Debug.Assert vs. Specific Thrown Exceptions

Question

I've just started skimming 'Debugging MS .Net 2.0 Applications' by John Robbins, and have become confused by his evangelism for Debug.Assert(...).

He points out that well-implemented Asserts store the state, somewhat, of an error condition, e.g.:

Debug.Assert(i > 3, "i > 3", "This means I got a bad parameter"); 

Now, personally, it seems crazy to me that he so loves restating his test without an actual sensible 'business logic' comment, perhaps "i <= 3 must never happen because of the flobittyjam widgitification process".

So, I think I get Asserts as a kind-of low-level "Let's protect my assumptions" kind of thing... assuming that one feels this is a test one only needs to do in debug - i.e. you are protecting yourself against colleague and future programmers, and hoping that they actually test things.

But what I don't get is, he then goes on to say that you should use assertions in addition to normal error handling; now what I envisage is something like this:

Debug.Assert(i > 3, "i must be greater than 3 because of the flibbity widgit status"); if (i <= 3) {     throw new ArgumentOutOfRangeException("i", "i must be > 3 because... i=" + i.ToString()); } 

What have I gained by the Debug.Assert repetition of the error condition test? I think I'd get it if we were talking about debug-only double-checking of a very important calculation...

double interestAmount = loan.GetInterest(); Debug.Assert(debugInterestDoubleCheck(loan) == interestAmount, "Mismatch on interest calc"); 

...but I don't get it for parameter tests which are surely worth checking (in both DEBUG and Release builds)... or not. What am I missing?

Answer 1

Assertions are not for parameter checking. Parameter checking should always be done (and precisely according to what pre-conditions are specified in your documentation and/or specification), and the ArgumentOutOfRangeException thrown as necessary.

Assertions are for testing for "impossible" situations, i.e., things that you (in your program logic) assume are true. The assertions are there to tell you if these assumptions are broken for any reason.

Hope this helps!