What is the test for a cause of action seeking restitution in a claim for breach of contract?

"There are several potential bases for a cause of action seeking restitution. For example, restitution may be awarded in lieu of breach of contract damages when the parties had an express contract, but it was procured by fraud or is unenforceable or ineffective for some reason. [Citations.] Alternatively, restitution may be awarded where the defendant obtained a benefit from the plaintiff by fraud, duress, conversion, or similar conduct. In such cases, the plaintiff may choose not to sue in tort, but instead to seek restitution on a quasi-contract theory . . . . [Citations.] In such cases, where appropriate, the law will imply a contract (or rather, a quasi-contract), without regard to the parties' intent, in order to avoid unjust enrichment." (McBride v. Boughton (2004) 123 Cal.App.4th 379, 388, fn. omitted.)