Non-generic default values in C#? Why not?!

A relatively obscure new keyword in C# 2.0 is the default(T) keyword.
It’s shorthand for checking if the given generic parameter T is a value type or reference type. If it’s a reference type, it returns null. Otherwise it returns 0/false/0.0 or whatever the default value for the type is.

For some reason, though, it can only be used on generic parameters. Why can’t I use default(typeof(int)) to get 0? Or, more realistically, default(myUnknownType) to return the default value?

A workaround is to check if it’s a value type with reflection, but it’s cumbersome:

if (!myType.IsValueType)
   return null;
   return Activator.CreateInstance(myType);

Luckily, value types (including structs) always have a default parameterless constructor. We can’t even override it – it will always initialize all struct members to their default values.

This little code snippet can now be wrapper in some sort of function – call it Default() or GetDefault() or even @default() if you want to keep it as close as possible to the generic keyword.

It’s interesting to note that the generic keyword relies on the InitObj IL opcode to initialize the object. If we could embed IL directly in our code we could call this directly. We can do it via Reflection.Emit and DynamicMethods but I feel that’s going a bit far overboard when the solution above works just as well.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.