Re: XML deserialisation - backwards compatability

[Marc Gravell]

Maybe not straightforward , but: if you add a hidden Area property, and
tell it never to serialize, it should work.
Note I've used C# 3 auto-implemented properties for Width/Height -
just replace with standard properties if you are using C# 2; but don't
use public fields... pretty much "ever".

Marc

using System;
using System.Componen tModel;
using System.IO;
using System.Xml.Seri alization;

[Serializable]
public sealed class MyClass
{
public int Width { get; set; }
public int Height { get; set; }

[EditorBrowsable (EditorBrowsabl eState.Never), Browsable(false )]
public int Area
{
get { return Width * Height; }
set { Width = Height = (int)Math.Sqrt( value); }
}
[EditorBrowsable (EditorBrowsabl eState.Never), Browsable(false )]
public bool ShouldSerialize Area() { return false; }

public override string ToString()
{
return string.Format(" {0}x{1}", Width, Height);
}
}

static class Program
{
static void Main(string[] args)
{
XmlSerializer xs = new XmlSerializer(t ypeof(MyClass)) ;
MyClass item = new MyClass { Width = 3, Height = 5 };

StringWriter writer = new StringWriter();
xs.Serialize(wr iter, item);
string xml = writer.ToString ();

Console.WriteLi ne(xml);

using (StringReader sr = new StringReader(@" <MyClass><Area> 25</
Area></MyClass>"))
{
MyClass oldStyle = (MyClass)xs.Des erialize(sr);
Console.WriteLi ne(oldStyle);
}
using (StringReader sr = new StringReader(@" <MyClass><Width >7</
Width><Height>3 </Height></MyClass>"))
{
MyClass newStyle = (MyClass)xs.Des erialize(sr);
Console.WriteLi ne(newStyle);
}
}
}

[Dylan Nicholson]

Re: XML deserialisation - backwards compatability

On Apr 11, 4:53 pm, Marc Gravell <marc.grav...@g mail.comwrote:

Maybe not straightforward , but: if you add a hidden Area property, and
tell it never to serialize, it should work.
Note I've used C# 3 auto-implemented properties for Width/Height -
just replace with standard properties if you are using C# 2; but don't
use public fields... pretty much "ever".
>
Marc
>

Thanks...I didn't know about the ShouldSerialize ...() trick - that
solved the problem of the deprecated field being written to new files.
But ideally I don't want the deprecated field to be public either.
Not a biggie, though.
As for public fields - in this case the class is just a holder for
fairly raw data, where no particular validation is needed.
And the beauty of C# is you can change public fields to properties at
any point and existing code will continue to compile.

[Marc Gravell]

Re: XML deserialisation - backwards compatability

But ideally I don't want the deprecated field to be public either.

Well, it is pretty well hidden by the two attributes - but ultimately
XmlSerializer works on public properties. If you have .NET 3.0,
another option is DataContractSer ializer (shown below), since this can
serialize private members; and a third option is custom serialization
(IXmlSerializab le), but this is considerably more work.

As for public fields - in this case the class is just a holder for
fairly raw data, where no particular validation is needed.

Then in VS2008 / C# 3, auto implemented properties are definitely the
way to go.

And the beauty of C# is you can change public fields to properties at
any point and existing code will continue to compile.

That isn't entirely true... first: it forces you to rebuild everything
that references it, and second: there are cases when this simply isn't
true; two obvious examples come to mind (there are others):

* using the field as a "ref" or "out" parameter to a method [can't use
properties as "ref" or "out"]
* if the field represents a mutable struct [which you shouldn't have
anyway], and you are changing nested properties [change gets discarded
with a property]

It will almost certainly break binary serialization, too.

Marc

## DataContract example ##

using System;
using System.IO;
using System.Runtime. Serialization;
using System.Xml;

[DataContract(Na mespace="")]
public sealed class MyClass
{
[DataMember(IsRe quired = false)]
public int Width { get; set; }
[DataMember(IsRe quired=false)]
public int Height { get; set; }

[DataMember(Name ="Area", IsRequired=fals e,
EmitDefaultValu e=false)]
private int? AreaFacade
{
get { return null; }
set {
if (value.HasValue )
{
Width = Height = (int)Math.Sqrt( value.Value);
}
}
}

public override string ToString()
{
return string.Format(" {0}x{1}", Width, Height);
}
}

static class Program
{
static void Main(string[] args)
{
DataContractSer ializer xs = new
DataContractSer ializer(typeof( MyClass));
MyClass item = new MyClass { Width = 3, Height = 5 };

StringWriter writer = new StringWriter();
using (XmlWriter xw = XmlWriter.Creat e(writer))
{
xs.WriteObject( xw, item);
xw.Close();
}
string xml = writer.ToString ();

Console.WriteLi ne(xml);

using (XmlReader xr = XmlReader.Creat e(new
StringReader(@" <MyClass><Area> 25</Area></MyClass>")))
{
MyClass oldStyle = (MyClass)xs.Rea dObject(xr);
Console.WriteLi ne(oldStyle);
}
using (XmlReader xr = XmlReader.Creat e(new
StringReader(@" <MyClass><Heigh t>3</Height><Width>7 </Width></
MyClass>")))
{
MyClass newStyle = (MyClass)xs.Rea dObject(xr);
Console.WriteLi ne(newStyle);
}
}
}