Wednesday, February 14, 2007

Introduction of Nullable<T> was a great thing that many of us have been waiting for for a long time. One of the benefits is the implementation of lazy loaded properties.

There are however few gotchas that you have to be aware of. Something that surprised me the most was the implementation of the ToString method:

public override string ToString()
{
    if (!this.HasValue)
    {
        return ""
    }
    return this.value.ToString();
} 

What is unusual with it? Given the following code:

int? i = null;
string s = i.ToString(); 

What would you expect to happen? I would expect a NullReferenceException to be thrown. After some thinking though, I'm not so sure anymore. If we take into consideration a fact that ValueTypes are not ReferenceTypes i.e. there is no reference to a ValueType unless it is boxed, than having no NullReferenceException is perfectly legal and obvious in the described scenario.
The ToString implementation above clearly fulfils this assert so there will be no NullReferenceException even if Nullable value is null.
Is it good or bad? Now when I know about it, it doesn't really matter that much for me, but I was very surprised nonetheless. I think though that since null is not really null in case of ValueTypes, maybe there should only be a HasValue property? That would however made using Nullable types a lot more cumbersome... Nothing is ever easy :-).

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