C#字符串连接Null时的一个小坑

C#里String.Concat方法有一个特别的地方，参数为Null时是不会抛出NullReferenceException异常的，而且，如果两个参数皆为Null,返回值是“”！有图有真相：

MSDN上的解释： In string concatenation operations, the C# compiler treats a null string the same as an empty string, but it does not convert the value of the original null string.

stackoverflow上的相关问题

虽然不是什么大坑，但还是值得mark一下，我本来是定义了几个类，因为需要放BindingList里头做数据源，定义的类重载了Equals方法和GetHashCode方法以方便查找，代码大概是这样的：

    public class Person     {         public string FirstName { get; set; }         public string SecondName { get; set; }         public int Age { get; set; }

        public override bool Equals(object obj)         {             if (base.Equals(obj))                 return true;             var d = obj as Person;             if (d == null)                 return false;             return FirstName == d.FirstName && SecondName == d.SecondName;         }

        public override int GetHashCode()         {             return string.Concat(FirstName, SecondName).GetHashCode();         }     }

    public class Book     {         public string Id { get; set; }         public double Price { get; set; }

        public override bool Equals(object obj)         {             if (base.Equals(obj))                 return true;             var d = obj as Book;             if (d == null)                 return false;             return Id == d.Id;         }

        public override int GetHashCode()         {             return Id.GetHashCode();         }     }