First create the coordinate class:
[DataContract]
class coord{
[DataMember]
public double lat{get;set;}
[DataMember]
public double lon{get;set;}
}
Now if you want to save a List<coord>
You may do:
DataContractSerializer ser = new DataContractSerializer(typeof(List<coord>));
ser.WriteObject(any_file_stream, instance_of_List<coord>);
But if you want to save just a couple of coordinates, not a list IsolatedStorageSetting would be a better option.