I'm working on a simple plug-in framework. WCF client need to create an instance of 'ISubject' and then send back to service side. The 'ISubject' can be extended by the user. The only thing client knows at runtime is ID of a subclass of 'ISubject'.
Firstly, client need to get type information of a specific subclass of 'ISubject'. Secondly, client using reflection to enumerate all members to create a custom property editor so that each member can be asigned with proper value. Lastly, client create an instance of that subclass and send back to service.
The problem is how does client get the type information through WCF communication?
I don't want client to load that assembly where the subclass (of 'ISubject') exists.
Thanks
First, you need to be aware that there is no magic way that WCF will provide any type information to your client in the scenario you have descibed. If you are going to do it, you will have to provide a mechanism yourself.
Next, understand that WCF does not really pass objects from server to client or vice versa. All it passes are XML infosets. Often, the XML infoset passed includes a serialized representation of some object which existed on the sender's side; in this case, if the client knows about that type (i.e. can load the type's metadata from its assembly), it can deserialize the XML to instantiate an identical object on the client side. If the client doesn't have the type metadata, it can't: this is the normal case with WCF unless data contract types are in assemblies shared by both server and client implementations (generally not a good idea).
The way WCF is normally used (for example if the client is implemented using a "Service Reference" in Visual Studio), what happens is that the service publishes WSDL metadata describing its operations and the XML schemas for the operation parameters and return values, and from these a set of types is generated for use in the client implementation. These are NOT the same .NET types as the data contract types used by the service implementation, but they are "equivalent" in the sense that they can be serialized to the same XML data passed over the network. Normally this type generation is done at design time in Visual Studio.
In order to do what you are trying to do, which is essentially to do this type generation at runtime, you will need some mechanism by which the client can get sufficient knowledge of the structure of the XML representing the various types of object implementing ISubject so that it can understand the XML received from the service and generate the appropriate XML the service is expecting back (either working with the XML directly, or deserializing/serializing it in some fashion). If you really, really want to do this, possible ways might be:
some out-of-band mechanism whereby the client is preconfigured with the relevant type information corresponding to each subclass of ISubject that it might see. The link provided in blindmeis's answer is one way to do that.
provide a separate service operation by which the client can translate the ID of the subclass to type metadata for the subclass (perhaps as an XSD schema from which the client could generate a suitable serializable .NET type to round trip the XML).
it would also be feasible in principle for the service to pass type metadata in some format within the headers of the response containing the serialized object. The client would need to read, interpret and act on the type infomation in an appropriate fashion.
Whichever way, it would be a lot of effort and is not the standard way of using WCF. You will have to decide if it's worth it.
I think you might be missing something :)
A major concept with web services and WCF is that we can pass our objects across the network, and the client can work with the same objects as the server. Additionally, when a client adds a service reference in Visual Studio, the server will send the client all the details it needs to know about any types which will be passed across the network.
There should be no need for reflection.
There's a lot to cover, but I suggest you start with this tutorial which covers WCF DataContracts - http://www.codeproject.com/KB/WCF/WCFHostingAndConsuming.aspx
To deserialize an object the receiving side will need to have the assembly the type is defined in.
Perhaps you should consider some type of remoting or proxying setup where the instance of ISubject lives on one side and the other side calls back to it. This may be problematic if you need to marshal large amounts of data across the wire.
wcf needs to know the real object(not an interface!) which should be sent across the wire. so you have to satisfy the server AND the clientproxy side from the WCF service that they know the types. if you dont know the object type while creating the WCF service, you have to find a way to do it in a dynamic way. i use the solution from here to get the knownTypes to my WCF service.
[ServiceContract(SessionMode = SessionMode.Required]
[ServiceKnownType("GetServiceKnownTypes", typeof(KnownTypeHelper))]//<--!!!
public interface IWCFService
{
[OperationContract(IsOneWay = false)]
object DoSomething(object obj);
}
if you have something "universal" like the code above, you have to be sure that whatever your object at runtime will be, your WCF service have to know this object.
you wrote your client create a subclass and sent it back to the service. if you want to do that, WCF(clientproxy and server!) needs to know the real type of your subclass.
Related
I am writing an application that is consuming an in-house WCF-based REST service and I'll admit to being a REST newbie. Since I can't use the "Add Service Reference", I don't have ready-made proxy objects representing the return types from the service methods. So far the only way I've been able to work with the service is by sharing the assembly containing the data types exposed by the service.
My problem with this arrangment is that I see only two possibilities:
Implement DTOs (DataContracts) and expose those types from my service. I would still have to share an assembly but this approach would limit the types contained in the assembly to the service contract and DTOs. I don't like to use DTOs just for the sake of using them, though as they add another layer of abstraction and processing time to convert from domain object to DTO and vice versa. Plus, if I want to have business rules, validation, etc. on the client, I'd have to share the domain objects anyways, so is the added complexity necessary.
Support serialization of my domain objects, expose those types and share that assembly. This would allow me to share business and validation logic with the client but it also exposes parts of my domain objects to the client that are meant only for the service app.
Perhaps an example would help the discussion...
My client application will display a list of documents that is obtained from the REST service (a GET operation). The service returns an array of DocumentInfo objects (lightweight, read-only representation of a Document).
When the user selects one of the items, the client retrieves the full Document object from the REST service (GET by id) and displays a data entry form so the user can modify the object. We would want validation rules for a rich user experience.
When the user commits the changes, the Document object is submitted to the REST service (a PUT operation) where it is persisted to the back-end data store.
If the state of the Document allows, the user may "Publish" the Document. In this case, the client POSTs a request to the REST service with the Document.ID value and the service performs the operation by retrieving the server-side Document domain object and calling the Publish method. The Publish method should not be available to the client application.
As I see it, my Document and DocumentInfo objects would have to be in a shared assembly. Doing this makes Document.Publish available to the client. One idea to hide it would be to make the method internal and add an InternalsVisibleTo attribute that allows my service app to call the method and not the client but this seems "smelly."
Am I on the right track or completely missing something?
The classes you use on the server should not be the same classes you use on the client (apart from during the data transfer itself). The best approach is to create a package (assembly/project) containing DTOs, and share these between the server and the client. You did mention that you don't want to create DTO's for the sake of it, but it is best practice. The performance impact of adding extra layers is negligible, and layering actually helps make your application easier to develop and maintain (avoiding situations like yours where the client has access to server code).
I suggest starting with the following packages:
Service: Resides on server only, exposes the service and contains server application logic.
DTO: Resides on both server and client. Contains simple classes which contain data which need to be passed between server and client. Classes have no code apart from properties. These are short lived objects which survive long enough only to transfer data.
Repository: Resides on client only. Calls the server, and turns Model objects into DTO's (and vice versa).
Model: Resides on client only. Contains classes which represent business objects and relationships. Model objects stay in memory throughout the life of the application.
Your client application code should call into Repository to get Model objects (you might also consider looking into MVVM if your not sure how to go about this).
If your service code is sufficiently complex that it needs access to Model classes, you should create a separate Model package (obviously give it a different name) - the only classes which should exist both on server and client are DTO classes.
I thought that I'd post the approach I took while giving credit to both Greg and Jake for helping guide me down the path.
While Jake is correct that deserializing the data on the client can be done with any type as long as it implements the same data contract, enforcing this without WSDL can be a bit tricky. I'm in an environment where other developers will be working with my solution both to support and maintain the existing as well as creating new clients that consume my service. They are used to "Add Service Reference" and going.
Greg's points about using different objects on the client and the server were the most helpful. I was trying to minimize duplicate by sharing my domain layer between the client and the server and that was the root of my confusion. As soon as I separated these into two distinct applications and looked at them in isolation, each with their own use cases, the picture became clearer.
As a result, I am now sharing a Contracts assembly which contains my service contracts so that a client can easily create a channel to the server (using WCF on the client-side) and data contracts representing the DTOs passed between client and service.
On the client, I have ViewModel objects which wrap the Model objects (data contracts) for the UI and use a service agent class to communicate with the service using the service contracts from the shared assembly. So when the user clicks the "Publish" button in the UI, the controller (or command in WPF/SL) calls the Publish method on the service agent passing in the ID of the document to publish. The service agent relays the request to the REST API (Publish operation).
On the server, the REST API is implemented using the same service contracts. In this case, the service works with my domain services, repositories and domain objects to carry out the tasks. So when the Publish service operation is invoked, the service retrieves the Document domain object from the DocumentRepository, calls the Publish method on the object which updates the internal state of the object and then the service passes the updated object to the Update method of the repository to persist the changes.
I am pleased with the outcome as I believe this gives me a more robust and extensible architecture to work with. I can change the ViewModels as needed to support the UI with no concern over poluting the service(s) and, likewise, change the internal implementation of the service operations (domain layer) without affecting the client application(s). All that binds the two are the contracts they share. Pretty clean.
You can serialize your domain objects and then de-serialize them into different types on the client. Both types need to implement the same data contract. All serializable types have at least a default data contract that includes all public read/write properties and fields.
Consider this scenario that two WCF clients connect to one WCF service(server), this service will receive an object from one client and send it to the other one through some operation contract and client callbacks, both clients have the type for this object but we do not want the WCF service(server) to be dependent on this type.
The project is much bigger than this, but I wonder if you can send an object with an unknown type to a service and somehow receive it back on the other client. I saw this but it does not help me at all: Can WCF service transmit type (client doesn't know this type) information?
Thanks in advance.
You can do certain things with the "raw" Message data type - but it's really not pretty programming...
Read about it here:
How to pass arbitrary data in a Message object using WCF
WCF : Untyped messages on WCF operations.
Sending an "object" with unknown type is not possible in WCF because WCF requires a full compatibility with WSDL - and WSDL requires transparent type definition.
Having said that, if you use a type of object I believe there is a way for this to be loaded as a string and in WSDL it is defined as xs:anyType.
I personally would prefer defining the type as string and passing an XML which can be serialised using plain XML Serialization. I have used this in our company and it works really well, especially since we will be storing the XML as document in database.
I'm looking to use the MsmqIntegrationBinding to integrate with a legacy queue which has a serialized object as the message body. Has anyone come up with a way to obtain the "metadata" of the message body and create a service side class to use within the service?
For example, if I put in a serialized Product object from System A and my service needs to consume it, how do I provide MsmqMessage the type if I do not have the Product class on my side? I was thinking of reading off a message in a separate program, deserializing, and then emitting via the code dom. Ideas?
I wholeheartedly recommend against attempting to emit the deserialized type at runtime in the message destination. Either work with the XML at the destination to obtain the data you desire, or build data contracts that both the source and destination can adhere to.
Hmm... in WCF, you can define service methods which take (and optionally return) an untyped Message type. This seems to fit your bill quite nicely.
Other than with strongly typed messages, you'll have to do all the putting together of the message on the client and the taking apart on the server by means of reading the raw XML - but that seems to be what you're looking for, right?
Find more information and samples here:
WCF - Handling Generic Messages
How to pass a generic object through WCF
Untyped messages on WCF
Untyped messages have some restrictions, e.g. you can only read them once on the server, but you should be able to manage your scenario with this, I think.
Marc
Greetings!
I am using a WCF library on an application server, which is referenced by an IIS server (which is therefore the client). I would like to put my validation in a place so that I can just call .Validate() which returns a string array of errors (field too short, missing, etc). The problem is, such functions don't cross the WCF boundary and I really don't want to code the same logic in the WCF service and in IIS/WCF client. Is there a way to use extension methods or something similar so both side can use use a .Validat() method which calls the same code?
Many thanks for any ideas!
Steve
If you control both sides of the wire, i.e. the server-side (service) and the client-side, then you could do the following:
put all your service and data contracts into a shared assembly
reference that "Contracts" assembly from both the server and the client
manually create the client proxy (by deriving from ClientBase<T> or by creating it from a ChannelFactory<T>) - do not use "Add Service Reference" or svcutil.exe!
put all validation logic into a shared assembly
reference that shared validation assembly from both projects
If you want to use a shared validation assembly, you must make sure the data types used on your server and client are identical - this can only be accomplished if you also share service and data contracts. Unfortunately, that requires manual creation of the client proxy (which is really not a big deal!).
If you'd use "Add Service Reference", then Visual Studio will inspect the service based on its metadata, and create a new set of client-side objects, which look the same in terms of their fields and all, but they're a separate, distinct type, and thus you wouldn't be able to use your shared validation on both the server-side and the client-side objects.
Do you have a problem with sending the data over to the server to be validated? In other words, your service interface actually offers the "Validate" method and takes a data contract full of data, validates it and returns a List where T is some kind of custom ValidationResult data contract that contains all the info you need about validation warnings/errors.
In a service architecture, you can't trust the client, who could theoretically be some other company altogether, to have done proper data validation for you. You always need to do it at the service layer and design for communication of those validation issues back to your client. So if you're doing that work at the server anyway, why not open that logic up to the clients so they can use it directly? Certainly the clients can (should) still do some kind of basic input validation such as checking for null values, empty strings, values out of range, etc, but core business logic checks should be shipped off to the service.
i have recently been involved in developing a WCF service which acts as a kind of multicast relay (i.e. accepts some incoming data, does some processing and then sends it off to multiple other external services). this service (which i will refer to as "my service") is fed data by a second internal service.
this data is going to be relayed from my service as XML held in a string. therefore my service could simply accept a string as an parameter to a method request - but this is not ideal as we lose type safety.
the second service has a class that encapsulates all of the information which my service requires to be processed, and eventually relayed to the external services.
the second service exposes this class in it's data contract. Ideally, in order to maintain type safety, and without requiring lots of changes to the second service's implementation, i should accept this type of class as an argument to my service operation.
what would be the best way for me to say in my datacontract that i require this type of class without duplicating code? could i add a service reference to this second class, and then use the proxy class which is created in my data contract?
i just can't get my head around this, even though it seems like a trivial problem!
cheers for any help!
If you are trying to avoid duplication of classes, put your class declaration in its own assembly and share that dll between all parties in the WCF Service. When you create your service reference you can choose which assemblies are shared (assuming you use the VS GUI service utility).
The use of a proxy class might be a good avenue as well. If you expose your main data class as a data contract, then create a proxy of that, the proxy will have a version of the exposed class that can be used by your other services.