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C# okay with comparing value types to null
If I try to assign null to a non-nullable type in C#:
System.DateTime time = null;
I'll get a compile-time error:
error CS0037: Cannot convert null to 'System.DateTime' because it is a non-nullable value type
which makes sense. But if compare the same type against null:
System.DateTime time = obtainFromSomewhere();
if( time == null ) {
//whatever;
}
there's no compile-time error. This doesn't make sense to me - if I can't assign null then why would it ever be null?
Why am I allowed to compare a non-nullable type with null?
919
The HasValue property returns true if the variable contains a value, or false if it is null. You can only use == and != operators with a nullable type. For other comparison use the Nullable static class.
Comparing NULL values Since you can't use a equality operator in the WHERE clause (remember, NULL values can't be equated or compared), the right way to compare NULL values is to use the IS and IS NOT operators.
Nullable variables may either contain a valid value or they may not — in the latter case they are considered to be nil . Non-nullable variables must always contain a value and cannot be nil . In Oxygene (as in C# and Java), the default nullability of a variable is determined by its type.
Nullable reference types are a compile time feature. That means it's possible for callers to ignore warnings, intentionally use null as an argument to a method expecting a non nullable reference. Library authors should include runtime checks against null argument values.
The reason this works for DateTime, is because DateTime defines it own == operator. Because it does so, it gets a lifted version of the operator which may be used with DateTime?. Since both DateTime and null may be implicitly converted to DateTime?, the comparison compiles but will always evaluate to false at runtime.
Thanks to Matt Ellen for pointing out the fact that my original answer didn't cover the example in the question.
196
It's due to Boxing.
DateTime can be boxed as object and thus it becomes a reference which can be compared with null (though it will always be false).
However, an object (null) can't be unboxed back to DateTime, thus it can't be assigned to DateTime.
Example: you could do
object now = DateTime.Now;
bool isNull = now == null
EDIT:
As Brian Rasmussen pointed out, I was wrong with the boxing theory. Boxing would only occur if explicitly casted to object like in my example or in (object)DateTime.Now == null.
5
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