Design By Contract with Code Contracts by Microsoft is a great technique allowing to push quality of the software we build to a next level.

1. DESIGN BY
CONTRACT
WITH CODE CONTRACTS

2. CONFESSION :(

3. Confession :(
“How many of you
do write unit tests?”

4. Confession :(
“How many of you do
write documentation?”

5. Confession :(
“How many of you do
write asserts?”

6. JUSTIFICATION :)

7. Justification :)
THE GOOD PART
“At some extent all of these
tools don`t work in a real life.”
- me

8. Justification :)
WATCH OUT
Documentation
No documentation is better
than bad documentation
CODE SNIPPET
//declare variable foo as an integer and
//set it to three.
private int foo = 3;

9. Justification :)
WATCH OUT
CODE SNIPPET
Unit tests
Are limited and time
consuming to
support
[Test]
public void PressEquals_AddingTwoPlusTwo_ReturnsFour()
{
// Arrange
decimal value1 = 2m;
decimal value2 = 2m;
decimal expected = 4m;
var calculator = new Calculator();
// Act
calculator.Enter(value1);
calculator.PressPlus();
calculator.Enter(value2);
calculator.PressEquals();
decimal actual = calculator.Display;
// Assert
Assert.AreEqual(expected, actual,
"When adding {0} + {1}, expected {2} but found
{3}.", value1, value2, expected, actual);
}

10. Justification :)
WATCH OUT
CODE SNIPPET
public string Substring(int startIndex, int length)
Asserts
Make little use for
calling code
CODE SNIPPET
public string Substring(int startIndex, int length)
{
if (startIndex < 0)
throw new ArgumentOutOfRangeException("startIndex");
if (startIndex > this.Length)
throw new ArgumentOutOfRangeException("startIndex");
if (length < 0)
throw new ArgumentOutOfRangeException("length");
if (startIndex > this.Length - length)
throw new ArgumentOutOfRangeException("length");
if (length == 0)
return string.Empty;
else
return this.InternalSubStringWithChecks(startIndex, length, false);
}

11. Consequences
ABANDONING
“If so, why wouldn`t I
abandon all this crap?”

12. Consequences
PROGRAMMING BY COINCIDENCE
“We should avoid programming by
coincidence - relying on luck and
accidental successes - in favor of
programming deliberately.”
- Dave Thomas

13. Design by Contract
WHAT IS IT?
“A way of designing software, which implies formal and precise
specifications for software components with pre-conditions,
post-conditions and invariants in source code itself.”
Bertrand Meyer
EIFFEL PL, 1986

14. Design by Contract
EIFFEL
CODE SNIPPET
Pre-conditions
Post-conditions
connect_to_server (server: SOCKET)
-- Connect to a server.
require
server /= Void and then server.address /= Void
do
server.connect
ensure
connected: server.is_connected
end
CODE SNIPPET
class
Invariants
DATE
invariant
valid_day: 1 <= day and day <= 31
valid_hour: 0 <= hour and hour <= 23
end

15. Design by Contract
RULES
Metaphor : Client, Supplier agree on a Contract
1
2
3
The supplier must provide a certain product
(obligation) and is entitled to expect that the client
has paid its fee (benefit).
The client must pay the fee (obligation) and is
entitled to get the product (benefit).
Both parties must satisfy certain obligations, such as
laws and regulations, applying to all contracts.

16. Design by Contract
WHY?
“What are the benefits?”
Discoverability of your
API
Improved testability
Runtime & Static
Checking
Automatic generation
of documentation

17. Design by Contract
IMPLEMENTATIONS FOR .NET
“Do we have similar concept in modern programming
languages? Lets ask Microsoft.”

19. Microsoft Research

20. Code Contracts
WHAT IS IT?
“Microsoft`s implementation of
Design by Contract for .NET.
Proposed back in 2008.”

21. Code Contracts
WHAT IS IT?
CODE SNIPPET
Pre-conditions
class WebService
{
private IWarehouse store;
public WebService(IWarehouse store)
{
Contract.Requires(store != null);
Contract.Ensures(this.store != null);
Post-conditions
this.store = store;
}
[ContractInvariantMethod]
private void ObjectInvariant()
{
Contract.Invariant(this.store != null);
}
Invariants
}

22. Code Contracts
COMPLETE API
“Mostly it is nice and easy, but
occasionally it can be mind
blowing.”

23. Code Contracts
COMPONENTS
CCRewrite
CCCheck
CCDocGen
Binary Rewriter
Static Checker
XML Doc Extender

24. Code Contracts
RUNTIME CHECKING
WebService.cs
public WebService(IWarehouse store) {
Contract.Requires(store != null);
Contract.Ensures(this.store != null);
this.store = store;
WebService.dll
IL from requires
}
csc/vbc/…
+
ccrewrite
IL from body
IL from ensures

25. Code Contracts
RUNTIME CHECKING (GENERAL CLIENTS)
WebService.cs
public WebService(IWarehouse store) {
Contract.Requires(store != null);
Contract.Ensures(this.store != null);
this.store = store;
}
WebService.dll
IL from requires
csc/vbc/…
+
ccrewrite
IL from body

26. Code Contracts
RUNTIME CHECKING (TRUSTED CLIENTS)
WebService.cs
public WebService(IWarehouse store) {
Contract.Requires(store != null);
Contract.Ensures(this.store != null);
this.store = store;
}
WebService.dll
csc/vbc/…
IL from body

27. Code Contracts
DOCUMENTATION GENERATION
WebService.xml
<member
name="M:PDC.WebService.#ctor(PDC.
IWarehouse)">
<summary>Constructs a new
instance for processing orders
against the specified
warehouse.</summary>
<param name="store">The warehouse
this instance is to use. </param>
</member>
WebService.xml
ccdocgen
WebService.Contracts.dll
IL from requires
IL from ensures
<member
name="M:PDC.WebService.#ctor(PDC.IWarehouse)">
<summary>Constructs a new instance for
processing orders against the specified
warehouse.</summary>
<param name="store">The warehouse this
instance is to use. </param>
<requires> store != null </requires>
<ensures> this.store != null </ensures>
</member>

28. Code Contracts
CONTRACT REFERENCE ASSEMBLIES
“Companion assemblies generated
at compile time and contain only
contract portion of types.”

29. Code Contracts
ANNOYANCES
1
2
3
Static analysis is usually slow
Tools are failing from time to time
No way to execute post-conditions under lock
statement

30. References
Code Contracts
http://msdn.microsoft.com/en-us/magazine/ee236408.aspx
Code Contracts on Microsoft Research
http://research.microsoft.com/en-us/projects/contracts/
Code Contracts on MSDN
http://msdn.microsoft.com/en-us/library/dd264808.aspx
Code Contracts in C#
http://www.infoq.com/articles/code-contracts-csharp

31. THANK YOU
Questions?