Code contracts static checker: Possibly calling a method on a null reference — why?

Question

I have the following class:

public class AndSpecification<TEntity> : ISpecification<TEntity>
{
    protected ISpecification<TEntity> First { get; private set; }
    protected ISpecification<TEntity> Second { get; private set; }

    public bool IsSatisfiedBy(TEntity entity)
    {
        return First.IsSatisfiedBy(entity) && Second.IsSatisfiedBy(entity);
    }

    public AndSpecification(ISpecification<TEntity> first, ISpecification<TEntity> second)
    {
        Contract.Requires(first != null);
        Contract.Requires(second != null);
        Contract.Ensures(First != null);
        Contract.Ensures(Second != null);

        First = first;
        Second = second;
    }
}

Note that I use Contract.Ensures() to ensure that after the constructor is invoked, First and Second are never null. However, Code Contracts give me a warning in IsSatisfiedBy() method implementation: CodeContracts: Possibly calling a method on a null reference 'this.First' (same for the Second). Can't figure out what's wrong, is that me or the static checker?

Answer 1

It appears you are missing some contracts and object invariants. I took your code above and ran Code Contracts static analysis on it and got exactly what you got. Then I worked my way through the code to arrive at an implementation that satisfies the static analyzer.

[ContractClass(typeof(ISpecificationContracts<>))]
public interface ISpecification<TEntity> 
    where TEntity : class
{
    bool IsSatisfiedBy(TEntity entity);
}

[ContractClassFor(typeof(ISpecification<>))]
abstract class ISpecificationContracts<TEntity> 
    : ISpecification<TEntity> where TEntity : class
{
    public bool IsSatisfiedBy(TEntity entity)
    {
        Contract.Requires(entity != null);
        throw new NotImplementedException();
    }
}

public class AndSpecification<TEntity> 
    : ISpecification<TEntity> where TEntity : class
{
    private readonly ISpecification<TEntity> _first;
    private readonly ISpecification<TEntity> _second;

    protected ISpecification<TEntity> First {
        get
        {
            Contract.Ensures(Contract.Result<ISpecification<TEntity>>() != null);
            return _first;
        }
    }
    protected ISpecification<TEntity> Second
    {
        get
        {
            Contract.Ensures(Contract.Result<ISpecification<TEntity>>() != null);
            return _second;
        }
    }

    public bool IsSatisfiedBy(TEntity entity)
    {
        return First.IsSatisfiedBy(entity) && Second.IsSatisfiedBy(entity);
    }

    public AndSpecification(ISpecification<TEntity> first,
                            ISpecification<TEntity> second)
    {
        Contract.Requires(first != null);
        Contract.Requires(second != null);
        Contract.Ensures(_first == first);
        Contract.Ensures(_second == second);

        _first = first;
        _second = second;
    }

    [ContractInvariantMethod]
    [System.Diagnostics.CodeAnalysis.SuppressMessage(
        "Microsoft.Performance", 
        "CA1822:MarkMembersAsStatic", 
        Justification = "Required for code contracts.")]
    private void ObjectInvariant()
    {
        Contract.Invariant(_first != null);
        Contract.Invariant(_second != null);
    }

}