Why Async function returning System.Threading.Tasks.Task`1[System.String]?

Question

I have a VB.NET function as below:

Public Shared Async Function GetIdDoc() As Task(Of String)
    Dim result As String = ""
    'Dim Uri As String = "http://localhost:53917/api/Documenti/GetNextIdDocumenti"
    Dim Uri As String = apiUri & ApiEndPoints.GetNextIdDocumenti

    Using client = New HttpClient()
        Using response = Await client.GetAsync(Uri)
            If response.IsSuccessStatusCode Then
                Dim DocumentiIDJsonString = Await response.Content.ReadAsStringAsync()
                result = DocumentiIDJsonString.ToString()

            End If
        End Using
    End Using
    Return result
End Function

I'm trying to return the Document ID from the DB but I'm getting

System.Threading.Tasks.Task`1[System.String]

Where actually it should return "2". Please help me on this: what am I doing wrong with this function?

Update

here is the function called:

 txtIDDoc_Detail.Text = ApiData.GetIdDoc().ToString()

But inside the textbox I'm getting the above text. thanks.

Please edit your question to also show the code where the function is called. — Andrew Morton
– Andrew Morton, Commented Oct 27, 2016 at 18:34
You should probably rename GetIdDoc to GetIdDocAsync to follow the generally recommended pattern for async methods. — Bradley Uffner
– Bradley Uffner, Commented Oct 27, 2016 at 19:56

Sebi · Accepted Answer · 2016-10-27 19:01:51Z

9

I'm from C# but should work the same. In newer .Net Versions (>= 4.5) async/await is implemented. So if a method is marked as async and returns a Task (which should always be the case), you need to await it. This implicate that you have to mark your Method as async too. So your call should look like this:

txtIDDoc_Detail.Text = await ApiData.GetIdDoc();

The await waits till the long running Task is ready and returns it's inner value. All async Methods should return Task. If the Method is void it would be Task. Else it could be Task<int> or any other type. So await it and you can keep running ;)

edited Oct 27, 2016 at 19:01

answered Oct 27, 2016 at 18:52

Sebi

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2 Comments

Bradley Uffner Over a year ago

Or txtIDDoc_Detail.Text = ApiData.GetIdDoc.Result if you want to run it synchronously (for whatever reason).

Fabio Over a year ago

@BradleyUffner - ApiData.GetIdDoc.Result can cause a deadlock.

Bradley Uffner · Accepted Answer · 2016-10-27 20:07:03Z

0

@Sebi gives a great explanation of how to properly use async and await in this case, but I'm going to expand on exactly why you are getting the result that you see.

 txtIDDoc_Detail.Text = ApiData.GetIdDoc().ToString()

Is returning

System.Threading.Tasks.Task`1[System.String]

Because you are calling .ToString on the instance of the Task Task(Of String), not the actual result. Types that don't override .ToString inherit the behavior of Object, which just returns the name of the type as a string.

You probably want this (asynchronous call):

txtIDDoc_Detail.Text = await ApiData.GetIdDoc()

Or this (synchronous call):

txtIDDoc_Detail.Text = ApiData.GetIdDoc().Result

Either of these calls will actually result of the task after it has completed.

edited Oct 27, 2016 at 20:07

answered Oct 27, 2016 at 20:00

Bradley Uffner

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3 Comments

Fabio Over a year ago

ApiData.GetIdDoc().Result can cause a deadlock, check this for more information: Don't Block on Async Code

Sebi Over a year ago

It will cause a deadlock if the GetIdDoc(); Method returns a result in MainThread. Because the Method want to return it while Result try to access it.

barsan Over a year ago

Thanks for explaining my issue.

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