Question

Say I have the following

public T Example(Func<T> f)
{
     Contract.Requires(f != null);
     Contract.Requires(f() != null); // no surprise, this is an error
...
}

Is there any way to specify that my Func<T> parameters must obey some contracts?

Answer 1

In general, that seems like a problematic requirement since invoking f may introduce side effects so specifying a contract could affect what the method does.

As the implementer of the Code Contracts library one could introduce wrapper code that checks to see if the value of f obeys the contract when it is invoked in the context of your method (to avoid introducing a spurious method call). This is problematic for a couple of reasons:

1. The method might never invoke f, so the caller could violate the contract and not get busted.
2. The method could call f only after doing some other work which might be invalid given that the call to f didn’t satisfy the specification.

If f had no side effects then these wouldn’t be problems, but in the presence of side effects dealing with 1 by always calling f wouldn’t work and calling f before doing any work to deal with 2 also wouldn’t fly.

So, in conclusion I don’t think that this is possible (in the context of native code contracts) and with good reason.