October 17, 2002 spout

Type Safety in a Loosely Coupled World

Tim Ewald gave a very rousing keynote address at the WinDev conference in Boston yesterday. During his talk, he did something I’ve never really heard anyone do convincingly before — he defended the typeless recordset/rowset/dataset style of programming. His justification was that you don’t always need type-safe object models and, when getting a subset of data, they’re often more trouble than they’re worth (do you really want a set of types for every query in your app?).

My standard objection to the just the data, ma’am” style of programming is that I don’t get compile-time type checking. Of course, I can write the code to check all the data I get at runtime, but I don’t like to do that. Instead, I like things like the type safe dataset generator built into VS.NET. However, that damn tool fooled me. I looked at those type safe wrappers and considered that compile-time type checking. Of course, it’s not. Instead, it’s a hunk of code that pretends to offer compile-time type checking, but only really offers run-time type checking, because the data still needs to be coerced at *runtime*.

I have always turned up my nose at run-time type checking until I realized (and this is the insight I got from Tim’s talk) that *all* marshaling-based type checking is done at run-time. Even the type-checking done in COM between apartments needs to coerce the data to and from a serialized format, which means that it’s possible for the data to be mismatched between two endpoints, causing a type exception at runtime. Since I’m not willing to give up the loose coupling involved with components talking to each other across apartment/thread/process/machine/context/appdomain/whatever boundaries, I need to accept the fact that type checking needs to happen at run-time as well as at compile-time. As far as type checking is concerned, unmarshaling an RPC call stack is really no different than coercing/converting dataset columns or applying an XSD to an XML document.

Once I’ve accepted run-time type checking, I can take Don’s advice to free your mind” (he’s moved on to other parts of The Matrix not involving medication) and embrace things like structural typing, which is much more flexible than the nominal typing that OO languages rely on. I told you that nobody makes it the first time,” Don, but I think I’ve finally made it across. : )