Question

Perhaps my Google-Fu has failed me, but I haven’t been able to determine if comparing a nullable in .NET will always be less than something else.

I’ve got some code similar to this

MyClass findLatest(List<MyClass> items){
    DateTime? latest_tstamp = null;
    MyClass latest_item = null;
    foreach(var item in items){
        if (latest_tstamp < item.tstamp){
            latest_tstamp = item.tstamp;
            latest_item = item;
        }
    }
    return latest_item;
}

It’s seemed to work in the few limited cases I’ve tried (item.tstamp is declared DateTime? tstamp as well, of course).

Is this guaranteed behavior?

Conclusion(?)

Based on the answers (and Jon Skeet’s [answer on another question]), I’ve gone with the following check:

if (item.tstamp != null &&
    (latest_tstamp == null || latest_tstamp < item.tstamp)){
    // do stuff
}

Answer 1

This is behavior is guaranteed by the C# specification. The result of < on nullable value-types is false if any of them is null. Reference types on the other hand might exhibit different behavior.

Still I wouldn’t recommend using this. It’s hard to understand this code. I’d prefer an explicit null check, or just a boolean flag isFirstElement instead of using a nullable in the first place.

7.2.7 Lifted operators

Lifted operators permit predefined and user-defined operators that operate on non-nullable value types to also be used with nullable forms of those types. Lifted operators are constructed from predefined and user-defined operators that meet certain requirements, as described in the following:

…

• For the relational operators
       <   >   <=   >=
  a lifted form of an operator exists if the operand types are both non-nullable value types and if the result type is bool. The lifted form is constructed by adding a single ? modifier to each operand type. The lifted operator produces the value false if one or both operands are null. Otherwise, the lifted operator unwraps the operands and applies the underlying operator to produce the bool result.

(Quoted from C# Language Specification Version 3.0)