A few patterns come up repeatedly across real D365 CE projects. Recognizing them helps you reach for a plugin confidently instead of reinventing the approach each time.

Cross-field validation

Enforcing a rule business rules can’t express — “the End Date must be after the Start Date,” “this discount can’t exceed 20% without manager approval already recorded elsewhere” — registered at Pre-operation so the invalid save never happens at all.

Calculated values too complex for a Calculated column

Dataverse’s built-in Calculated columns only support simple formulas. Anything involving a loop, an external lookup, or multi-step logic (e.g. a weighted score combining several unrelated tables) needs a plugin instead, typically at Post-operation, updating the record after the main save.

Cascading updates across related records

Updating a parent record’s rollup-style field whenever a child changes, beyond what a native Rollup column supports (e.g. needing it to update instantly rather than on the rollup’s scheduled recalculation) — a Post-operation plugin on the child table, updating the parent synchronously.

Preventing invalid deletes or updates

Blocking a delete when business conditions aren’t met (an open Case still has unresolved related records) — registered on the Delete message at Pre-operation, throwing InvalidPluginExecutionException if the condition fails.

Integration triggers

Firing off to an external system when something happens in Dataverse — though for most new integration work, it’s worth seriously asking whether Power Automate (calling the same external API from a cloud flow triggered by a Dataverse trigger) is a better fit than a plugin, especially if the external call might be slow or unreliable.

A genuinely useful interview framing. For almost every scenario above, a good answer explains not just “use a plugin” but which stage (pre vs. post), sync vs. async, and why — that’s what distinguishes someone who’s actually built plugins from someone who’s only read about them.