Question

I came across this today in a WCF contract:

[DataMember(IsRequired = true)]
public DateTime? LastModified { get; set; } 

What are the consequences of IsRequired = True and a nullable DateTime? They appear to be contradictory to each other.

Answer 1

It can make sense if you want to initialize it with null and let user to set a valid date. So before submitting it can validate user input.

Here is a similar contradictory that may answer your question.

Interaction with IsRequired

The DataMemberAttribute attribute has an IsRequired property (the
default is false). The property indicates whether a given data member
must be present in the serialized data when it is being deserialized.
If IsRequired is set to true, (which indicates that a value must be
present) and EmitDefaultValue is set to false (indicating that the
value must not be present if it is set to its default value), default
values for this data member cannot be serialized because the results
would be contradictory. If such a data member is set to its default
value (usually null or zero) and a serialization is attempted, a
SerializationException is thrown.