C# Default使用

在泛型类和泛型方法中产生的一个问题是，在预先未知以下情况时，如何将默认值分配给参数化类型 T：

• T 是引用类型还是值类型。

• 如果 T 为值类型，则它是数值还是结构。

给定参数化类型 T 的一个变量 t，只有当 T 为引用类型时，语句 t = null 才有效；只有当 T 为数值类型而不是结构时，语句 t = 0 才能正常使用。 解决方案是使用 default 关键字，此关键字对于引用类型会返回 null，对于数值类型会返回零。 对于结构，此关键字将返回初始化为零或 null 的每个结构成员，具体取决于这些结构是值类型还是引用类型。 对于可以为 null 的值类型，默认返回 System.Nullable(Of T)，它像任何结构一样初始化。

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            // Test with a non-empty list of integers.
            GenericList<int> gll = new GenericList<int>();
            gll.AddNode(5);
            gll.AddNode(4);
            gll.AddNode(3);
            int intVal = gll.GetLast();
            // The following line displays 5.
            System.Console.WriteLine(intVal);

            // Test with an empty list of integers.
            GenericList<int> gll2 = new GenericList<int>();
            intVal = gll2.GetLast();
            // The following line displays 0.
            System.Console.WriteLine(intVal);

            // Test with a non-empty list of strings.
            GenericList<string> gll3 = new GenericList<string>();
            gll3.AddNode("five");
            gll3.AddNode("four");
            string sVal = gll3.GetLast();
            // The following line displays five.
            System.Console.WriteLine(sVal);

            // Test with an empty list of strings.
            GenericList<string> gll4 = new GenericList<string>();
            sVal = gll4.GetLast();
            // The following line displays a blank line.
            System.Console.WriteLine(sVal);
        }
    }

    // T is the type of data stored in a particular instance of GenericList.
    public class GenericList<T>
    {
        private class Node
        {
            // Each node has a reference to the next node in the list.
            public Node Next;
            // Each node holds a value of type T.
            public T Data;
        }

        // The list is initially empty.
        private Node head = null;

        // Add a node at the beginning of the list with t as its data value.
        public void AddNode(T t)
        {
            Node newNode = new Node();
            newNode.Next = head;
            newNode.Data = t;
            head = newNode;
        }

        // The following method returns the data value stored in the last node in
        // the list. If the list is empty, the default value for type T is
        // returned.
        public T GetLast()
        {
            // The value of temp is returned as the value of the method. 
            // The following declaration initializes temp to the appropriate 
            // default value for type T. The default value is returned if the 
            // list is empty.
            T temp = default(T);

            Node current = head;
            while (current != null)
            {
                temp = current.Data;
                current = current.Next;
            }
            return temp;
        }
    }

}