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How to serialize an IList<T>?
I wish to serialize an class (let's call it S) that contains a property of the type IList<T> where T is another class which I have defined. I get an exception when I attempted to serialize an instance of class S to XML. This is understandable as the XmlSerializer doesn't know which concrete class to use. Is there a way, hopefully using attributes, to specify which concrete class to instantiate when serializing/deserializing an instance. My implementation of class S creates a instance of the List<T> class. Here is some code to illustrate my example:
using System;
using System.Xml.Serialization;
using System.IO;
[Serializable]
public class T { }
[Serializable]
public class S
{
public IList<T> ListOfTs { get; set; }
public S()
{
ListOfTs = new List<T>();
}
}
public class Program
{
public void Main()
{
S s = new S();
s.ListOfTs.Add(new T());
s.ListOfTs.Add(new T());
XmlSerializer serializer = new XmlSerializer(typeof(S));
serializer.Serialize(new StringWriter(), s);
}
}
I'm hoping there's an attribute I can place above the ListOfTs definition that says to the serialize, "assume an instance of List<T> when serializing/deserializing".
Change the
public IList<T> ListOfTs { get; set; }
to
public List<T> ListOfTs { get; set; }
Then it will work.
I think there's 2 ways that are generally approached for this kind of problem.
One would be to use DataContractSerializer, but since you're already using XmlSerializer, you probably don't want to do that.
The "alternative" is to have a property that is used specifically for the purpose of serialization, and to ignore the IList during serialization.
[Serializable]
public class S
{
IList<T> _listofTs;
[XmlIgnore]
public IList<T> ListOfTs { get _listofTs; set _listofTs = value; }
[XmlElement(Name="Not Exactly the Greatest Solution!")]
public List<T> ListOfTs { get _listofTs; set _listofTs = value; }
public S()
{
ListOfTs = new List<T>();
}
}
You can implement the IXmlSerializable interface and implement the serialization logic manually using Xml readers and writers.
Related
I want to create a method that serializes lists of different childs. Each child gets his own XML file. Doing this for a single object was easy:
interface ISerializable
{
}
public class Item : ISerializable
{
public string name { get; set; }
public int number { get; set; }
}
public class Weapon : ISerializable
{
public string name { get; set; }
public int damage {get; set;}
}
public static void SerializeToXML(ISerializable child)
{
XmlSerializer serializer = new XmlSerializer(child.GetType());
using (TextWriter textWriter = new StreamWriter("test.xml"))
{
serializer.Serialize(textWriter, child);
}
}
I can put anything derived from ISerializable into the serialize method to get a desired result. However when give List<ISerializable> as possible argument it does not compile Cannot convert List<item> to List<ISerializable>. Anyway to solve this?
public static void SerializeListToXML(List<ISerializable> listToXML)
{
XmlSerializer serializer = new XmlSerializer(listToXML.GetType());
using (TextWriter textWriter = new StreamWriter("test.xml"))
{
serializer.Serialize(textWriter, listToXML);
}
}
The base reason i'm doing this is to get XML templates of much larger structures so i an manually add XML to load into my program. I figured i would benefit from creating a serializer first so i have the correct XML structure to start with. Perhaps this can come in handy later down the line for saving user data too.
SerializeListToXML should take an IEnumerable<ISerializable> instead of a List<Item>.
A List<Item> cannot be casted to List<ISerializable>. If you could to that, then you'd be able to add any ISerializable to the list, not just Items. With an IEnumerable<>, you cannot modify the collection - only iterate over it. Since SerializeListToXML does not need to modify the collection, it can accept an IEnumerable<ISerializable>, which is less restrictive. Please see the following section and sub-sections about covariance and contravariance: Covariance and Contravariance
I would like to be able to do something like this :
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Test
{
public interface IFoo
{
IEnumerable<int> integers { get; set; }
}
public class Bar : IFoo
{
public List<int> integers { get; set; }
}
}
Why does the compiler complains..?
Error 2 'Test.Bar' does not implement interface member 'Test.IFoo.integers'. 'Test.Bar.integers' cannot implement 'Test.IFoo.integers' because it does not have the matching return type of 'System.Collections.Generic.IEnumerable<int>'.
I understand that the interface says IEnumerable and the class uses a List, but a List is an IEnumerable.....
what can I do? I do not want to specify IEnumerable in the class, I want to use a concrete type that implements IEnumerable, like List...
This is a Type Covariance/Contravariance issue (see http://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#C.23 ).
There's a workaround: use explicit interfaces, like so:
public class Bar : IFoo {
private IList<int> _integers;
IEnumerable<int> IFoo.integers {
get { return _integers };
set { _integers = value as IList<int>; }
}
public IList<int> integers {
get { return _integers; }
set { _integers = vale; }
}
}
Note that integers should be TitleCased to conform to .NET's guidelines.
Hopefully you can see the problem in the code above: IList<int> is compatible with IEnumerable<int> only for the accessor, but not for setting. What happens if someone calls IFoo.integers = new Qux<int>() (where Qux : IEnumerable<int> but not Qux : IList<int>).
Although List implements IEnumerable that's not the way interfaces work. The interface specifies exactly which types need to be exposed for properties. If you created a generic interface like
public interface IFoo<T> where T : IEnumerable<int>
{
T integers { get; set; }
}
You could then use IFoo<List<int>> to implement it in the way you expect.
You're not going to be able to use a concrete type unless you do it behind the scenes. The problem is that you can both Get and Set the Property.
Your interface specifies that the property is of type IEnumerable<int>. HashSet<int> implements IEnumerable<int>. That means the following should work just fine:
IFoo instance = new Bar();
instance.integers = new HashSet<int>();
But since you're trying to implement the interface using the concrete type List<int>, there's no way that assignment can work.
The easiest fix, assuming you don't constantly need to re-assign the collection, would be to only specify a getter for the collection:
public interface IFoo
{
IEnumerable<int> Integers { get; }
}
public class Bar
{
public List<int> Integers { get; private set; }
public Bar(List<int> list)
{
Integers = list;
}
}
My question is so simple.
Imagine we have a Foo class. and has a T property.
public class Foo<T>
{
public T Property { get; set; }
}
I want to implement in it IXmlSerializable and my problem is I have no idea to imitate the standard XmlSerialization. I really have no idea about how to write this property like the standart XmlSerlalization.
UPDATE: I did this and it's not working
public class Foo<T> : IFoo
where T : IXmlSerializable
{
public T Value { get; set; }
}
public class FooA : Foo<string> { }
public class FooB : Foo<int> { }
public class FooC : Foo<List<Double>> { }
When I serialized this classes (A, B and C). I wanna have something like this:
Using first class:
<FooA>
<Value>StringDemo</Value>
</FooA>
Using second class:
<FooB>
<Value>2</Value>
</FooB>
Using third class:
<FooC>
<Value>
<ArrayOfDouble xsi:..>
<Double>3.1416</Double>
<Double>4.2</Double>
</ArrayOfDouble>
</Value>
</FooC>
Something like this is what I wanted, I don't want to implement in all of this a custom XmlSerializer, I mean the default which the standard serialization use.
You would need to implement IXmlSerializable in Foo<T>, and T (using where T : IXmlSerializable. The implementation in Foo<T> would then be responsible for writing a wrapper element with type information (in WriteXml) and call WriteXml on T, and recreate the instance from the type information (in ReadXml) and call ReadXml on T.
Basic types like String and Double is not IXmlSerializable, so you cant have where T : IXmlSerializable if you want to support them. You can also provide default implementations of ReadXml and WriteXml. This works since you need to create a Foo<T> before calling ReadXml, forcing you to know T.
public class Foo<T> : IXmlSerializable {
public T Value { get; set; }
XmlSchema IXmlSerializable.GetSchema() {
return GetSchema();
}
protected virtual XmlSchema GetSchema() {
return null;
}
public virtual void ReadXml(XmlReader reader) {
reader.ReadStartElement();
var xmlSerializer = new XmlSerializer(typeof(T));
Value = (T)xmlSerializer.Deserialize(reader);
reader.ReadEndElement();
}
public virtual void WriteXml(XmlWriter writer) {
var xmlSerializer = new XmlSerializer(typeof(T));
xmlSerializer.Serialize(writer, Value);
}
}
Any good sample on how to serialize list of generic objects with abstract base class. Samples with non abstract base class are listed in XML Serialize generic list of serializable objects. My base class is similar to Microsoft.Build.Utilities.Task
Another alternative is to use the XmlElementAttribute to move the list of known types to the generic list itself...
using System;
using System.Xml;
using System.Xml.Serialization;
using System.Collections.Generic;
public abstract class Animal
{
public int Weight { get; set; }
}
public class Cat : Animal
{
public int FurLength { get; set; }
}
public class Fish : Animal
{
public int ScalesCount { get; set; }
}
public class AnimalFarm
{
[XmlElement(typeof(Cat))]
[XmlElement(typeof(Fish))]
public List<Animal> Animals { get; set; }
public AnimalFarm()
{
Animals = new List<Animal>();
}
}
public class Program
{
public static void Main()
{
AnimalFarm animalFarm = new AnimalFarm();
animalFarm.Animals.Add(new Cat() { Weight = 4000, FurLength = 3 });
animalFarm.Animals.Add(new Fish() { Weight = 200, ScalesCount = 99 });
XmlSerializer serializer = new XmlSerializer(typeof(AnimalFarm));
serializer.Serialize(Console.Out, animalFarm);
}
}
... which will also result in a better looking XML output (without the ugly xsi:type attribute)...
<?xml version="1.0" encoding="ibm850"?>
<AnimalFarm xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<Cat>
<Weight>4000</Weight>
<FurLength>3</FurLength>
</Cat>
<Fish>
<Weight>200</Weight>
<ScalesCount>99</ScalesCount>
</Fish>
</AnimalFarm>
It is often useful to have abstract classes with several derived types to allow use of strongly typed lists and the such.
For example you might have a DocumentFragment class which is abstract and two concrete classes called TextDocumentFragment and CommentDocumentFragment (this example from Willis).
This allows the creation of a List property which can contain objects only of those two types.
If you attempt to create a WebService that returns this list you get an error but this is easy to get around with the code below....
[Serializable()]
[System.Xml.Serialization.XmlInclude(typeof(TextDocumentFragment))]
[System.Xml.Serialization.XmlInclude(typeof(CommentDocumentFragment))]
public abstract class DocumentFragment {
...}
The XmlInclude attributes tell the class that it might be serialized to those two derived classes.
This generates an attribute in the DocumentFragment element specifying the actual type, as below.
<DocumentFragment xsi:type="TextDocumentFragment">
Any additonal properties specific to the derived class will also be included using this method.
I am trying to create a base class where I can inherit from it (to add properties to the derived classes) and the utilized the Load and Save methods from the base class. I find myself writing the Load and Save over and over and I'd like to apply some DRY to it...
namespace Common
{
using System;
using System.IO;
using System.Xml.Serialization;
public abstract class ApplicationSettings
{
protected ApplicationSettings()
{
}
public static ApplicationSettings Load(string fileName)
{
if (!File.Exists(fileName))
{
return null;
}
XmlSerializer serializer = new XmlSerializer(typeof(ApplicationSettings));
using (StreamReader reader = new StreamReader(fileName))
{
ApplicationSettings param = (ApplicationSettings)serializer.Deserialize(reader);
reader.Close();
return param;
}
}
public void Save(string fileName)
{
XmlSerializer serializer = new XmlSerializer(typeof(ApplicationSettings));
using (StreamWriter writer = new StreamWriter(fileName))
{
serializer.Serialize(writer, this);
writer.Close();
}
}
}
}
Given this abstract class, I then want to derive a class such as:
namespace Common
{
using System;
public class ApplicationParameters : ApplicationSettings
{
public ApplicationParameters()
{
}
public string AuthorizationCode
{
get;
set;
}
public string ReferenceNumber
{
get;
set;
}
}
}
For the Derived class, I should be able to do something like
ApplicationParameters parameters =
ApplicationParmeters.Load("settings.xml");
However, in the implementation above, an compiler error occurs when I attempt to cast the ApplicationSettings to the ApplicationParameters class when I call the Load method in the base class.
Is there a way to do this?
Try replacing typeof(ApplicationSettings) with GetType().
Using this mechanism you will also tell the serializer that ApplicationParameters is a child class of ApplicationSettings. You do this via XmlInclude
[XmlInclude(typeof(ApplicationParameters))]
class ApplicationSettings
The latter is a requirements of the serializer because otherwise it won't know what class to instantiate.
Why are you using XmlSerializer ?
Unless you must control the way the output XML looks, DataContractSerializer is recommended
See here, for example
Make the top level class generic so that the Save/Load methods can support multiple types:
public abstract class ApplicationSettings<T>
{
public static T Load(string xml){ // Implementation }
public static void Save (T obj) { // Implementation }
}
public class ApplicationParameters : ApplicationSettings<ApplicationParameters>
{
}
Or you could just make the static methods themselves generic:
public abstract class ApplicationSettings
{
public static T Load<T>(string xml){ // implementation }
public static void Save<T>(T obj){ // implementation }
}
You will now notice that the Save/Load methods from the abstract parent class are strongly typed to the child so that the following line will work as expected:
ApplicationParameters parameters = ApplicationParameters.Load("settings.xml");
or
ApplicationParameters parameters =
ApplicationSettings.Load<ApplicationParameters>("settings.xml");
Depending on which method you use.
How about a constructor in your ApplicationParameters class that takes an ApplicationSettings as an argument and copy the shared properties from one to another? And then just set the not shared properties to be null or the default...