Assertion Task Or Something Like It

I am playing around trying to develop a thing in SSIS that would be like an assertion elsewhere -- e.g., test a precondition and raise an exception if the precondition isn't true. I'm posting this to see if folks have already done something like this or have suggestions how to go about it in a semi-general way that could be re-used.

My toy thingamabob right now has the goal of comparing the grand totals for a numeric field in two xml files and raising an exception if they aren't equal. I've implemented it as two XML tasks, each of which uses XSLT to compute the total and post it as a string into a variable, then a script task to convert the string variables into numbers, compare them, and take appropriate action.

chw