DataMember attribute set to field or property?

Question

In which way should I use DataMemeber attribute ?

I.

 [DataMember]
 internal protected string _FirstName="";

[DataMember]
public string FirstName { get { return _FirstName; } 
internal protected set { _FirstName=(value!=null?value:""); } }

II.

internal protected string _FirstName="";

    [DataMember]
    public string FirstName { get { return _FirstName; } 
    internal protected set { _FirstName=(value!=null?value:""); } }

III.

[DataMember]
internal protected string _FirstName="";


    public string FirstName { get { return _FirstName; } 
    internal protected set { _FirstName=(value!=null?value:""); } }

Answer 1

1st is definitely not correct, as serialization will happen twice. Between 2nd and 3rd I personally prefer 2nd, as encapsulating implementation.

Answer 2

The second one. This exposes only the property as a data member. That's what you want. You don't want to have the field exposed.