How do I the non-amdatory fields in xml.serialisation

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.NET

.NET4.5

By XML.Serialisation:
I need to set 'Mandatory' Fields with XMLElement.
Means, the non-amdatory fields have no tag in the xml-file, when the Value is NULL.
It doesn't help me the isnullable with the attribut: xsi:nil="true".
the java peoples on other site want not find this tag in the file, when an not-mandatory field is NULL
I not found, how to make in .NET.
3 yeahrs ago, we write the xml-file 'manually' and doesn't use the serialisation from .NET becaus this...
Somebody now?
thanks?