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Consider this immutable type: public class Settings { public string Path { get; private set; } [ContractInvariantMethod] private void ObjectInvariants() { Contract.Invariant(Path != null); } public Settings(string path) { Contract.Requires(path != null); Path = path; } } Two things to notice here: There is a contract invariant which ensures the Path property can never be null The constructor checks the path argument value to respect the previous contract invariant At this point, a Setting instance can never have a null Path property. Now, look at this type: public class Program { private readonly string _path; [ContractInvariantMethod] private void ObjectInvariants() { Contract.Invariant(_path != null); } public Program(Settings settings) { Contract.Requires(settings != null); _path = settings.Path; } // <------ "CodeContracts: invariant unproven: _path != null" } Basically, it has its own contract invariant (on _path field) that can't be satisfied during static checking (cf. comment above). That sounds a bit weird to me, since it's easy to prove it: settings is immutable settings.Path can't be null (because Settings has the corresponding contract invariant) so by assigning settings.Path to _path , _path can't be null Did I miss something here?

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