The 'Async' modifier can only be used on Subs, or on Functions that return Task or Task(Of T)

Question

I'm having trouble writing this DecompressionHandler using the ContinueWith lambda sub function. The error I'm recieving is "The 'Async' modifier can only be used on Subs, or on Functions that return Task or Task(Of T)." within the ContinueWith sub function. I don't know what the problem or how to resolve it.

   Public Class DecompressionHandler
    Inherits DelegatingHandler
    Public Compressors As List(Of ICompressor)
    Public Sub New()
        Compressors = New List(Of ICompressor)()
        Compressors.Add(New GZipCompressor())
        Compressors.Add(New DeflateCompressor())
    End Sub
    Protected Overrides Async Function SendAsync(request As HttpRequestMessage, cancellationToken As CancellationToken) As Task(Of HttpResponseMessage)
        Return Await MyBase.SendAsync(request, cancellationToken).ContinueWith(Of HttpResponseMessage)(Async Function(responseToCompleteTask)
                                                                                                           Dim response As HttpResponseMessage = responseToCompleteTask.Result
                                                                                                           If response.Content.Headers.ContentEncoding IsNot Nothing AndAlso response.Content IsNot Nothing Then
                                                                                                               Dim encoding = response.Content.Headers.ContentEncoding.First()
                                                                                                               Dim compressor = Compressors.FirstOrDefault(Function(c) c.EncodingType.Equals(encoding, StringComparison.InvariantCultureIgnoreCase))
                                                                                                               If compressor IsNot Nothing Then response.Content = Await DecompressedContentAsync(response.Content, compressor).ConfigureAwait(False)
                                                                                                           End If
                                                                                                           Return response
                                                                                                       End Function)
    End Function
    Private Shared Async Function DecompressedContentAsync(compressContent As HttpContent, compressor As ICompressor) As Task(Of StreamContent)
        Using compressContent
            Dim decompressed As New MemoryStream
            Await compressor.Decompress(Await compressContent.ReadAsStreamAsync, decompressed)
            Dim newContent As New StreamContent(decompressed)
            newContent.Headers.ContentType = compressContent.Headers.ContentType
            Return newContent
        End Using
    End Function
End Class

Answer 1

I solved it... If you remove the Async/Await function and change DecompressedContentAsync(response.content,compressed).Result()

That works... Dave was half way right...

Answer 2

VB is complaining about:

Async Function(responseToCompleteTask)

since it returns HttpResponseMessage

Change it to:

Function(responseToCompleteTask)

Answer 3

Just use Await instead of ContinueWith. My VB is rusty, but this should give you an idea:

Protected Overrides Async Function SendAsync(request As HttpRequestMessage, cancellationToken As CancellationToken) As Task(Of HttpResponseMessage)
  Dim response as HttpResponseMessage = Await MyBase.SendAsync(request, cancellationToken)
  If response.Content.Headers.ContentEncoding IsNot Nothing AndAlso response.Content IsNot Nothing Then
    Dim encoding = response.Content.Headers.ContentEncoding.First()
    Dim compressor = Compressors.FirstOrDefault(Function(c) c.EncodingType.Equals(encoding, StringComparison.InvariantCultureIgnoreCase))
    If compressor IsNot Nothing Then response.Content = Await DecompressedContentAsync(response.Content, compressor).ConfigureAwait(False)
  End If
  Return response
End Function