    Public Property Ping Ping
    Public Property int Timeout

End Class

For i = 1 To nPings
    Thread.Sleep(10)
    pPing As New System.Net.NetworkInformation.Ping
    AddHandler pPing.PingCompleted, AddressOf pingHandler
    pPing.SendAsync(ip, new PingAsyncState() With { .Ping = pPing, .Timeout = timeout })
Next i

Sub PingHandler(obj As Object)

    Dim state = CType(state, PingAsyncState)       

    ... code to do stuff ...

    state.Ping.Dispose()

End Sub

Also, if you inspect the code inside the Ping.Dispose(), you’ll see that it closes various EventHandles. This may not cause an exception if the Ping object is disposed before the async completes, but it certainly puts the Ping object in an unstable state. You might find events not firing or the async complete handler code not being called or other subtle things happening.