I was trying out the code contracts in C# 4.0 and came across something quite surprising every time I ran my unit tests I got an annoying message box popping up “Assert Failure”, Description must use the rewriter when using Contract.Requires<TException>.
After some googling around I found that in Project Properties you need to find the code contracts tab and set “Perform Runtime Contract Checking” to full.
I opened my project properties I realized I did not have the Code Contracts tab… So in order to enable this tab you will need to install Code Contracts from DevLabs (here: http://msdn.microsoft.com/en-us/devlabs/dd491992.aspx).
Rebuild and he complained about the Assembly Mode Setting not being appropiate. After Changing it I could run my tests without having to click every time a testmethod came in the method that had the contract.
One problem I came across is that when trying Interface contracts I had an assembly that ended in Contracts and apparently it does not work (Code Contracts will compile its contracts in a assembly named <assembly name>.Contracts.dll. I renamed the assembly to IContracts and everything worked as expected