difference in Contract.Requires and Contract.Ensures

Question

I've looked up the difference on the microsoft site but can't really see the difference. links to the website are below

• Contracts.Ensures
• Contract.Requires

Any additional explaination would be great

Using visual studio c#

Answer 1

Requires is a precondition, meaning that the condition specified must be true prior to the method being invoked. Ensures is a postcondition, meaning that the method guarantees that the condition specified will be true after the method call is complete.

Preconditions and/or postconditions may be violated while the method is executing: the tests are done upon entry to and exit from of the method, respectively. An invariate condition is a contract that says that the specified condition always holds true.

Read Bertrand Meyer's Object-Oriented Software Construction for more [much more] detail. This paper by Meyer is shorter [much shorter].