代码契约静态分析：证明者的局限性？

Question

我一直在玩代码契约，我真的很喜欢到目前为止所看到的。他们鼓励我评估并明确声明我的假设，这已经帮助我识别了一些我在添加合约的代码中没有考虑到的极端情况。现在我正在尝试强制执行更复杂的不变量。我有一个案例目前无法证明，我很好奇除了简单地添加 Contract.Assume 调用之外是否有一种方法可以解决这个问题。这是有问题的类，为了便于阅读而进行了精简：

public abstract class MemoryEncoder
{
    private const int CapacityDelta = 16;

    private int _currentByte;

    /// <summary>
    ///   The current byte index in the encoding stream.
    ///   This should not need to be modified, under typical usage,
    ///   but can be used to randomly access the encoding region.
    /// </summary>
    public int CurrentByte
    {
        get
        {
            Contract.Ensures(Contract.Result<int>() >= 0);
            Contract.Ensures(Contract.Result<int>() <= Length);
            return _currentByte;
        }
        set
        {
            Contract.Requires(value >= 0);
            Contract.Requires(value <= Length);
            _currentByte = value;
        }
    }

    /// <summary>
    ///   Current number of bytes encoded in the buffer.
    ///   This may be less than the size of the buffer (capacity).
    /// </summary>
    public int Length { get; private set; }

    /// <summary>
    /// The raw buffer encapsulated by the encoder.
    /// </summary>
    protected internal Byte[] Buffer { get; private set; }

    /// <summary>
    /// Reserve space in the encoder buffer for the specified number of new bytes
    /// </summary>
    /// <param name="bytesRequired">The number of bytes required</param>
    protected void ReserveSpace(int bytesRequired)
    {
        Contract.Requires(bytesRequired > 0);
        Contract.Ensures((Length - CurrentByte) >= bytesRequired);

        //Check if these bytes would overflow the current buffer););
        if ((CurrentByte + bytesRequired) > Buffer.Length)
        {
            //Create a new buffer with at least enough space for the additional bytes required
            var newBuffer = new Byte[Buffer.Length + Math.Max(bytesRequired, CapacityDelta)];

            //Copy the contents of the previous buffer and replace the original buffer reference
            Buffer.CopyTo(newBuffer, 0);
            Buffer = newBuffer;
        }

        //Check if the total length of written bytes has increased
        if ((CurrentByte + bytesRequired) > Length)
        {
            Length = CurrentByte + bytesRequired;
        }
    }

    [ContractInvariantMethod]
    private void GlobalRules()
    {
        Contract.Invariant(Buffer != null);
        Contract.Invariant(Length <= Buffer.Length);
        Contract.Invariant(CurrentByte >= 0);
        Contract.Invariant(CurrentByte <= Length);
    }
}

我感兴趣的是如何在 ReserveSpace 中构造 Contract 调用，以便可以证明类不变量。特别是，它抱怨 (Length <= Buffer.Length) 和 (CurrentByte <= Length)。对我来说，它看不到 (Length <= Buffer.Length) 得到满足是合理的，因为它正在创建一个新的缓冲区并重新分配引用。添加假设满足不变量是我唯一的选择吗？

Answer 1

在与这个问题斗争了一段时间后，我想出了这个可证明的解决方案（构造函数是一个虚拟的，以允许隔离测试）：

public abstract class MemoryEncoder
{
    private const int CapacityDelta = 16;

    private byte[] _buffer;
    private int _currentByte;
    private int _length;

    protected MemoryEncoder()
    {
        Buffer = new byte[500];
        Length = 0;
        CurrentByte = 0;
    }

    /// <summary>
    ///   The current byte index in the encoding stream.
    ///   This should not need to be modified, under typical usage,
    ///   but can be used to randomly access the encoding region.
    /// </summary>
    public int CurrentByte
    {
        get
        {
            return _currentByte;
        }
        set
        {
            Contract.Requires(value >= 0);
            Contract.Requires(value <= Length);
            _currentByte = value;
        }
    }


    /// <summary>
    ///   Current number of bytes encoded in the buffer.
    ///   This may be less than the size of the buffer (capacity).
    /// </summary>
    public int Length
    {
        get { return _length; }
        private set
        {
            Contract.Requires(value >= 0);
            Contract.Requires(value <= _buffer.Length);
            Contract.Requires(value >= CurrentByte);
            Contract.Ensures(_length <= _buffer.Length);
            _length = value;
        }
    }

    /// <summary>
    /// The raw buffer encapsulated by the encoder.
    /// </summary>
    protected internal Byte[] Buffer
    {
        get { return _buffer; }
        private set
        {
            Contract.Requires(value != null);
            Contract.Requires(value.Length >= _length);
            _buffer = value;
        }
    }

    /// <summary>
    /// Reserve space in the encoder buffer for the specified number of new bytes
    /// </summary>
    /// <param name="bytesRequired">The number of bytes required</param>
    protected void ReserveSpace(int bytesRequired)
    {
        Contract.Requires(bytesRequired > 0);
        Contract.Ensures((Length - CurrentByte) >= bytesRequired);

        //Check if these bytes would overflow the current buffer););
        if ((CurrentByte + bytesRequired) > Buffer.Length)
        {
            //Create a new buffer with at least enough space for the additional bytes required
            var newBuffer = new Byte[Buffer.Length + Math.Max(bytesRequired, CapacityDelta)];

            //Copy the contents of the previous buffer and replace the original buffer reference
            Buffer.CopyTo(newBuffer, 0);
            Buffer = newBuffer;
        }

        //Check if the total length of written bytes has increased
        if ((CurrentByte + bytesRequired) > Length)
        {
            Contract.Assume(CurrentByte + bytesRequired <= _buffer.Length);
            Length = CurrentByte + bytesRequired;
        }
    }

    [ContractInvariantMethod]
    private void GlobalRules()
    {
        Contract.Invariant(_buffer != null);
        Contract.Invariant(_length <= _buffer.Length);
        Contract.Invariant(_currentByte >= 0);
        Contract.Invariant(_currentByte <= _length);
    }
}

我注意到的主要事情是，在属性上放置不变量会变得混乱，但似乎使用不变量更容易解决字段。对财产获取者规定适当的合同义务也很重要。我必须继续试验，看看什么有效，什么无效。这是一个有趣的系统，但我绝对想知道更多是否有人有关于证明器如何工作的良好“备忘单”。