EvalStack.cs

class EvalStack
{
  private const char YesThis = '1';
  private const char NotThis = '0';
  private readonly string _;

  private EvalStack(string s) { _ = s; }

  public EvalStack() : this("") {}

  public bool Peek()
  {
    if (IsEmpty)
    {
      throw new Exception("Cannot Peek an empty stack.");
    }
    return _[0] == YesThis;
  }

  public EvalStack Pop()
  {
    if (IsEmpty)
    {
      throw new Exception("Cannot Pop an empty stack.");
    }
    return new EvalStack(_.Substring(1));
  }

  public EvalStack Push(bool b)
  {
    char c = b ? YesThis : NotThis;
    return new EvalStack(c + _);
  }

  public bool IsEmpty
  {
    get { return _.Length == 0; }
  }

  public override bool Equals(object that)
  {
    return Equals(that as EvalStack);
  }

  public bool Equals(EvalStack that)
  {
    return _.Equals(that._);
  }

  public override int GetHashCode()
  {
    return (_ != null ? _.GetHashCode() : 0);
  }

  public override string ToString()
  {
    return "[" + _ + "]";
  }
}