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<h1>Object Clone</h1>
<p>Platform implementations of #clone is not compatible with <span class="MCTextPopup"><a href="javascript:void(0);" class="MCTextPopupSpot" onclick="FMCTextPopup( event, this ); return false;">TP<img style="border: none;margin-left: 5px;" src="../../../../SkinSupport/ExpandingClosed.gif" MadCap:altsrc="../../../../SkinSupport/ExpandingOpen.gif" class="MCExpandingIcon" onload="if ( typeof( FMCPreloadImage ) == 'function' ) { FMCPreloadImage( '../../../../SkinSupport/ExpandingOpen.gif' ); }" /></a><span class="MCTextPopupBody" style="display: none; ">Transparent Persistence</span></span>.
</p>
<p>Both java and .NET object implementations provide
<code>#clone</code> method for default objects, which is enabled by
implementing <code>Cloneable/ICloneable</code> interface. This
implementation is a shallow clone, i.e. only the top-level object
fields are duplicated, all the referenced(children) objects are only
copied as references to the same object in the parent clone. But how it
affects Transparent Persistence?</p>
<p>If you remember
<a href="../transparent_persistence_implementation.htm">Transparent Persistence Implementation</a>you must know that a special <code>Activator</code> field is used to
bind an object to the object container. Consequently, the default clone
will copy this <code>Activatable</code> field to the object's
duplicate, which will produce ambiguity as the object container won't
know which object should be activated for write.</p>
<p>Let's look how it will affect db4o in practice. We will use a usual
<a href="car.htm">Car</a> class and make it cloneable. Use the following code to store a
car object and it's clone: </p>
<p MadCap:conditions="Global.Primary:java" />
<p MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages">
<pre class="prettyprint" xml:space="preserve">TPCloneExample.cs: StoreCar
private static void StoreCar()
{
File.Delete(Db4oFileName);
IObjectContainer container = Database(Db4oFactory.NewConfiguration());
if (container != null)
{
try
{
// create a car
Car car = new Car("BMW", new Pilot("Rubens Barrichello"));
container.Store(car);
Car car1 = (Car)car.Clone();
container.Store(car1);
}
finally
{
CloseDatabase();
}
}
}</pre>
</p>
<p MadCap:conditions="Global.Primary:cs" />
<p MadCap:conditions="Primary..NET,Primary.VB.NET,Primary.All languages">
<pre class="prettyprint lang-vb" xml:space="preserve">TPCloneExample.vb: StoreCar
Private Shared Sub StoreCar()
File.Delete(Db4oFileName)
Dim container As IObjectContainer = _
Database(Db4oFactory.NewConfiguration())
If container IsNot Nothing Then
Try
' create a car
Dim car As New Car("BMW", New _
Pilot("Rubens Barrichello"))
container.Store(car)
Dim car1 As Car = DirectCast(car.Clone(), Car)
container.Store(car1)
Finally
CloseDatabase()
End Try
End If
End Sub</pre>
</p>
<p MadCap:conditions="Global.Primary:vb" />
<p>So it works for the first store, but what if we will clone an object retrieved from the database?</p>
<p MadCap:conditions="Global.Primary:java" />
<p MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages">
<pre class="prettyprint" xml:space="preserve">TPCloneExample.cs: TestClone
private static void TestClone()
{
IConfiguration configuration = ConfigureTP();
IObjectContainer container = Database(configuration);
if (container != null)
{
try
{
IObjectSet result = container.QueryByExample(new Car(null, null));
ListResult(result);
Car car = null;
Car car1 = null;
if (result.Size() > 0)
{
car = (Car)result[0];
System.Console.WriteLine("Retrieved car: " + car);
car1 = (Car)car.Clone();
System.Console.WriteLine("Storing cloned car: " + car1);
container.Store(car1);
container.Commit();
}
}
finally
{
CloseDatabase();
}
}
}</pre>
</p>
<p MadCap:conditions="Global.Primary:cs" />
<p MadCap:conditions="Primary..NET,Primary.VB.NET,Primary.All languages">
<pre class="prettyprint lang-vb" xml:space="preserve">TPCloneExample.vb: TestClone
Private Shared Sub TestClone()
Dim configuration As IConfiguration = ConfigureTP()
Dim container As IObjectContainer = Database(configuration)
If container IsNot Nothing Then
Try
Dim result As IObjectSet = container.QueryByExample( _
New Car(Nothing, Nothing))
ListResult(result)
Dim car As Car = Nothing
Dim car1 As Car = Nothing
If result.Size() > 0 Then
car = DirectCast(result(0), Car)
System.Console.WriteLine("Retrieved car: " _
+ car.ToString())
car1 = DirectCast(car.Clone(), Car)
System.Console.WriteLine("Storing cloned car: " _
+ car1.ToString())
container.Store(car1)
container.Commit()
End If
Finally
CloseDatabase()
End Try
End If
End Sub</pre>
</p>
<p MadCap:conditions="Global.Primary:vb" />
<p>The code above throws an exception when the cloned object is being
bound to the object container. Luckily we can easily fix it by
overriding #clone method and setting activator to null: </p>
<p MadCap:conditions="Global.Primary:java" />
<p MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages">
<pre class="prettyprint" xml:space="preserve">Car.cs: Clone
public object Clone()
{
Car test = (Car)base.MemberwiseClone();
test._activator = null;
return test;
}</pre>
</p>
<p MadCap:conditions="Global.Primary:cs" />
<p MadCap:conditions="Primary..NET,Primary.VB.NET,Primary.All languages">
<pre class="prettyprint lang-vb" xml:space="preserve">Car.vb: Clone
Public Function Clone() As Object Implements ICloneable.Clone
Dim test As Car = DirectCast(MyBase.MemberwiseClone(), Car)
test._activator = Nothing
Return test
End Function</pre>
</p>
<p MadCap:conditions="Global.Primary:vb" />
<p MadCap:conditions="Primary.Online">Download example code:</p>
<p MadCap:conditions="Primary.Online">
<MadCap:conditionalText MadCap:conditions="Primary..NET,Primary.VB.NET,Primary.All languages"><a href="tpclonevb.zip">VB.NET </a>
</MadCap:conditionalText>
<MadCap:conditionalText MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages"><a href="tpclonecs.zip">c# </a>
</MadCap:conditionalText>
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