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Class Name Format In .NET

Db4o uses full class name to distinguish classes within the database file. In .NET full class name has the following format:

Namespace.ClassName, AssemblyName

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:

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:

Test.cs
/**//* Copyright (C) 2004 - 2007 Versant Inc. http://www.db4o.com */
namespace Db4objects.Db4odoc.ClassNameFormat
 {
    class Test
     {
        public override string ToString()
         {
            return "Test";
        }
    }
}

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

Test1 application will store one object of Test class to the database:

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();
            }
        }

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

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:

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();
            }
        }

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

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.

In order to make your classes readable from another assembly you should use one of the existing workarounds:

Download example code:

VB.NET c#