Question

Does anyone have concrete information on how C# handles comparisons with Nullable<T> types when one side of the comparison is null?

As I understand from experimenting with the compiler, it seems that the comparison always returns false, but I can’t find any documentation to back that up. Is this a real feature of the language (and thus something I can count on), or is this an implementation detail that might change in future versions?

In other words, does the following method returning true imply y.HasValue, and can you point me to some documentation that proves that it does?

    public bool foo(int x, int? y)
    {
        return x < y;
    }

Answer 1

Does anyone have concrete information on how C# handles comparisons with Nullable types when one side of the comparison is null?

Yes – the C# language specification, section 7.3.7. In this case, it’s a relational operator:

For the relation operators < > <= >= a lifted form of an operator exists if the operand types are both non-nullable 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.

There are similarly detailed sections for other operators.

When in doubt about how some aspect of the language works (and whether it’s guaranteed or implementation-specific), the C# language specification should be your first port of call.