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<h1>Activation In Action</h1>
<p>Let's see db4o's activation in action. To see activation you need a deep object-graph. To keep this example simple we create a person-class with a mother-field. This allows us to simply create a very deep object graph.</p>
<p>First the Person class:</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">internal class Person
{
private Person mother;
private string name;
public Person(string name)
{
this.name = name;
}
public Person(Person mother, string name)
{
this.mother = mother;
this.name = name;
}
public Person Mother
{
get { return mother; }
}
public string Name
{
get { return name; }
}
}</pre>
<div class="codesnippet-meta">Person.cs: Person with a reference to the mother
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Friend Class Person
Private m_mother As Person
Private m_name As String
Public Sub New(ByVal name As String)
m_mother = m_mother
Me.m_name = name
End Sub
Public Sub New(ByVal mother As Person, ByVal name As String)
Me.m_mother = mother
Me.m_name = name
End Sub
Public ReadOnly Property Mother() As Person
Get
Return m_mother
End Get
End Property
Public ReadOnly Property Name() As String
Get
Return m_name
End Get
End Property
End Class</pre>
<div class="codesnippet-meta">Person.vb: Person with a reference to the mother
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<p>After that we store a deep hierarchy of persons. Let's say we store a hierarchy of seven people. Then we query for it and traverse this object graph. When we hit the sixth person, that object won't be activated, because it's outside the activation depth. That object will have all fields set to null.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">Person jodie = QueryForJodie(container);
Person julia = jodie.Mother.Mother.Mother.Mother.Mother;
// This will print null
// Because julia is not activated
// and therefore all fields are not set
Console.WriteLine(julia.Name);
// This will throw a NullPointerException.
// Because julia is not activated
// and therefore all fields are not set
string joannaName = julia.Mother.Name;</pre>
<div class="codesnippet-meta">ActivationDepthPitfall.cs: Run into not activated objects
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</div>
<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim jodie As Person = QueryForJodie(container)
Dim julia As Person = jodie.Mother.Mother.Mother.Mother.Mother
' This will print null
' Because julia is not activated
' and therefore all fields are not set
Console.WriteLine(julia.Name)
' This will throw a NullPointerException.
' Because julia is not activated
' and therefore all fields are not set
Dim joannaName As String = julia.Mother.Name</pre>
<div class="codesnippet-meta">ActivationDepthPitfall.vb: Run into not activated objects
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<h2>Use Explicit Activation</h2>
<p>When we traverse deep object graphs, we know that we might run into not activated objects. Therefore you can activate objects explicitly.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">Person julia = jodie.Mother.Mother.Mother.Mother.Mother;
container.Activate(julia,5);
Console.WriteLine(julia.Name);
string joannaName = julia.Mother.Name;
Console.WriteLine(joannaName);</pre>
<div class="codesnippet-meta">ActivationDepthPitfall.cs: Fix with explicit activation
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim julia As Person = jodie.Mother.Mother.Mother.Mother.Mother
container.Activate(julia, 5)
Console.WriteLine(julia.Name)
Dim joannaName As String = julia.Mother.Name</pre>
<div class="codesnippet-meta">ActivationDepthPitfall.vb: Fix with explicit activation
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<h2>Configure Activation</h2>
<p>You can configure db4o to increase the activation depth. You can increase it <a href="../../configuration/common/activation_depth.htm">globally</a> or for <a href="../../configuration/objectclass/minimum_activation_depth.htm">certain classes</a>. Or you can <a href="../../configuration/objectclass/cascade_on-activate.htm">cascade activate</a> certain objects.</p>
<p>However remember that activation is there to improve the performance and save memory. If you set the activation depth to high it will hurt the performance.</p>
<h2>Transparent Activation</h2>
<p>If you have a very complex model or don't want to deal with all the activation hassle then transparent activation is the best option. Transparent activation will manage the activation for you. <a href="transparent_activation_framework.htm" target="" title="" alt="" class="MCXref">See "Transparent Activation"</a></p>
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