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How to: Enable Streaming

Windows Communication Foundation (WCF) can send messages using either buffered or streamed transfers. In the default buffered-transfer mode, a message must be completely delivered before a receiver can read it. In streaming transfer mode, the receiver can begin to process the message before it is completely delivered. The streaming mode is useful when the information that is passed is lengthy and can be processed serially. Streaming mode is also useful when the message is too large to be entirely buffered.

To enable streaming, define the OperationContract appropriately and enable streaming at the transport level.

To stream data

  1. To stream data, the OperationContract for the service must satisfy two requirements:

    1. The parameter that holds the data to be streamed must be the only parameter in the method. For example, if the input message is the one to be streamed, the operation must have exactly one input parameter. Similarly, if the output message is to be streamed, the operation must have either exactly one output parameter or a return value.

    2. At least one of the types of the parameter and return value must be either Stream, Message, or IXmlSerializable.

    The following is an example of a contract for streamed data.

    <ServiceContract(Namespace:="http://Microsoft.ServiceModel.Samples")> _
    Public Interface IStreamingSample
        <OperationContract()> _
        Function GetStream(ByVal data As String) As Stream
        <OperationContract()> _
        Function UploadStream(ByVal stream As Stream) As Boolean
        <OperationContract()> _
        Function EchoStream(ByVal stream As Stream) As Stream
        <OperationContract()> _
        Function GetReversedStream() As Stream
    
    End Interface
    
    

    The GetStream operation receives some buffered input data as a string, which is buffered, and returns a Stream, which is streamed. Conversely UploadStream takes in a Stream (streamed) and returns a bool (buffered). EchoStream takes and returns Stream and is an example of an operation whose input and output messages are both streamed. Finally, GetReversedStream takes no inputs and returns a Stream (streamed).

  2. Streaming must be enabled on the binding. You set a TransferMode property, which can take one of the following values:

    1. Buffered,

    2. Streamed, which enables streaming communication in both directions.

    3. StreamedRequest, which enables streaming the request only.

    4. StreamedResponse, which enables streaming the response only.

    The BasicHttpBinding exposes the TransferMode property on the binding, as does NetTcpBinding and NetNamedPipeBinding. The TransferMode property can also be set on the transport binding element and used in a custom binding.

    The following samples show how to set TransferMode by code and by changing the configuration file. The samples also both set the maxReceivedMessageSize property to 64 MB, which places a cap on the maximum allowable size of messages on receive. The default maxReceivedMessageSize is 64 KB, which is usually too low for streaming scenarios. Set this quota setting as appropriate depending on the maximum size of messages your application expects to receive. Also note that maxBufferSize controls the maximum size that is buffered, and set it appropriately.

    1. The following configuration snippet from the sample shows setting the TransferMode property to streaming on the basicHttpBinding and a custom HTTP binding.

      <basicHttpBinding>
        <binding name="HttpStreaming" maxReceivedMessageSize="67108864"
                 transferMode="Streamed"/>
      </basicHttpBinding>
      <!-- an example customBinding using Http and streaming-->
      <customBinding>
        <binding name="Soap12">
          <textMessageEncoding messageVersion="Soap12WSAddressing10" />
          <httpTransport transferMode="Streamed" maxReceivedMessageSize="67108864"/>
        </binding>
      </customBinding>
      
      
    2. The following code snippet shows setting the TransferMode property to streaming on the basicHttpBinding and a custom HTTP binding.

      Public Shared Function CreateStreamingBinding() As Binding
          Dim b As New BasicHttpBinding()
          b.TransferMode = TransferMode.Streamed
          Return b
      End Function
      
      
    3. The following code snippet shows setting the TransferMode property to streaming on a custom TCP binding.

      Public Shared Function CreateStreamingBinding() As Binding
          Dim transport As New TcpTransportBindingElement()
          transport.TransferMode = TransferMode.Streamed
          Dim binding As New CustomBinding(New BinaryMessageEncodingBindingElement(), _
                                           transport)
          Return binding
      End Function
      
      
  3. The operations GetStream, UploadStream, and EchoStream all deal with sending data directly from a file or saving received data directly to a file. The following code is for GetStream.

    Public Function GetStream(ByVal data As String) As Stream Implements IStreamingSample.GetStream
        'this file path assumes the image is in
        ' the Service folder and the service is executing
        ' in service/bin 
        Dim filePath = Path.Combine(System.Environment.CurrentDirectory, ".\..\image.jpg")
        'open the file, this could throw an exception 
        '(e.g. if the file is not found)
        'having includeExceptionDetailInFaults="True" in config 
        ' would cause this exception to be returned to the client
        Try
            Return File.OpenRead(filePath)
        Catch ex As IOException
            Console.WriteLine(String.Format("An exception was thrown while trying to open file {0}", filePath))
            Console.WriteLine("Exception is: ")
            Console.WriteLine(ex.ToString())
            Throw ex
        End Try
    End Function
    
    

Writing a custom stream

  1. To do special processing on each chunk of a data stream as it is being sent or received, derive a custom stream class from Stream. As an example of a custom stream, the following code contains a GetReversedStream method and a ReverseStream class-.

    GetReversedStream creates and returns a new instance of ReverseStream. The actual processing happens as the system reads from the ReverseStream object. The ReverseStream.Read method reads a chunk of bytes from the underlying file, reverses them, then returns the reversed bytes. This method does not reverse the entire file content; it reverses one chunk of bytes at a time. This example shows how you can perform stream processing as the content is being read to or written from the stream.

    Friend Class ReverseStream
        Inherits Stream
    
        Private inStream As FileStream
    
        Friend Sub New(ByVal filePath As String)
            'opens the file and places a StreamReader around it
            inStream = File.OpenRead(filePath)
        End Sub
    
        Public Overrides ReadOnly Property CanRead() As Boolean
            Get
                Return inStream.CanRead
            End Get
        End Property
    
        Public Overrides ReadOnly Property CanSeek() As Boolean
            Get
                Return False
            End Get
        End Property
    
        Public Overrides ReadOnly Property CanWrite() As Boolean
            Get
                Return False
            End Get
        End Property
    
        Public Overrides Sub Flush()
            Throw New Exception("This stream does not support writing.")
        End Sub
    
        Public Overrides ReadOnly Property Length() As Long
            Get
                Throw New Exception("This stream does not support the Length property.")
            End Get
        End Property
    
        Public Overrides Property Position() As Long
            Get
                Return inStream.Position
            End Get
            Set(ByVal value As Long)
                Throw New Exception("This stream does not support setting the Position property.")
            End Set
        End Property
    
        Public Overrides Function Read(ByVal buffer() As Byte, _
                                       ByVal offset As Integer, _
                                       ByVal count As Integer) As Integer
    
            Dim countRead = inStream.Read(buffer, _
                                          offset, _
                                          count)
            ReverseBuffer(buffer, _
                          offset, _
                          countRead)
            Return countRead
        End Function
    
        Public Overrides Function Seek(ByVal offset As Long, _
                                       ByVal origin As SeekOrigin) As Long
            Throw New Exception("This stream does not support seeking.")
        End Function
    
        Public Overrides Sub SetLength(ByVal value As Long)
            Throw New Exception("This stream does not support setting the Length.")
        End Sub
    
        Public Overrides Sub Write(ByVal buffer() As Byte, _
                                   ByVal offset As Integer, _
                                   ByVal count As Integer)
            Throw New Exception("This stream does not support writing.")
        End Sub
    
        Public Overrides Sub Close()
            inStream.Close()
            MyBase.Close()
        End Sub
    
        Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
            inStream.Dispose()
            MyBase.Dispose(disposing)
        End Sub
    
        Private Sub ReverseBuffer(ByVal buffer() As Byte, _
                                  ByVal offset As Integer, _
                                  ByVal count As Integer)
    
            Dim i = offset
            Dim j = offset + count - 1
    
            Do While i < j
                Dim currenti = buffer(i)
                buffer(i) = buffer(j)
                buffer(j) = currenti
                i += 1
                j -= 1
            Loop
    
        End Sub
    End Class
    
    

See Also


© 2007 Microsoft Corporation. All rights reserved.
Last Published: 2010-03-21

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