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DataContractSerializer doesn't serialize private fields of all base data contracts. But public fields are serialized successfully.
Additional information is described in the following MSDN article:
http://msdn.microsoft.com/en-us/library/ms733127.aspx.
Citation:
"Member accessibility levels (internal, private, protected, or public) do not affect the data contract in any way".

Reproducible: Always

Steps to Reproduce:
1. Declare a new data contract named 'A' with private read-only field 'fa';
2. Derive another data contract 'B' from 'A'.
3. Create a new instance of 'B';
4. Serialize instance of 'B' with DataContractSerializer.
Actual Results:  
Normal serialization/deserialization.

Expected Results:  
Serialization/deserialization of public fields and fields declared at the top of data contract hierarchy.