An isolation system is described for converting original product code into corresponding modified code. The isolation system operates by identifying a subset of original methods to be converted. For each such original method, the isolation system generates a modified part having at least one property with a type-safe delegate type which matches a signature of the original method. Test code, which tests the product code, can then associate a delegate instance to the thus-defined property of the original method. This prompts an execution system to dynamically execute detour code associated with the delegate instance, rather than an instrumentation of the original method, thus avoiding dependency on potentially non-deterministic functionality which would be otherwise invoked by an instrumentation of the original method.