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<head><title>Class Name Format In .NET </title>
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<h1>Class Name Format In .NET</h1>
<p>Db4o uses full class name to distinguish classes within the database file. In .NET full class name has the following format:</p>
<p><code>Namespace.ClassName, AssemblyName</code>
</p>
<p>Effectively that means that the same class definition within different assemblies (applications or libraries) will be recognized as two different classes by db4o. You should keep this in mind in the following cases:</p>
<ul>
<li value="1">2 or more applications working with the same database file;</li>
<li value="2">client/server application with the classes deployed on both the client and the server;</li>
<li value="3"><a href="asp.net.htm">ASP.NET2</a> application with dynamic compilation</li>
</ul>
<p>Let's use an example to see what happens in these cases. We will create 2 applications Test1.exe and Test2.exe. Both will have a simplest class definition:</p>
<p MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages">
<pre class="prettyprint" xml:space="preserve">Test.cs
/**//* Copyright (C) 2004 - 2007 Versant Inc. http://www.db4o.com */
namespace Db4objects.Db4odoc.ClassNameFormat
{
class Test
{
public override string ToString()
{
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">Test.vb
' Copyright (C) 2004 - 2007 Versant Inc. http://www.db4o.com
Namespace Db4objects.Db4odoc.ClassNameFormat
Class TestClass Test
Public Overloads Overrides Function ToString() As String
Return "Test"
End Function
End Class
End Namespace</pre>
</p>
<p MadCap:conditions="Global.Primary:vb" />
<p>Test1 application will store one object of Test class to the database:</p>
<p MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages">
<pre class="prettyprint" xml:space="preserve">ClassNameExample1.cs: StoreObjects
private static void StoreObjects()
{
File.Delete(Db4oFileName);
IObjectContainer container = Db4oFactory.OpenFile(Db4oFileName);
try
{
// Store a simple class to the database
Test test = new Test();
container.Store(test);
}
finally
{
container.Commit();
}
}</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">ClassNameExample1.vb: StoreObjects
Private Shared Sub StoreObjects()
File.Delete(Db4oFileName)
Dim container As IObjectContainer = _
Db4oFactory.OpenFile(Db4oFileName)
Try
' Store a simple class to the database
Dim test As Test = New Test
container.Store(test)
Finally
container.Commit()
End Try
End Sub</pre>
</p>
<p MadCap:conditions="Global.Primary:vb" />
<p>Another application (Test2) will try to read this object from the same database file. To check how the Test object was actually stored in the database we will use StoredClass API:</p>
<p MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages">
<pre class="prettyprint" xml:space="preserve">ClassNameExample2.cs: CheckDatabase
public static void CheckDatabase()
{
IObjectContainer container = Db4oFactory.OpenFile(Db4oFileName);
try
{
// Read db4o contents from another application
IObjectSet result = container.QueryByExample(typeof(Test));
ListResult(result);
// Check what classes are actualy stored in the database
IStoredClass[] storedClasses = container.Ext().StoredClasses();
foreach (IStoredClass storedClass in storedClasses)
{
System.Console.WriteLine("Stored class: " + storedClass.GetName());
}
}
finally
{
container.Commit();
}
}</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">ClassNameExample2.vb: CheckDatabase
Private Shared Sub CheckDatabase()
Dim container As IObjectContainer = Db4oFactory.OpenFile(Db4oFileName)
Try
' Read db4o contents from another application
Dim result As IObjectSet = container.QueryByExample(GetType(Test))
ListResult(result)
' Check what classes are actualy stored in the database
Dim storedClasses As IStoredClass() = container.Ext.StoredClasses
Dim storedClass As IStoredClass
For Each storedClass In storedClasses
System.Console.WriteLine("Stored class: " + storedClass.GetName)
Next
Finally
container.Commit()
End Try
End Sub</pre>
</p>
<p MadCap:conditions="Global.Primary:vb" />
<p>From the example we can see that though the class has been stored to the database, it cannot be retrieved from the Test2 application, as the assembly name is different from the original.</p>
<p>In order to make your classes readable from another assembly you should use one of the existing workarounds:</p>
<ul>
<li value="1">keep your persistent classes in a separate class library, which should be available for your application assemblies (for the example above compile the Test class into Persistent.dll);</li>
<li value="2">use db4o <a href="../configuration/common/alias.htm">Aliases</a>.</li>
</ul>
<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="classnameformatvb.zip">VB.NET </a>
</MadCap:conditionalText>
<MadCap:conditionalText MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages"><a href="classnameformatcs.zip">c# </a>
</MadCap:conditionalText>
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