Every time I meet this attribute, I always see such usage:
[DataContract]
class DataTransferObject
{
[DataMember]
public int Value {get;set;}
}
And in this example all inherited members should apply DataMember attribute to every property or field, this can lead to VERY clumsy and poilerplate code. But, recently I found (maybe secret feature?) a very elegant way of using it:
[DataContract]
public abstract class DTOBase
{
}
public class MyDTO : DTOBase
{
public int Value {get;set;}
public MyDTO(){} //important part is here
}
Important part: You should always explicitly define parameterless constructor, otherwise it won't serialize properly.
And yeah. It will serialize all its public members, no matter how deep will be inheritance, without need to apply attributes to members or class definitions.
Is this somehow documented somewhere (I didn't found)? Because, I were very supprized how much of boilerplate can be avoided.
Actually, you don't need to use DataContract and DataMember attributes if you don't want to, however they give you flexibility in defining what needs to be serialized and how.
I suggest starting with article Serializable Types on MSDN, it has a lot of information how Data Contract serializer works. Here are first 2 paragraphs, proving that you don't need to use attributes:
By default, the DataContractSerializer serializes all publicly visible
types.
All public read/write properties and fields of the type are
serialized. You can change the default behavior by applying the
DataContractAttribute and DataMemberAttribute attributes to the types
and members This feature can be useful in situations in which you have
types that are not under your control and cannot be modified to add
attributes. The DataContractSerializer recognizes such "unmarked"
types.
The main rules that apply to your case are:
DataContract attribute is not inherited. You can either apply it or not on your base class DTOBase, it is ignored in child class MyDTO. You can remove DataContract attribute from DTOBase class and result will be the same.
If you use DataContract attribute on a class, then only members that have DataMember attribute will be serialized. This is what happened in class DataTransferObject in your first sample.
If you do not use DataContract attribute on a class, then all public members of a class are serialized. This is what happened with your class MyDTO.
Related
I was trying to create an attribute that implies [Serializable] but I noticed that this SerializableAttribute class is sealed.
In Java it was possible to create an interface (say, MyInterface) that is inherited from Serializable interface and so all the subclasses of MyInterface would also be serializable, even its sub-sub classes would be so.
Let's say I am creating an ORM and I want customers to annotate their entity classes as [DatabaseEntity] but in order to make sure that entities are serializable, I also need to ask them to attribute their classes with extra [Serializable] which does not look quite compact and neat.
I am wondering why SerializableAttribute class is sealed and why has Inherited=false which implies that subclasses of serializable class will not be serializable unless it is explicitly stated. What motives are behind these design choices?
The SerializableAttribute is only used by the BinaryFormatter. If you are writing your own serialiser then don't worry about.
The sealed keyword is applied to the attribute not the class associated with the attribute. It is saying that the SerializableAttribute cannot be subclassed.
The BinaryFormatter uses an opt-in model. Any class (or subclass) must specify that it is serializable. This why the Inherited=false is used.
It's suggested best practice that all .Net attributes should be sealed, according to Microsoft:
The .NET Framework class library provides methods for retrieving custom attributes. By default, these methods search the attribute inheritance hierarchy; for example System.Attribute.GetCustomAttribute searches for the specified attribute type, or any attribute type that extends the specified attribute type. Sealing the attribute eliminates the search through the inheritance hierarchy, and can improve performance. [my emphasis]
So [Serializable] is sealed because it's quicker for .Net reflection to check the attributes. The cost is that you can't inherit and extend SerializableAttribute.
You can make your own un-sealed attributes if you want (you'll get code analysis warnings though).
This gets a little confusing with how attributes are used in inheritance for the classes that they apply to. It's probably best to use an example:
[Serializable]
public class A
{
public int SimpleSerialisableProperty { get; set;}
}
public class B : A
{
public C ComplexReferenceProperty { get; set; }
}
[Serializable]
public class D : A
{
public bool AnotherSerialisableProperty { get; set;}
}
You asked why SerializableAttribute.Inherited = false and this is why:
Class A is marked as [Serializable], and it is.
However class B inherits A and extends it with properties that are not serialisable. If .Net tries to serialise B it will encounter an error.
That Inherited = false tells .Net that just because A has been marked as [Serializable] not every class that inherits it will be serialisable too.
Now class D inherits A and is serialisable, so it gets its own [Serializable] attribute.
Finally, in terms of design attributes are a great way of extending behaviour (nice UI editors in property grids, etc). However they are terrible at enforcing it. If you need your customers to implement their entity classes in a particular way then an abstract base class or an interface is a much better way to go. If you make it an attribute then you're basically letting them know that [Serializable] is an option that you can handle either way.
Serialization is not a magic thing and you don't need any attribute to serialize an object. It is a process of writing your class' properties and fields to a stream (and attributes are only directives to serializers about how to behave while outputting an object).
See this over-simplified serializer code which totally ignores all attributes including NonSerializable
object obj = yourObject;
var props = obj.GetType()
.GetProperties()
.ToDictionary(p => p.Name, p => p.GetValue(obj, null));
string serializedText = String.Join("\n",
props.Select(kv => kv.Key + "=" + kv.Value ?? kv.Value.ToString()));
Above code, for example, would give
IsEmpty=False
X=3
Y=5
for object obj = new Point(3,5);
Deserialization process would be to read these values and set the properties back accordingly.
Put the [Serializable] attribute on top of the class you want serialized. Serialization is opt-in process. You have to manually do that for each class you want serialized. There are bunch of other keywords.
I find the [DataContract] and [DataMember] attributes a bit messy and would rather do this with code in a config method or something. Is this possible?
You don't have to use these attributes at all. DataContractSerializer will serialize all public properties with getter and setter but in case of serializing entities with navigation properties you will easily end with exception due to "cyclic reference".
To avoid that exception you must either use [DataContract(IsReference = true)] on your entity class with DataMember on every property you want to serilize or IgnoreDataMember on every property you don't want to serialize.
The last and the most complex option is avoiding attributes completely and custom classes implementing IDataContractSurrogate to control serialization outside of the type.
You can also write your completely custom serialization process or use XML serialization or binary serialization with all its requirements.
No, the DataContractSerializer is an opt-in serializer - you have to tell it what you want included.
With other serializers you need to use things like NonSerializedAttribute or XmlIgnoreAttribute to tell the serializer to leave things alone.
I know this is a rather old post, but I came here thinking the same thing if there is a way to set all member attributes automatically on some legacy code with public fields and no getters and setters.
What makes it look just a little bit less messy is shortening up the name DataMember:
using DM = System.Runtime.Serialization.DataMemberAttribute;
[DataContract]
public class SomeClass
{
[DM] public bool IsMO;
[DM] public string LabCode;
[DM] public string OrderNumber;
}
So I've been studying the use of various Serializers in the .NET Framework and while trying to experiment on preventing certain objects in a class from being serialized I was thrusted back to some very basic programming questions that I "thought" I knew. Given this example:
public class Example
{
public string examName;
[XmlIgnore]
public int exampleNumber;
public Example()
{ }
[XmlIgnore]
public int ExampleNumberTwo { get; set; }
}
I can create an instance of this class and using the XMLSerializer can output the content of this class in XML format. The [XmlIgnore] attribute actually does what I'd expected; it prevents the serialization of the referenced items.
So venturing further I replaced the [XmlIgnore] declaration for "exampleNumber" with [NonSerializable] expecting the similar results but the output did not change. After searching through resources, it was stated that the [NonSerializable] attribute should only be used on fields and [XmlIgnore] attributes should be used on properties.
Yet another post stated that the [NonSerializable] attribute has no effect when using the XMLSerializer but will produce the expected results when using the SOAP or BinaryFormatter. So I'm lost on the concept at this point.
But this brought me to the basic question, what defines a field vs. a property? I know its a basic question and I've even viewed other discussions here but the degree of clarity I am looking for still wasn't really clear.
I can use the [XmlIgnore] attribute on the property (ExampleNumberTwo) or the variable (exampleNumber) so the statement that it can ONLY be used on Properties doesn't seem correct.
But then again, I have always referred to the objects in my example such as (examName) and (exampleNumber) as being member variables. So what exactly is the signature of a "Field"
Can anyone shed some light on this?
The MSDN documentation supports the idea that [NonSerialized] only gives the expected results with the binary and SOAP serializers:
When using the BinaryFormatter or SoapFormatter classes to serialize
an object, use the NonSerializedAttribute attribute to prevent a field
from being serialized. For example, you can use this attribute to
prevent the serialization of sensitive data.
The target objects for the NonSerializedAttribute attribute are public
and private fields of a serializable class. By default, classes are
not serializable unless they are marked with SerializableAttribute.
During the serialization process all the public and private fields of
a class are serialized by default. Fields marked with
NonSerializedAttribute are excluded during serialization. If you are
using the XmlSerializer class to serialize an object, use the
XmlIgnoreAttribute class to get the same functionality. Alternatively,
implement the ISerializable interface to explicitly control the
serialization process. Note that classes that implement ISerializable
must still be marked with SerializableAttribute.
In terms of "field" vs. "property", fields are straight data variables contained by a class. Properties are actually specially named methods on the class (get_PropName() and set_PropName()). In your code, the compiler allows you to use properties the same way you would use a field, and then inserts the appropriate get/set call for you.
Oftentimes, properties will be simple wrappers around a field:
private int myField;
public int MyProperty
{
get { return myField; }
set { myField = value; }
}
But they don't have to be:
public int TodaysDate
{
get { return DateTime.Today; }
}
In general, you want all your fields to be private, since they're supposed to be implementation details. Any simple data that you'd like to expose should be done via a property, since you can easily surround the data access with (changeable) logic.
In C#, the short answer is that properties have get and/or set methods, while fields do not. VB.NET makes it a little more evident by requiring the "Property" qualifier to be used to differentiate one.
With C#, you can just append " { get; set; }" to the end of a field's definition and it's now a property.
Where this really comes into play is in reflection. Fields and Properties are segregated from one another into different enumerable collections.
This answer to What are the differences between the XmlSerializer and BinaryFormatter will help you get started in the right direction.
I have a whole list of entity classes which I need to make Serializable (due to storing session state in SQL, but that's another story).
I have added the attribute [Serializable] and all seems to be fine.
All of my entity classes extend from the same base class.
If I mark the base class as Serializable, does this mean all children are marked as Serializable too?
Thanks
No, attribute is not inherited.
When you extend the class, it's possible to add features that might not be serializable by nature therefore .NET framework cannot assume for you that everything what extends serializable base class is also serializable.
That's why you must explicitly state [Serializable] attribute on every class individually.
Nope, each one will have to be marked as [Serializable] specifically.
Also if you intend to serialize an object to XML which is of a derived type as though it is the base type you'll also need a [XmlInclude] attribute.
EG:
[Serializable]
public class BaseClass : ParentClass
{
}
[Serializable]
[XmlInclude(typeof(BaseClass))]
public class ParentClass
{
}
(Binary serialization, like what is used for sessions, do not need this)
I've been reading a text about an extension to C# and at one point it says that "An attribute decoration X may only be applied to fields of type Y."
I haven't been able to find a definition for attribute decoration, and I'm not making much sense out of this by exchanging the two.
It's probably referring to the Attribute class. For example, you can mark a type as serializable via the SerializableAttribute. When you apply an attribute, you can leave off the "Attribute" suffix.
[Serializable]
public class SomeClass {
}
Attributes provide a means to add meta-data about the code.
Attributes are used to add metadata to .NET (C#) code in a structured manner. What a lot of people don't realise, though, is that there are actually two types of attribute.
The simplest is custom attributes, where you define an attribute that specific classes look for to alter the way they work. A common example is the System.Xml.Serialization attributes which are read by the XmlSerializer to alter its output, e.g. a class could be marked up something like the following to specify its namespace and that the field should be an attribute:
[XmlType(Namespace = "http://mycompany.com/")]
public class MyClass
{
[XmlAttribute]
public string MyField;
}
Custom attributes like this have no meaning to the compiler or the runtime, they are just added to the class as part of its metadata, and can be retrieved by a call to Type.GetCustomAttributes.
The other main group of attributes is pseudo-custom attributes, which actually have meaning to either the compiler or the runtime. The example in the post by Haacked with SerializableAttribute is actually an example of a pseudo-custom attribute. It is actually stored as part of the type definition and cannot be retrieved using Type.GetCustomAttributes. You cannot create your own pseudo-custom attributes.
So it's likely what you're dealing with here is a custom attribute which is being looked for by a specific tool.