VB API Result converts decimal numbers - vb.net

Question

I have a simple API Post like this
<HttpPost>
Public Function testPost() As IEnumerable(Of sp_select)
Dim _tmplist As New List(Of sp_select)
Dim _Content = jsonSerializer.Deserialize(Of myparams)(Request.Content.ReadAsStringAsync().Result)
Dim mandant = _Content.mandant
Using mycon As New dal_globalDataContext
Dim _result = mycon.sp_select(CType(mandant, Decimal)).ToList
If Not _result.Any = False Then
For Each entry In _result
_tmplist.Add(entry)
Next
End If
End Using
Return _tmplist
End Function
All good. But If I parse the result like this:
Public Sub testPost()
Dim client = New System.Net.Http.HttpClient()
client.BaseAddress = New Uri("http://localhost:62068")
client.DefaultRequestHeaders.Accept.Clear()
client.DefaultRequestHeaders.Accept.Add(New System.Net.Http.Headers.MediaTypeWithQualityHeaderValue("application/json"))
Dim _datasource As IEnumerable(Of Object) = Nothing
Dim _content = New With {.mandant = 0}
Dim serializer As New JavaScriptSerializer()
Dim arrayJson As String = serializer.Serialize(_content)
Dim responseTask = client.PostAsync("/actionapi/dataSource/testPost/", New StringContent(arrayJson))
responseTask.Wait()
Dim result = responseTask.Result
If result.IsSuccessStatusCode Then
Dim readTask = result.Content.ReadAsAsync(Of IList(Of sp_select))()
readTask.Wait()
_datasource = readTask.Result
Any decimal property of the class has an decimal point. So 1 will be 1.0
Do you know how to prevent this behaviour?