Synchronous and Asynchronous Operations

Article
08/07/2009

Describes local asynchronous implementations and invocations as well as the synchronous use of asynchronous message exchanges.

Many applications call methods asynchronously because it enables the application to continue doing useful work while the method call runs. Windows Communication Foundation (WCF) services and clients can participate in asynchronous operation calls at two distinct levels of the application, which provide WCF applications even more flexibility to maximize throughput balanced against interactivity.

Types of Asynchronous Operations

All service contracts in WCF, no matter the parameters types and return values, use WCF attributes to specify a particular message exchange pattern between client and service. WCF automatically routes inbound and outbound messages to the appropriate service operation or running client code.

The client possesses only the service contract, which specifies the message exchange pattern for a particular operation. Clients can offer the developer any programming model they choose, so long as the underlying message exchange pattern is observed. So, too, can services implement operations in any manner, so long as the specified message pattern is observed.

The independence of the service contract from either the service or client implementation enables the following forms of asynchronous execution in WCF applications:

Clients can invoke request/response operations asynchronously using a synchronous message exchange.
Services can implement a request/response operation asynchronously using a synchronous message exchange.
Message exchanges can be one-way, regardless of the implementation of the client or service.

Suggested Asynchronous Scenarios

Use an asynchronous approach in a service operation implementation if the operation service implementation makes a blocking call, such as doing I/O work. When you are in an asynchronous operation implementation, try to call asynchronous operations and methods to extend the asynchronous call path as far as possible. For example, call a BeginOperationTwo() from within BeginOperationOne().

Use an asynchronous approach in a client or calling application in the following cases:
If you are invoking operations from a middle-tier application. (For more information about such scenarios, see Middle-Tier Client Applications.)
If you are invoking operations within an ASP.NET page, use asynchronous pages.
If you are invoking operations from any application that is single threaded, such as Windows Forms or Windows Presentation Foundation (WPF). When using the event-based asynchronous calling model, the result event is raised on the UI thread, adding responsiveness to the application without requiring you to handle multiple threads yourself.
In general, if you have a choice between a synchronous and asynchronous call, choose the asynchronous call.

Client-Side Asynchronous Invocations

A WCF client application can use one of two asynchronous calling models, both described in Asynchronous Programming Design Patterns:

Asynchronous operations that use events.
Asynchronous operations that use System.IAsyncResult objects.

The first approach, the event-based asynchronous pattern, is recommended for calling applications because it only requires adding an event handler to receive a notification of the response -- and the resulting event is raised on the user interface thread automatically. To use this approach, specify both the /async and /tcv:Version35 command options with the ServiceModel Metadata Utility Tool (Svcutil.exe), as in the following example.

svcutil https://localhost:8000/servicemodelsamples/service/mex /async /tcv:Version35

When this is done, Svcutil.exe generates a WCF client class with the event infrastructure that enables the calling application to implement and assign an event handler to receive the response and take the appropriate action. For a complete example, see How to: Call WCF Service Operations Asynchronously.

The event-based asynchronous model, however, is only available in .NET Framework version 3.5. In addition, it is not supported even in .NET Framework 3.5 when a WCF client channel is created by using a System.ServiceModel.ChannelFactory. With WCF client channel objects, you must use System.IAsyncResult objects to invoke your operations asynchronously. To use this approach, specify the /async command option with the ServiceModel Metadata Utility Tool (Svcutil.exe), as in the following example.

svcutil https://localhost:8000/servicemodelsamples/service/mex /async

This generates a service contract in which each operation is modeled as a <Begin> method with the AsyncPattern property set to true and a corresponding <End> method. For a complete example using a ChannelFactory, see How to: Call Operations Asynchronously Using a Channel Factory.

In either case, applications can invoke an operation asynchronously even if the service is implemented synchronously, in the same way that an application can use the same pattern to invoke asynchronously a local synchronous method. How the operation is implemented is not significant to the client; when the response message arrives, its content is dispatched to the client's asynchronous <End> method and the client retrieves the information.

Asynchronous Operation Implementations

Likewise, a service operation can be implemented in an asynchronous fashion using the .NET Framework asynchronous programming pattern and marking the <Begin> method with the AsyncPattern property set to true. In this case, the asynchronous operation is exposed in metadata in the same form as a synchronous operation: It is exposed as a single operation with a request message and a correlated response message. Client programming models then have a choice. They can represent this pattern as a synchronous operation or as an asynchronous one, so long as when the service is invoked a request-response message exchange takes place.

For an example, see How to: Implement an Asynchronous Service Operation.

To define a contract operation X that is executed asynchronously regardless of how it is called in the client application:

Define two methods using the pattern BeginOperation and EndOperation.
The BeginOperation method includes in and ref parameters for the operation and returns an IAsyncResult type.
The EndOperation method includes an IAsyncResult parameter as well as the out and ref parameters and returns the operations return type.

For example, see the following method.

int DoWork(string data, ref string inout, out string outonly)

Function DoWork(ByVal data As String, ByRef inout As String, _
out outonly As out) As Integer

To create an asynchronous operation, the two methods would be:

[OperationContract(AsyncPattern=true)]
IAsyncResult BeginDoWork(string data, 
                          ref string inout, 
                          AsyncCallback callback, 
                          object state);
int EndDoWork(ref string inout, out string outonly, IAsyncResult result);

<OperationContract(AsyncPattern := True)>  _
Function BeginDoWork(ByVal data As String, _
                 ByRef inout As String, _
                 ByVal callback As AsyncCallback, _
                 ByVal state As Object) _
As IAsyncResult 

Function EndDoWork(ByRef inout As String, _
        ByRef outonly As String, _
        ByVal result As IAsyncResult) _
As Integer

Note:
The OperationContractAttribute attribute is applied only to the `BeginDoWork` method. The resulting contract has one WSDL operation named `DoWork`.

One-Way Message Exchange Patterns

You can also create an asynchronous message exchange pattern in which one-way operations (operations for which the System.ServiceModel.OperationContractAttribute.IsOneWay is true have no correlated response) can be sent in either direction by the client or service independently of the other side. (This uses the duplex message exchange pattern with one-way messages.) In this case, the service contract specifies a one-way message exchange that either side can implement as asynchronous calls or implementations, or not, as appropriate. Generally, when the contract is an exchange of one-way messages, the implementations can largely be synchronous because once a message is sent the application does not wait for a reply and can continue doing other work.

Event-based Asynchronous Clients and Message Contracts

The design guidelines for the event-based asynchronous model state that if more than one value is returned, one value is returned as the Result property and the others are returned as properties on the EventArgs object. One result of this is that if a client imports metadata using the event-based asynchronous command options and the operation returns more than one value, the default EventArgs object returns one value as the Result property and the remainder are properties of the EventArgs object.

If you want to receive the message object as the Result property and have the returned values as properties on that object, use the /messageContract command option. This generates a signature that returns the response message as the Result property on the EventArgs object. All internal return values are then properties of the response message object.

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