Custom configuration section: How to get null for non required properties?

[style]

In the following sample I declared a custom configuration section within my
app.config. As you can see, the lastName attribute of the second employee
element is missing:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<section name="Employees" type="CustomConfigurationTest.MyConfigSection,
CustomConfigurationTest" />
</configSections>
<Employees>
<Employee EmployeeID="1" FirstName="John" LastName="Doe" />
<Employee EmployeeID="2" FirstName="Jane" />
</Employees>
</configuration>

The corresponding ConfigurationElement derived class looks as follows:

public class EmployeeElement : ConfigurationElement
{
[ConfigurationProperty("EmployeeID", IsKey = true, IsRequired = true)]
public int EmployeeID
{
get { return (int)this["EmployeeID"]; }
set { this["EmployeeID"] = value; }
}
[ConfigurationProperty("FirstName", IsRequired = true)]
public string FirstName
{
get { return (string)this["FirstName"]; }
set { this["FirstName"] = value; }
}
[ConfigurationProperty("LastName", IsRequired = false)]
public string LastName
{
get { return (string)this["LastName"]; }
set { this["LastName"] = value; }
}
}

Please note that the LastName property is marked as "IsRequired=false". At
runtime the LastName property contains an empty string for the second
employee because the lastName attribute isn't specified in the app.config.
But I'd rather like to get a null value instead. Does the .NET configuration
mechanism somehow allow to get a null value for non required configuration
element properties when the corresponding attribute isn't defined? Or is
this just not possible?

Any hints are appreciated.

Best regards