Contracting the Facebook API

In recent years, there has been an explosive growth in the popularity of online social networks such as Facebook. In a new twist, third party developers are now able to create their own web applications which plug into Facebook and work with Facebook’s “social” data, enabling the entire Facebook user base of more than 400 million active users to use such applications. These client applications can contain subtle errors that can be hard to debug if they misuse the Facebook API. In this paper we present an experience report on applying Microsoft’s new code contract system for the .NET framework to the Facebook API.We wrote contracts for several classes in the Facebook API wrapper which allows Microsoft .NET developers to implement Facebook applications. We evaluated the usefulness of these contracts during implementation of a new Facebook application. Our experience indicates that having code contracts provides a better and quicker software development experience.

💡 Research Summary

The paper “Contracting the Facebook API” presents an experience report on applying Microsoft’s Code Contracts framework to a .NET wrapper for the Facebook API. The authors begin by noting the explosive growth of Facebook, now exceeding 400 million active users, and the parallel rise of third‑party applications that integrate directly with Facebook’s social data. While the Facebook platform exposes a RESTful API, .NET developers typically interact with it through a higher‑level wrapper library that abstracts HTTP calls and JSON parsing. This wrapper, however, lacks explicit specifications about valid inputs, expected outputs, and object invariants, which makes it easy for developers to introduce subtle bugs—such as passing null identifiers, assuming non‑empty collections, or misinterpreting server responses. Traditional debugging in this context relies on log inspection and runtime exceptions that often surface far from the point of failure, especially when network latency, token expiration, or API version changes are involved.

To address these challenges, the authors adopt Microsoft’s Code Contracts, a design‑by‑contract system that allows developers to annotate methods with preconditions, postconditions, and invariants. Preconditions assert requirements on method arguments before execution; postconditions describe guarantees about return values or state after execution; invariants capture conditions that must hold for the lifetime of an object. The Code Contracts tooling provides both static analysis (to catch obvious violations at compile time) and runtime checking (to throw ContractException when a contract is breached). By embedding contracts directly in the source code, the specifications become self‑documenting, and test generation tools can automatically create boundary‑condition tests.

The study focuses on five core classes of the Facebook .NET wrapper—FacebookClient, FacebookUser, FacebookFriend, FacebookPhoto, and FacebookEvent. In total, 45 contracts were written. For example, the method GetUser(string userId) receives a precondition userId != null && userId.Length > 0 and a postcondition that the returned FacebookUser object’s Id equals the supplied userId. The `UploadPhoto(byte