Is using "as" in C# a safe way of casting?

Question

I am wondering if using the keyword "as" in the following code is a safe way (i.e. won't blow up) of casting in C#:

public void abc(ref Object dataSource) {     DataTable table = dataSource as DataTable; } 

Is there a safer way of casting?

Answer 1

It won't blow up... but that doesn't necessarily mean it's safe.

Typically when I use a cast for a reference conversion, it's because I really, really think that the execution-time type is the one I'm specifying. If it's not, that indicates a bug in my code - and I'd rather that manifested itself as an exception.

If you've got bad data in your system, then continuing as if everything was fine is the dangerous path, not the safe path. That's the way that as will take you, whereas a cast would throw an InvalidCastException, aborting whatever you're doing before you get the chance to cause mayhem with the bad data.

as is good if it's valid for the object not to be of the given type - if it doesn't indicate a bug. You almost always see the pattern of:

Foo x = y as Foo; if (x != null) {     ... } 

See MSDN for more details about what as does.

Note also that you probably don't want to use ref in your method. See my article on parameter passing for more details. Most of the time if I see people using ref a lot, it's because they don't understand what it really means :)