Use ReturnValues if you want to get the item attributes as they appeared before they were updated with the PutItem request.
            For PutItem, the valid values are: 
- NONE - If ReturnValues is not specified, or if its value is NONE, then nothing is returned. (This is the default for ReturnValues.)
- ALL_OLD - If PutItem overwrote an attribute name-value pair, then the content of the old item is returned.
Constraints:
 Declaration Syntax
 Declaration Syntax| C# | 
public string ReturnValues { get; set; }