Code contracts

Design by Contract for Mainstream .NET
Code Contracts
AshicMahtabhttp://www.heartysoft.com@ashic

Static Languages
Why do we code in C#?
So type errors don’t manifest at runtime?
Intellisense?
Fear of learning Ruby?

Contract
A promise that something will behave in a certain way.
(Sounds like an interface, right?)

A joint account can be opened only if the number of holders is greater than two.
public void OpenJointAccount(AccountHolder[] holders)
An account holder’s balance may never fall below -100.
Go ahead…try writing interfaces for them.

Design by Contract
Based on the work of Alan Turing, Tony Hoare, EdsgerDijkstra, David Gries
Popularized / advocated by Bertrand Meyer and originally implemented in Eiffel.
Works on the basis of constraints like preconditions, post conditions and invariants which the supplier provides.

Preconditions: I require this to be true before I execute.
Post condition: I ensure this to be true when I am done executing.
Invariant: A condition that holds true during the lifetime of an object. It may not hold during the execution of a method but will do so before and after the method executes.

Demo
Basics
Preconditions, Post conditions, Invariants
Subtle edge case violations
Contract Blocks, [Pure]
Contracts on Interfaces and Abstract Classes
[ContractArgumentValidator], [ContractAbbreviator]

Integrating into legacy apps
[ContractVerification(false)]
Baseline

Drawbacks
Slower build with static checking
Public properties 
Interface contracts have weird syntax
Currently no contracts on delegates
Static checker can’t understand collections
Algorithmic performance may be compromised
IL mangling…no edit and continue
Must have buy in – something whole team uses or nobody does

Resources
http://research.microsoft.com/en-us/projects/contracts/

Code contracts

by Ashic Mahtab on Oct 10, 2011

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