Ryan Rinaldi

• Contract based development
• Add contracts to code that ensure adherence to spec and implementation
• Contract.Requires( 0 < x);
• Contract.Ensures(Contract.Result<string>() != null)
• System.Diagnostics.Contracts
• .NET 4.0
• Contracts document design decisions
• Contracts are executable
• Checked documentation
• Contracts help static verification
• Assertions help, but not perfect
• Not a contract between 2 parties
• Demo of contracts
• Creating a Rational number
• You can get a preview of code contracts now for 2008
• By default it looks at IL and can generate warnings of possible runtime errors even without any contracts
• Adding contracts make the contracts part of the method signature
• Contracts look at the type as a whole, not just the standard constructor
• Use ObjectInvariant method
• This binds the object to a state that has to be true at the end of each method and each constructor
• Using Pex with Contracts
• Pex finds all the values that would violate the contract
• What can I do with Contracts?
• Static Checking
• Runtime Checking
• Test Generation (Pex)
• Documentation
• What contracts are there?
• Requires
• Ensures
• what is true at method exit
• Invariants
• what is true at ANY method exit
• What can be put in a contract?
• Any boolean expression (including method calls)
• Contract.Requires( value != null);
• Contract.Result
• Contract.Ensures(Contract.Result<int>() == Contract.OldValue(Count));
• (SOMETHING ELSE that i missed. Grr)
• Contracts are declarative
• They are executed at the proper point in the chain no matter where they are called
• Demo of an ArrayList full of contracts (Runtime validations)
• There is a subtype that overrides Add and returns -1 which violates the Add contract
• Contracts get populated into subtypes
• The tool takes care of contract injections
• Probably don't want to put contracts out there in Release builds
• If you still want them in Release there are other methods to use to put them into release builds
• If you don't want them in Release the standard methods will exclude the contracts (faster)
• Static Contract Checking
• Analyzes all execution paths
• Guides development
• Not only explicit contracts
• De-referencing null
• indexing arrays
• Arithmetic exceptions
• What gets shipped?
• Release assemblies + Contract Reference assemblies
• Contract Reference Assemblies are all contracts, no code
• Interface Contracts
• ContractClass attribute
• ContractClassFor attribute
• Summary
• Contract library class for all .NET languages
• Contracts are being used in the BCL today
• Same contracts used for
• Runtime checking
• Static checking
• Documentation generation
• PEX
• Testing before Pex
• Testing is tedious
• too easy to miss cases
• old tests get stale
• too much legacy code - what does it do?
• Tests for free: Demo
• Using Pex Explorations you can see all the branches and all the values that execute those branches
• Any Pex exploration can be saved as a unit test
• Just right click in code -> explore -> get test cases for free
• Demo: Using Pex to explore a class in NHibernate (Typename Parser)
• Generates 31 different test cases
• All MSTest cases
• Pex gives you 100% Code Coverage
• Once you start to understand code, you should start adding contracts and running more explorations to validate those contracts
• Pex gives you high code coverage and generates automagically pretty good test suites
• Pex finds contract violations
• Integrates with VS Team Test
• can be used as Build Verification tests
• Very targeted small tests
• PexMethods
• Parameterized Unit Test
• A unit test method that takes parameters
• Pex generates traditional Unit Tests in C# which fix particular values
• How to test this code? Demo
• ResourceReader
• It takes a stream
• It gets down to reading bytes from a stream that can be ugly (includes magic numbers)
• With parameterized tests you need to do some setup and create a viable scenario for testing
• Everytime Pex finds a branch it creates a test case.
• You can tell Pex what exceptions are acceptable and it will only flag un-expected exceptions as errors
• Pex understands code
• Pex does not guess
• Pex does not enumerate all possible values
• Pex does not make you write test input generators
• Pex partitions inputs into equivalance classes
• One equivalence class per branchin behavior
• Test inputs computed by Z3, a constraint solver from MSR

Pex and Contracts are the shit.  Seriously.  After watching this demo I'm all about it.  Downloading and installing as soon as I get back home.

URL: http://research.microsoft.com/pex