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<h1><a name="kanchor73"></a>db4o Meta-Information</h1>
<p>Db4o meta information API provides an access to the actual structure of db4o database file. Its primary use is <a href="refactoring_and_schema_evolution.htm">refactoring</a>.</p>
<p>You can access the meta information via extended object container. You can ask the object container for all stored classes or for a specific class. To find the meta information for a specific class you can provide the full name, the class itself or an instance of a particular type.</p>
<p>Note that db4o also returns information about internal db4o instances which have been stored. </p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">// Get the information about all stored classes.
IStoredClass[] classesInDB = container.Ext().StoredClasses();
foreach (IStoredClass storedClass in classesInDB)
{
Console.WriteLine(storedClass.GetName());
}
// Information for a certain class
IStoredClass metaInfo = container.Ext().StoredClass(typeof (Person));</pre>
<div class="codesnippet-meta">MetaInfoExample.cs: All stored classes
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">' Get the information about all stored classes.
Dim classesInDB As IStoredClass() = container.Ext().StoredClasses()
For Each storedClass As IStoredClass In classesInDB
Console.WriteLine(storedClass.GetName())
Next
' Information for a certain class
Dim metaInfo As IStoredClass = container.Ext().StoredClass(GetType(Person))</pre>
<div class="codesnippet-meta">MetaInfoExample.vb: All stored classes
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<p>The stored class interface provides all meta information db4o knows about. You can get the name of the class, ask for the instance count, ask for a list of the ids and get the meta info for super classes.</p>
<p>The most important information about the stored classes meta info is the list of the field which are stored. You can get a list of all fields or ask for specific fields. Note that the meta information might return information for fields which don't exist anymore. This is <a href="refactoring_and_schema_evolution/field_type_change.htm">useful for refactoring</a>. </p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IStoredClass metaInfoForPerson = container.Ext().StoredClass(typeof (Person));
// Access all existing fields
foreach (IStoredField field in metaInfoForPerson.GetStoredFields())
{
Console.WriteLine("Field: " + field.GetName());
}
// Accessing the field 'name' of any type.
IStoredField nameField = metaInfoForPerson.StoredField("name", null);
// Accessing the string field 'name'. Important if this field had another time in previous
// versions of the class model
IStoredField ageField = metaInfoForPerson.StoredField("age", typeof (int));
// Check if the field is indexed
bool isAgeFieldIndexed = ageField.HasIndex();
// Get the type of the field
String fieldType = ageField.GetStoredType().GetName();</pre>
<div class="codesnippet-meta">MetaInfoExample.cs: Accessing stored fields
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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 metaInfoForPerson As IStoredClass = container.Ext().StoredClass(GetType(Person))
' Access all existing fields
For Each field As IStoredField In metaInfoForPerson.GetStoredFields()
Console.WriteLine("Field: " & field.GetName())
Next
' Accessing the field 'name' of any type.
Dim nameField As IStoredField = metaInfoForPerson.StoredField("name", Nothing)
' Accessing the string field 'name'. Important if this field had another time in previous
' versions of the class model
Dim ageField As IStoredField = metaInfoForPerson.StoredField("age", GetType(Integer))
' Check if the field is indexed
Dim isAgeFieldIndexed As Boolean = ageField.HasIndex()
' Get the type of the field
Dim fieldType As String = ageField.GetStoredType().GetName()</pre>
<div class="codesnippet-meta">MetaInfoExample.vb: Accessing stored fields
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<p>On a the field meta information you can find out the name, type and if the field has an index. And you also can access the values of a object via the stored field. This allows you to access information which is stored in the database but has been removed from the class model. This is <a href="refactoring_and_schema_evolution/field_type_change.htm">useful for refactoring</a>.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IStoredClass metaForPerson = container.Ext().StoredClass(typeof (Person));
IStoredField metaNameField = metaForPerson.StoredField("name", null);
IList<Person> persons = container.Query<Person>();
foreach (Person person in persons)
{
string name = (string) metaNameField.Get(person);
Console.WriteLine("Name is " + name);
}</pre>
<div class="codesnippet-meta">MetaInfoExample.cs: Access via meta data
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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 metaForPerson As IStoredClass = container.Ext().StoredClass(GetType(Person))
Dim metaNameField As IStoredField = metaForPerson.StoredField("name", Nothing)
Dim persons As IList(Of Person) = container.Query(Of Person)()
For Each person As Person In persons
Dim name As String = DirectCast(metaNameField.Get(person), [String])
Console.WriteLine("Name is " & name)</pre>
<div class="codesnippet-meta">MetaInfoExample.vb: Access via meta data
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