Question

I have the following code to convert a list of messages into json :

Public Class MessageService
    <OperationContract()> _
    Public Function GetMessage(Id As Integer) As String



        Dim msg As New Message()
        Dim stream As New MemoryStream()
        Dim serializer As New DataContractJsonSerializer(GetType(NewMessage))

        Dim dtMessages = msg.getUnreadMessages("3A3458")

        Dim list As New ArrayList

        Dim m As New NewMessage()
        m.Id = 1
        m.Text = "mmsg"
        list.Add(m)
        list.Add(m)
        serializer.WriteObject(stream, list)

        stream.Position = 0
        Dim streamReader As New StreamReader(stream)
        Return streamReader.ReadToEnd()
    End Function
End Class

But I got the following error :

    The server was unable to process the request due to an internal error. 
 For more information about the error, either turn on IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute or from the <serviceDebug> configuration behavior) on the server in order to send the exception information back to the client, 
or turn on tracing as per the Microsoft .NET Framework 3.0 SDK documentation and inspect the server trace logs.

Answer 1

Here:

Dim serializer As New DataContractJsonSerializer(GetType(NewMessage))

You indicate to the serializer that you want to serialize a NewMessage instance.

And here:

serializer.WriteObject(stream, list)

you are passing an Arraylist. This obviously is ambiguous. I would recommend you modifying your method so that it returns directly a strongly typed collection and leave the JSON serialization to the binding instead of writing plumbing code in your data contract:

Public Class MessageService
    <OperationContract()> _
    Public Function GetMessage(Id As Integer) As List(Of NewMessage)
        Dim list As New List(Of NewMessage)
        Dim m As New NewMessage()
        m.Id = 1
        m.Text = "mmsg"
        list.Add(m)

        Return list
    End Function
End Class