Question

Just trying to get my head around Generics by reading this enlightening article by Juval Lowy

Paraphrasing.. When you define a Generic class definition, it is compiled into IL.

• For value-types, as soon as you request for a specific value-type, it substitutes the T with your specific value type to obtain the IL for that specific configuration e.g. MyList<int> Benefit: No boxing and unboxing penalties.
• All good.. for reference types, the compiler replaces all instances of T in your definition with Object and creates the IL which is used for all ref types. Instances however are allocated based on the actual requested ref type e.g. MyList<String>

Now pre-generics we could have written methods that take Object parameters. Generics claims 100% performance improvement because ‘it avoids the performance penalty you incur when you downcast the object type to your specific type when you want to use it’

 // assume GetItem returns an Object  string sMyPreciousString = (string) obList.GetItem();  

What is this performance hit when you downcast from Object to specific reference type? Also it seems like up-casting to Object (even Generics would do this) isn’t a performance hit.. why?

Answer 1

Upcasting to object doesn’t require an execution time check – it will always work, and is just a no-op basically.

Downcasting requires an execution time check to make sure you’re not casting a Stream to a String for example. It’s a pretty small penalty, and very unlikely to be a bottleneck – but avoiding it is just one extra benefit for generics.