String类是密封类,因此是不允许继承的:
[SerializableAttribute] [ComVisibleAttribute(true)] public sealed class String : IComparable, ICloneable, IConvertible, IComparable<string>, IEnumerable<char>, IEnumerable, IEquatable<string>
具体的看:http://msdn.microsoft.com/zh-cn/library/system.string(v=vs.110).aspx