Code Contract is a pretty easy way to reinforce the overall quality of your code. The following slides give you an overview of what Code Contracts are and how to make it work in a .Net environment. You’ll find a lot of links to let you go further, as well.

1. Code Contracts
Mini-Training
01/2014

2. Agenda
What is this?
Are they different from Unit-Tests?
How that works?
When should we use them?
Going further…
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3. What is this?
• Design by Contract has been invented by B. Meyer in 1986 when
he specified the Eiffel language
• The principle is to specify what must be true at some point in the
execution of a program
• In many programming languages, contracts are implemented with
assert
• Assertions represent a contract specifying the responsibilities
between the customer and the provider in a piece of code
• Design by contract does not replace regular testing strategies…
we’ll see that later.
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4. What is this?
• Code Contracts is a coding practice
• A contract is an assertion in the code
• Contracts take the form of preconditions, postconditions, and
object invariants (a condition which is always true)
• Contracts are expressed using static method calls at method
entries
• A contract is a form of documentation
• Runtime checking
• Works well with static analyzer tool like Resharper ;-)
But remember…
• A contract is not a behavior!
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5. Are they different from Unit-Tests?
Not really 
As Patrick Smacchia said: “Code Contracts and Automated Testing
are pretty much the same thing”
– They check often for code behavior correctness
– They trigger obvious failure in case of broken correctness found
– They document code behavior
Code Contracts assertions raise obvious failure when violated during
the execution of some automated tests
When using massively contracts and unit-tests, maintenance cost is
cut by an order of magnitude
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6. Are they different from Unit-Tests?
Contracts are not a replacement for Tests and vice-versa: both should
be practiced seriously:
• Contracts need Automated Tests to be exercised often in a
repeatable way.
• Automated Tests need Contracts as a much finer way to check
code behavior correctness, inside the code itself.
Chances that a bug remains undetected are pretty close to 0 when
the code is both 100% covered by tests and contains an optimal set
of contract assertions.
Read full Patrick Smacchia article.
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7. How that works?
In .Net, there’s 2 way to do it:
• Microsoft Code Contracts Framework
• System.Diagnostics Debug.Assert()
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8. When should we use them?
Contracts in C# are more than 60% about avoiding
NullReferenceException but there are other situations when it’s cool to
do so:
–
–
–
–
–
str.Length > 2
str[0] == ‘.’
i > 10
validate an invariant state inside a loop
…
But as Patrick Smacchia says: remember that null reference check
represent like half of the bugs on earth! (aka the Billion $ mistake)
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9. When should we use them?
Code Contract vs. Debug.Assert()?
Code Contract is a really nice framework that allows you a lot of things
(contract on interfaces members, Require(), Ensure(), Invariant(),
ValueAtReturn(), …)
But it doubles compilation time! Code Contracts assemblies get
rewritten once compiled by the C#. (but it might change in C# 6!)
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10. When should we use them?
In practice use contracts on the public surface API of your application and
Debug.Assert() everywhere else!
Stephen Cleary suggests that:
Set this on your Release project properties (code contracts tab)
This will output an “<applicationName>.Contracts.dll
That way: Projects consuming your library should reference your Release build. In
their Debug configuration, they should check "Perform Runtime Contract Checking"
and "Call-site Requires Checking"; this will ensure that the code contracts for your
library's public API are enforced at runtime (the "Call-site" option uses the
".Contracts.dll" assembly that you built). In their Release configuration, they should
leave code contracts disabled, which allows all assemblies to run at full speed.
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11. Give a try to…
http://pexforfun.com/default.aspx?language=CSharp&sample=StringTrimSuffix
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12. Give a try to…
• Microsoft Code Digger
– analyzes possible execution paths
through your .NET code. The result
is a table where each row shows a
unique behavior of your code. The
table helps you understand the
behavior of the code, and it may
also uncover hidden bugs.
• Code Contracts Editor Extensions
– provides
contract
support
programmers as they edit code
for
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13. Questions?
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14. Some links…
Design by contract on Wikipedia
Code Contracts @Microsoft
Code Contracts User Manual (pdf)
Code Contracts is the next coding practice you should learn and use
Better Than Unit Tests (to go further with other test techniques, a bit off-topic but worth a read)
Patrick Smacchia (NDepend) @Twitter
Michael Nygard @ThinkRelevance & @Twitter
Code Contracts for .NET VisualStudio Gallery
Pretty interesting question on SO and nice answer by Jon Skeet himself!
Simple and Easy Code Contracts - Stephen Cleary’s guide to get started with some useful information
about build settings
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15. Find out more
• On https://techblog.betclicgroup.com/
•
@__MaxS__

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