CodeContracts with collection types

class Test3
{
  protected readonly List<A> _children = new List<A>();
  public List<A> Children
  {
    get
    {
      Contract.Ensures(Contract.ForAll(Contract.Result<List<A>>(), i => i != null));
      Contract.Assume(Contract.ForAll(_children, i => i != null));
      return _children; // No warning here when using List instead of ObservableCollection
    }
  }
  [ContractInvariantMethod]
  private void ObjectInvariant()
  {
    Contract.Invariant(_children != null);
  }
}

  Test1 t1 = new Test1();
  foreach (A child in t1.Children)
  {
    child.SomeMethod(); // CodeContracts: Possibly calling a method on a null reference 'child'
  }
  Test2 t2 = new Test2();
  foreach (A child in t2.Children)
  {
    child.SomeMethod(); // CodeContracts: Possibly calling a method on a null reference 'child'
  }
  Test3 t3 = new Test3();
  foreach (A child in t3.Children)
  {
    child.SomeMethod(); // CodeContracts: Possibly calling a method on a null reference 'child'
  }

我可以定义一个合约，以避免每次使用Children属性时都写Contract.Assume(child != null)吗?

我试图实现Enumerator，确保在Current属性getter中不为空的条件，正如 phog 所建议的那样。但是警告仍然存在(对我来说很意外)。

public class NotNullEnumerable<T> : IEnumerable<T>
{
    private IEnumerable<T> _enumerable;
    public NotNullEnumerable(IEnumerable<T> enumerable)
    {
        _enumerable = enumerable;
    }
    #region IEnumerable<T> Members
    public IEnumerator<T> GetEnumerator()
    {
        return new NotNullEnumerator<T>(_enumerable.GetEnumerator());
    }
    #endregion
    #region IEnumerable Members
    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
    #endregion
}
public class NotNullEnumerator<T> : IEnumerator<T>
{
    private readonly IEnumerator<T> _enumerator;
    public NotNullEnumerator(IEnumerator<T> enumerator)
    {
        _enumerator = enumerator;
    }
    #region IEnumerator<T> Members
    public T Current
    {
        get
        {
            Contract.Ensures(Contract.Result<T>() != null);
            return _enumerator.Current;
        }
    }
    #endregion
    #region IDisposable Members
    public void Dispose()
    {
        _enumerator.Dispose();
    }
    #endregion
    #region IEnumerator Members
    object System.Collections.IEnumerator.Current
    {
        get
        {
            Contract.Ensures(Contract.Result<object>() != null);
            return _enumerator.Current;
        }
    }
    public bool MoveNext()
    {
       return _enumerator.MoveNext();
    }
    public void Reset()
    {
        _enumerator.Reset();
    }
    #endregion
}

在代码中的用法:

        Test1 t1 = new Test1();
        var NonNullTest1 = new NotNullEnumerable<A>(t1.Children);
        foreach (A child in NonNullTest1)
        {
            child.SomeMethod(); // CodeContracts: Possibly calling a method on a null reference 'child'
        }

任何想法?