CodeContract think assigned readonly field can be null

Question

I've got this code:

public class CodeContractSample
{
    private readonly List<object> _items = new List<object>();

    public IEnumerable<object> Query()
    {
        Contract.Ensures(Contract.Result<IEnumerable<object>>() != null);
        //if (_items == null) throw new Exception();
        return _items;
    }
}

CodeContracts gives this warning:

CodeContracts: ensures unproven: Contract.Result>() != null

If I uncomment the middle row it stops complaining. But why does it complain to start with? _items should never be null..?

Pieter van Ginkel · Accepted Answer · 2011-02-01T06:35:12.593

5

Contracts are not 100% and there are still gaps in what it understands.

You are right: there is no reason for the result being unproven. See http://social.msdn.microsoft.com/Forums/en-US/codecontracts/thread/f82aa25c-e858-4809-bc21-0a08de260bf1 for more information on this specific issue.

For now, you can solve this using:

Contract.Assume(_items != null);

You can also achieve this using contract invariants:

[ContractInvariantMethod]
void Invariants()
{
    Contract.Invariant(_items != null);
}

edited Feb 01 '11 at 06:35

answered Jan 31 '11 at 15:09

Pieter van Ginkel

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Nice, this is also useful after some of my Asserts in my unit tests... :) – Allrameest Jan 31 '11 at 15:16
1

It would be cleaner to solve it with an invariant, like this: `[ContractInvariantMethod] void Invariants() { Contract.Invariant(_items != null); }`. Otherwise, you need to Assume all over the place :) – porges Feb 01 '11 at 00:14

score 0 · Answer 2 · answered Jan 31 '11 at 14:47

0

Why do you think items will never be null? You could have another method in that class that sets it to null ...

answered Jan 31 '11 at 14:47

blowdart

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It's readonly. Only the constructor could set it to null. But I don't have any constructor. – Allrameest Jan 31 '11 at 14:58

score 0 · Answer 3 · answered Feb 01 '11 at 13:47

0

Actually i can imagine this:

CodeContractSample s = new CodeContractSample();
s.GetType().GetField("_items", BindingFlags.NonPublic | BindingFlags.Instance).SetValue(s, null);
var q = s.Query();

What do You think ?

answered Feb 01 '11 at 13:47

ZFE

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Yes, that could happen. But then the contract would throw an exception at runtime. When doing things with reflection I can no longer expect that the static compile-time checker would give me warnings. – Allrameest Feb 01 '11 at 13:57

score -1 · Answer 4 · answered Jan 31 '11 at 14:36

-1

The Contract.Ensures line is making a promise that the method will never return null. But there is nothing in the code that prevents this until you uncomment the line.

answered Jan 31 '11 at 14:36

Jack Allan

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...except that `_items` is initialised in the class the constructor and `Query` is a non-static method. – fearofawhackplanet Jan 31 '11 at 14:48
I don't think that kind of functionality has been implemented yet. It would require a change to the c# language. See http://channel9.msdn.com/Forums/TechOff/494357-Design-by-Contract-in-C/0ef9306b38314b1aa4c09deb0009144e – Jack Allan Jan 31 '11 at 14:54

CodeContract think assigned readonly field can be null

4 Answers4