This document contains information about conference DVDs and an introduction to code contracts in .NET. It summarizes that code contracts are a way to define preconditions, postconditions, and class invariants for software components to specify their interfaces and ensure correctness. It provides examples of using the System.Diagnostics.Contracts namespace in C# to add preconditions and postconditions to methods. Contracting types can make them more robust by validating arguments and object states.

1. David McCarter
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3. Overview
Introducing Code Contracts
Pre-conditions & Post-conditions
Contract Your Types With Contracts
Summary

4. Overview

8. 


Always check for valid parameter
arguments!
Perform argument validation for every public
or protected method
Throw meaningful exceptions to the
developer for invalid parameter arguments


Use the System.ArgumentException class
Or your own class derived from
System.ArgumentException

9. public string FormatPhoneNumber(string str, bool bTakeOutDash)
{
string str1 = str.Trim();
if (bTakeOutDash)
{
if (11 == str.Length && -1 != str.IndexOf('-'))
{
str1 = "";
for (int i = 0; i < str.Length; i++)
{
if ("-" != str.Substring(i, 1))
{
str1 = string.Concat(str1, str.Substring(i, 1));
}
}
}
}
return str1;
}

10. public string FormatPhoneNumber(string str, bool bTakeOutDash)
{
if (string.IsNullOrWhiteSpace(str))
{
throw new ArgumentNullException("str", "Phone number must be supplied.");
}
string str1 = str.Trim();
if (bTakeOutDash)
{
// Code Removed for brevity
}
return str1;
}

11. Introducing
Code Contracts
Build Better Types

12. 


Is an approach for designing software.
It prescribes that software designers should
define formal, precise and verifiable interface
specifications for software components, which
extend the ordinary definition of abstract data
types with preconditions, post-conditions and
invariants.
These specifications are referred to as
"contracts", in accordance with a conceptual
metaphor with the conditions and obligations of
business contracts.

13. 
A discipline of
analysis, design, implementation, management

Applications throughout the software lifecycle:

Getting the software right:







analysis, design , implementation
Debugging & testing
Automatic documentation
Getting inheritance right
Getting exception handling right!
Maintenance
Management

14. 
Every software element is intended to
satisfy a certain goal, or contract


For the benefit of other software elements (and
ultimately of human users)
The contract of any software element should
be


Explicit
Part of the software element itself

15. Precondition

What does it expect?
Postcondition

What does it promise?

What does it maintain?
Class
invariant

16. 




New API + tools from Microsoft for Designby-Contract
System.Diagnostics.Contracts
MSIL rewriting
Inspired by Spec#
Included in .NET 4.0/ 4.5

Latest version released 9/13

http://research.microsoft.com/en-us/projects/contracts/

17. Pre-conditions
Post-conditions
Oh My!

18. 



Usually for method parameter checking
Caller’s responsibility to make sure the
precondition is true on entry into method
Does not need to be compiled in production
code
Can also include throwing of Exceptions

These must be compiled into assembly

19. public static string FormatPhoneNumber(string str, bool bTakeOutDash)
{
Contract.Requires(string.IsNullOrWhiteSpace(str) == false);
string str1 = str.Trim();
// code removed for brevity
return str1;
}

24. public static string FormatPhoneNumber(string str, bool bTakeOutDash)
{
Contract.Requires<ArgumentNullException>
(string.IsNullOrWhiteSpace(str) == false);
string str1 = str.Trim();
// code removed for brevity
return str1;
}

25. 


Postconditions are contracts for the state of
a method when it terminates
The postcondition is checked just before
exiting a method
The run-time behavior of failed
postconditions is determined by the runtime
analyzer.

26. public static string FormatPhoneNumber(string str, bool bTakeOutDash)
{
Contract.Requires(string.IsNullOrWhiteSpace(str) == false);
Contract.Ensures(string.IsNullOrEmpty(Contract.Result<string>())
==false);
string str1 = str.Trim();
// code removed for brevity
return str1;
}

28. 


EnsuresOnThrow<T>
OldValue
ForAll


Ensures


Out parameters
EndContractBlock


Collections
Legacy parameter checking
Abstract Methods, Overloaded Methods and more!

29. Contract Your
Types
With Contracts
Creates MUCH Better Types!

30. 
ContractInvariantMethod




Mark methods that contain object invariant
specifications
The methods marked with this attribute must
be nullary methods with void return types and
contain nothing other than calls to Contract
You may not call these methods from within
your code.
Force coders to do proper value checking!

31. public Int32 Value
{
get { return _value; }
private set
{
if (this.Value != value)
{
if ((value <= this.Maximum) && (value >= this.Minimum))
{
this._value = value;
this.OnValueChanged();
}
}
}
}

32. public Int32 Value
{
get { return _value; }
private set
{
if (this.Value != value)
{
Contract.Requires<ArgumentOutOfRangeException>(value >= 0);
if ((value <= this.Maximum) && (value >= this.Minimum))
{
this._value = value;
this.OnValueChanged();
}
}
}
}

33. private Int32 _value;
[ContractInvariantMethod]
private void Invariants()
{
Contract.Invariant(_value >= 0,
"Value cannot be less than zero.");
}
-------------------------------------set
{
// Contract.Requires<ArgumentOutOfRangeException>(value >= 0);
if (this.Value != value)
{
// Code Removed for Brevity
}
}

36. Summary

37. 


Asserts
Inheritance
Attributes









ContractClass, ContractClassFor
Pure
RuntimeContracts
ContractPublicPropertyName
ContractVerification
ContractRuntimeIgnored
ContractOption
Contracting Ordering
Lots more!

38. 
Pex and Moles




Isolation and White box Unit Testing for .NET
Pex auto generates test suites with high code
coverage
Moles supports unit testing by way of detours
and stubs (delegates)
http://research.microsoft.com/en-us/projects/pex/

39. 
Microsoft Research


http://research.microsoft.com/enus/projects/contracts/
Channel 9

http://channel9.msdn.com/Search/?Term=code
%20contracts

40. Questions?
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