WCF Extensibility – Serialization Callbacks – Carlos Figueira MSDN blog. This post is part of a series about WCF extensibility points. For a list of all previous posts and planned future ones, go to the index page. Continuing on the topic about serialization, the serialization callbacks are methods which, when tagged appropriately, are called by the serializer during the serialization or deserialization of an object of the type where the methods are defined. Those callbacks predate WCF, being part of the . NET Framework since its 2. Binary. Formatter and the Soap. Formatter (interestingly, the Xml. Serializer, the first general- purpose serializer from the framework, doesn’t implement those, possibly because it already gives a lot of control to the user with types such as those which implement the IXml. Note; Json.NET attributes take precedence over standard.NET serialization attributes (e.g. WCF Data Contract Attributes This article will give you the details of wcf data contract attributes like Name, Namespace. DataContractSerializer Basics. To set up a class for serialization. This sample shows how.NET Framework attributes such as T:System.Runtime.Serialization.DataContractAttribute, T:System.Runtime.Serialization. WCF CollectionDataContract and DataMember attributes. We can mark our collection as DataContract and wrap another. The main purpose of this article is to discuss some of the uses for XML Serialization, and explore some of the basics of using the XmlSerializer and the attributes in. WCF Services : Data Contract - Attributes. If the type already contains DataMember attributes (but not a DataContract. Applying the serialization event attributes. Serializable or ISerializable interfaces). The idea behind those callbacks is that they’re invoked in many places during the serialization / deserialization of an object to allow some user initialization / finalization code to take place. Below is some code which shows the the “canonical” example of those callbacks. For types which are to be serialized, if you decorate an instance method with any of the serialization callback attributes (On. Remember that DataContract makes serialization opt-in, so you will also need to add DataMember attributes to the properties: DataContract Serialization is not working #146. So for the DataContract and DataMember attributes I'm not able to change the Order parameter value. System.Runtime.Serialization.DataContract. Initializes a new instance of the SqlRuleAction class with the specified SQL expression and. Datacontract Do Not SerializeDeserialized. Attribute, On. Deserializing. Attribute, On. Serialized. Attribute, On. Serializing. Attribute). ![]() The method must not have a return type (“void” in C#, Sub in VB), and it must have a single parameter of type Streaming. Context. When an object of the type My. Object is about to be serialized, its On. Serializing method is called (by the way, the method names do not have to match the attribute names, it’s just a convention that I like to use). After it’s been written out, its On. Serialized method is called. When an instance of that type is about to be deserialized from a stream (or a reader), its On. Deserializing is called, and finally when all its members from the stream have been deserialized, On. Deserialized is called. This parameter is never used by the main WCF serializers – in the Data. Contract. Serializer, Xml. Serializer and Data. Contract. Json. Serializer, the parameter is always an object with the Context property set to null (Nothing in VB) and the State property set to Streaming. Context. States. All). For the Net. Data. Contract. Serializer, you can specify an object while creating the serializer, and it will be passed to the callbacks when they’re called. But really, I’ve never seen it being used in the context of WCF. Ok, this is interesting, we can know when an object of the type is being serialized / deserialized. But why do we really need those callbacks, why do we care when the objects are being serialized or deserialized? Shouldn’t we simply decorate the types with the appropriate attributes to get full advantage of the declarative serialization and not have to do anything else? In most cases, this is true. Those attributes aren’t used in most instances, but there are some scenarios in which they come out handy. Here are some of the scenarios, along with how the serialization attributes can be used to solve the issues. Object Initialization. This is a commonissuefromtheforums: the type has some initialization (field initializer or constructor), but when the type is deserialized, those initializers are not applied. When an instance of the type below, for example, is read, the Name field is properly initialized from the data from the serialized stream, but the fields ID and Friends are set to their default values (0 and null / Nothing, respectively). Instead, the serializers use the Formatter. Services. Get. Uninitialized. Object method to create an empty instance of the object, with all of its members set to the default value for their type. This is really counter- intuitive for most people, since the constructor is the first “line- of- defense” for the type to maintain internal consistency (i. So why do the serializers skip that? Well, the WCF serializers could have called the constructor. That’s exactly what the Xml. Serializer does – it only works with classes which contain a public, parameter- less constructor. Other serializers which predated WCF (the . The advantage of the latter model is that it supports the serialization of most types, even those which don’t have a parameter- less constructor, which is why I think that model was chosen. A common follow up question: but why not call any constructor in the type, even if it meant passing some parameters to the constructor? That opens a new set of problems – how can we guarantee that the parameters are correct (that won’t cause the constructor to throw an exception)? What if there are multiple constructors, which one to choose? Any heuristic chosen by the WCF team would be wrong in some scenarios, so this option doesn’t work. Yet another follow up question: if the type has a default constructor, why doesn’t WCF invoke it? The answer for this one in consistency – once you go in the constructor- less route, you should go all in, or not go at all, so I think this decision makes sense. So I hope I have convinced you why the serializers skip the constructor (or at least given you a good reason why they don’t), the problem still exists. What to do if we want to initialize the object? That’s the main purpose of the On. Deserializing. Attribute. Right after the uninitialized object is created, the serializer will call any method in the type which is decorated with this attribute. There we can initialize any members which are needed, before the fields from the stream are read into the object. The type Person can be rewritten to be deserialization- aware as shown below. The types which are not decorated with anything (a. Plain Old Clr Object, or POCO types) behave differently – those are types which the creator didn’t think about serialization explicitly by decorating them appropriately. In this case, the parameter- less constructor is always invoked (if the type doesn’t have one, it isn’t serializable at all) when it’s being deserialized. Other kinds of objects also have their constructors called: types which implement the IXml. Serializable interface (it needs to have a parameter- less constructor), and types which implement the ISerializable interface (during deserialization constructor with a Serialization. Info and a Streaming. Context parameters is called). Object state validation. Besides initialization, another function of the constructor is to guarantee that the parameters passed to it are valid, and throw an exception if it’s not the case. In the example below, all three parameters are validated, and we also have a cross- parameter validation between two parameters. The problem with this approach is, the constructor is not invoked during serialization, so that logic needs to be moved somewhere else. That works for types decorated with . Normally we’d simply say to use . That’s where On. Deserialized. Attribute enters. When all the members of the object have been deserialized, the method decorated with this attribute will be invoked. There we can centralize the validation for serialization scenarios. Date. Time objects, for example, are always serialized in a fixed format (e. T1. 0: 3. 8: 3. 9. I’m writing this paragraph). If you want to change it to a different format (i. Date. Time type. One possible solution is to use a property to do the conversion, and store the field to be used in the data contract as a string, as in the example below. One alternative is to only do the conversion during serialization episodes. Prior to the serialization of the type (On. Serializing), the code would set the serializable member, and after the deserialization of the members are complete (On. Deserialized), we store the value from the stream into the public field. By populating it prior to the deserialization, we ensure that we have something which can be converted. Other scenarios. There are many other scenarios for which the callbacks can be useful, as some listed below. Object locking: the serialization of an object happens in a single thread, but if other threads are being executed and can modify the object causing it to be in an invalid state, we may want to protect it from modifications while it’s being serialized. On the On. Serializing callback we’d acquire a mutex (which is used in other methods / properties which modify the object), and it would only be released on the On. Serialized callback. Backup: The “archive flag”, which was common in the DOS world, would be set in a file whenever the file was changed (so it needed to be backed up), and reset whenever a backup was done for that file (or vice- versa, I can never remember it exactly). We can have something similar in serialization: if a property in the object is changed, the “archive flag” is set, and when the object is persisted (via serialization), an On. Serialized callback would reset that flag. Timestamp: to differentiate between two serialized versions of the same object, an On. Serializing callback can be used to save the current time in a serializable member, and this would be saved along with the object. There are many others, I’ll leave it for your imagination (or any real problems) to come up with those. Final thoughts about serialization callbacks. A few things about serialization callbacks which I think are important to mention: Collection types: the serialization callbacks do not work on collection types. This is a common request which may be implemented in a future release of the . NET Framework, but as of 4. Inheritance: if both base and derived types have a method decorated with the serialization callbacks, first the one from the base type is called, then the one from the derived type is called. This is the same for both serialization and deserialization callbacks. Xml. Serializer: the XML Serializer does not honor the serialization callbacks. The fact that the default constructor is called in this serializer solves many of the issues which are handled by the callbacks.
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