Software quality with Code Contracts and PEX - CodeCamp16oct2010

Software quality with Code
Contracts and PEX
Sorin DAMIAN

Contracts
Design by Contract ™
Code contracts and runtime checking (Foxtrot)
Static verification (Clousot)
Documentation
Automated unit testing (PEX)

Design by contract
Described in articles since 1986
Around since the Eiffel language
Bertand Meyer
Analogy with business contracts
Contracts
Preconditions (What does the method expect?)
Postconditions (What does it guarantee?)
Invariants (What does it maintain?)

Microsoft Code Contracts Library
Microsoft Research
Code contracts and runtime checking (Foxtrot)
Static checker (Clousot)
Available for .NET 2.0 as an external assembly
Included in .NET 4.0
Integration with Visual Studio 2008/2010
Available in all .NET programming languages

Microsoft Code Contracts Tools
Runtime checking
 ccrewrite.exe
Static program verification
 cccheck.exe
Documentation generation
 ccdoc.exe
Automatic testing tools like PEX can take advantage
of contracts

Expressing contracts
Requirement and specification documents
Code comments
Guards / defensive programming
CodeContracts
Contract.Requires(arg != null);
Contract.Ensures(Contract.Result<int>() > 0);
Contract.Invariant(this.Total > 0);
Assertions
Assumptions

Code Contracts
 Contracts vs. Validation
 Handling legacy code
 Pre and post conditions
 Documentation
 Handling contract failures
 Contract violation events
 Assert on contract failure
 Contracts on interfaces and base classes
 Contracts inheritance
 Adding contracts to external libraries
 Contract reference assemblies
 Using a baseline for large projects

Demo
using System.Diagnostics.Contracts;

Code Contracts
Preconditions
Legacy contracts
Post conditions
Invariants
Side effects and pure methods

Demo
Interfaces and contracts inheritance

Known issues
 Build slowdown due to the assembly rewriter
 No mechanism to provide contracts on delegates
 Static checker doesn’t work with closures and yield iterators
 You may get static checker errors for generated code
 No edit and continue support

Static verification
 Floyd-Hoare logic proposed in 1969
 Sir Charles Antony Richard Hoare
 Design by Contract
 Bertrand Meyer
 Spec#
 Microsoft Research (credits go here..)
 Code contracts library in .NET Framework 4.0

Static verification
 Verification vs. Testing
 Abstract interpretation
 Working with existing projects

Pex Explorer
 Automated white box testing tool for .NET
 Parameterized unit tests
 Supports multiple frameworks
 Test cases for free!!!

Input
• [runs the code +
monitors it]
Collects
observed
constraints
• [picks a branch]
Builds
constraints
system to solve
• [solve “Z3��]

Pex understands your code
 Pex does not guess
 No random inputs
 No brute force
 Pex analyzes
 Partitions inputs into equivalence classes
 One equivalence class per branching behavior
 Test inputs computed by Z3 (the constraint solver for program analysis from
Microsoft Research)
 Performs inter-procedural, path-sensitive analysis
 Results:
 Small test suite with high test coverage (new test == new branch in code)

Thank You!
And please fill the feedback forms 

Links
 http://research.microsoft.com/en-us/projects/contracts/
 http://research.microsoft.com/en-us/projects/pex/
 http://pexforfun.com/
 http://visualstudiogallery.msdn.microsoft.com/en-us/85f0aa38-
a8a8-4811-8b86-e7f0b8d8c71b

Software quality with Code Contracts and PEX - CodeCamp16oct2010

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Software quality with Code Contracts and PEX - CodeCamp16oct2010

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