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泛型程式碼中的預設關鍵字 (C# 程式設計手冊)

在泛型類別和方法中會發生一個問題,也就是當您無法預先知道下列資訊時,如何將預設值指派給參數化型別 T:

  • T 是參考型別或實值型別

  • 如果 T 是實值型別,則會是數字值或結構

當指定參數化型別 T 的變數 t 時,只有在 T 是參考型別時,陳述式 t = null 才有效,並且 t = 0 只有對數值型別 (而不是結構) 有效。 解決方法是使用 default 關鍵字,對參考型別傳回 null 而對數值型別傳回零。 這種方法會根據結構是實值或參考型別,為結構傳回初始化成零或 null 的每個結構成員。 對於可為 Null 的實質型別,預設值會傳回 System.Nullable<T>,該值會與任一結構一樣經過初始化。

下列 GenericList<T> 類別的程式碼範例示範如何使用 default 關鍵字。 如需詳細資訊,請參閱泛型概觀

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;
        }
    }
}

請參閱

參考

泛型 (C# 程式設計手冊)

泛型方法 (C# 程式設計手冊)

System.Collections.Generic

概念

C# 程式設計手冊

.NET Framework 中的泛型