How do wcf callbacks work

So, we need to develop an interesting application supporting the solution. Implementing the solution The first thing to know before implementing this approach is that not all bindings support callback operations and only bidirectional-capable bindings can be used. In order to support callbacks in your application, WCF provides the WSDualHttpBinding, which actually sets up two HTTP channels: one for the calls from the client to the server, and one for the calls from the server to the client.

Now, let's create a console application to host the service in Visual Studio. NET and add a reference to the System. ServiceModel assembly. Defining the callback contract The callback operations are part of the service contract. A service contract can have at most one callback contract. Once defined, the clients are required to support the callback and also provide the callback endpoint to service in every call.

In our example, we have a service which provides a math service doing some long term calculation. We want to be notified when the calculation operation begins. The service contract attribute annotates our contract and offers a CallbackContract property of the type Type setting up our callback contract, as shown in Listing 1.

Generic; using System. Text; using System. Definition of the service contract and the associated callback contract. The service implementation In order to invoke the client callback from the service, we need a reference to the callback object. When the client invokes the service operations, it supplies a callback channel for the communication with the server through the callback. This channel can be referenced from the server by calling the GetCallbackChannel operation on the global OperationContext instance, as shown in the Listing 2.

Sleep The MathCalculationService implementation. Rename IService1. Step 2: Open IService. In the above code, since IServiceCallback interface is set as a CallbackContract of the IService service contract, it will automatically serialize through the proxy, to the client application. The client application can then subscribe to this interface, to receive callback from the WCF service. Step 3: Open Service. Step 4: Open Web. Config file and write the following configuration as shown below.

Add the WCF service reference in it. Along with this, you will also find the IServiceCallback, the callback contract in the proxy. Step 7: Now this is the most important step. We need to implement the IServiceCallback contract in the client application so that the Callback from the WCF service can be received in the client. Here the client application acts as service and service acts as a client. Write the following code in it:.

This method displays count of the department in the department array object. Step 8: Design Form1. Step 9: Open Form1. Step Write the following code in the Form Loaded event:. The above code creates an instance of the RequestCallBack class created above in Step 7. Using this object, the context information of the service instance is acquired using InstanceContext class, then this context is passed to the Proxy object so that the WCF service can automatically send the request to the client application.

Step Write the following code in the click event of the Get Data button. This is a very simple piece of code for making call to the WCF service:. Step Run the application and click on the Get Data button. The result will be as shown below:. The dialog-box shows that the value is posted from client to the WCF service. After a second, the result shown below will be displayed indicating the Service has made a callback to the client:. Conclusion: In WCF, callback is used to implement the PUSH mechanism, so that delayed or long running operation results can be automatically pushed back to the client application.

The entire source code of this article can be downloaded over here. This way seems to me to be nasty. Especially for 3rd or 4th level children. So i want to learn is there a "best practice" or something like that to handle callbacks by many windows.

May be it is the silly question, but i really cant find an answer. What I have tried: I try to "google" it, but didn't get any close enough result. Posted Dec am ZlobnyZerg. Add a Solution. Accept Solution Reject Solution. I've got an answer, but I've also got an alternative recommendation for WCF callbacks Passing data throughout an application should be done via services in the DDD sense and a bus.

Any other object in the application desiring the information will then simply subscribe to the bus. For a simple application bus you can use a Reactive Subject. Reactive objects are much more powerful in general and are easier to work with. I've got some extensive articles on service buses if you want to check them out. It's much more easy to support loose coupling, asynchronous behaviors, scalability, etc. Let's say, for example, I have an enterprise application and I update a User.

Aren't their probably other applications that will need that updated information? How would it get to other applications if the WCF service call is local to the application? As a result, one-way operations can't return values, and any exception thrown on the service side will not make its way to the client.

One-way calls do not equate to asynchronous calls. When one-way calls reach the service, they may not be dispatched all at once and may be queued up on the service side to be dispatched one at a time, all according to the service configured concurrency mode behavior and session mode.

How many messages whether one-way or request-reply the service is willing to queue up is a product of the configured channel and the reliability mode. If the number of queued messages has exceeded the queue's capacity, then the client will block, even when issuing a one-way call.

However, once the call is queued, the client is unblocked and can continue executing while the service processes the operation in the background. This usually gives the appearance of asynchronous calls.

All the Windows Communication Foundation bindings support one-way operations. The OperationContract attribute offers the Boolean IsOneWay property, which defaults to false, meaning a request-reply operation. But setting IsOneWay to true configures the method as a one-way operation:. Because there is no reply associated with a one-way operation, there's no point in having any returned values or results, and the operation must have a void return type without any outgoing parameters.

Since Windows Communication Foundation does not have any hooks into the language compiler and can't verify proper usage at compile-time, it enforces this by verifying the method signature when loading up the host at run time and throwing an InvalidOperationException in case of a mismatch.

The fact that the client doesn't care about the result of the invocation does not mean the client doesn't care if the invocation took place at all. In general, you should turn on reliability for your services, even for one-way calls. This will ensure delivery of the requests to the service.

However, with one-way calls, the client may or may not care about the invocation order of the one-way operations. This is one of the main reasons why Windows Communication Foundation allows you to separate enabling reliable delivery from enabling ordered delivery and execution. Windows Communication Foundation supports allowing the service to call back to its clients. During a callback, in many respects the tables are turned: the service is the client and the client becomes the service see Figure 1.

The client also has to facilitate hosting the callback object. Not all bindings support callback operations. A service contract can have at most one callback contract. Once the callback contract is defined, the clients are required to support the callback and also to provide the callback endpoint to the service in every call.

You need to set it to the callback contract type and provide the definition of the callback contract, as shown here:. Note that the callback contract need not be marked with a ServiceContract attribute—it is implied. The client-side imported callback interface will not necessarily have the same name as in the original service-side definition. Instead, it will be have the name of the service contract interface suffixed with the word Callback. It is up to the client to host the callback object and expose a callback endpoint.

The innermost execution scope of the service instance is the instance context:. All the client needs to do to host a callback object is to instantiate the callback object and construct a context around it:. Whenever interacting with a service endpoint whose contract defines a callback contract, the client must use a proxy that will set up the bidirectional communication and pass the callback endpoint reference to the service.

The proxy will construct an endpoint around the callback context, while inferring the details of the callback endpoint from the service endpoint configuration. The callback endpoint contract is the one defined by the service contract callback type. The callback endpoint will use the same binding or transport, actually as the outgoing call. Windows Communication Foundation will use the client's machine name for the address and even select a port when using HTTP.

Simply by passing the instance context to the duplex proxy and using the proxy to call the service will expose that client-side callback endpoint. The client constructs a callback instance, hosts it in a context, creates a proxy and call the service, thus passing the callback endpoint reference:.

Note that as long as the client is expecting callbacks, it can't close the proxy. Doing so would close the callback endpoint and cause an error on the service side when the service tried to call back.

It is often the case that the client itself implements the callback contract, in which case the client will typically use a member variable for the proxy and close it when the client is disposed, as shown in Figure 4.

The client-side callback endpoint reference is passed along with every call the client makes to the service and is part of the incoming message. What exactly the service does with the callback reference and when it decides to use the reference is completely at the discretion of the service.

The service can extract the callback reference from the operation context and store it for later use or the service can use the reference during the service operation to call back to the client. Figure 5 demonstrates the first option. Using the same definition of the callback contract as shown earlier, the service uses a static generic list to store references to interfaces of the type IMyContractCallback. Because the service does not know which client is calling it and whether the client has called it already, the service checks in every call to see whether the list already contains the callback reference.

If the list does not contain the reference, the service adds the callback to the list. The service class also offers the static method CallClients. Any party on the host side can simply use that to call back to the clients:.

Invoked this way, the invoking party is using some host-side thread for the callback invocation. That thread is unrelated to any thread executing an incoming service call. The service may also want to invoke the callback reference passed in or invoke the list of callbacks during the execution of a contract operation.

However, such invocations are disallowed because by default the service class is configured for single-threaded access: the service instance is associated with a lock, and only one thread at a time can own the lock and access the service instance. Calling out to the clients during an operation requires blocking the service thread while invoking the callbacks.

The problem is that processing the reply message from the client once the callback returns requires ownership of the same lock, and so a deadlock would occur. To avoid a deadlock, if the single-threaded service instance tries to call back to its clients, Windows Communication Foundation will throw an InvalidOperationException.

There are three possible solutions. The first is to configure the service for multiple-threaded access, which would not associate it with a lock and would therefore allow callbacks.