新建一個.NET Core控制檯專案，敲入下面程式碼，我們來測試將值型別和引用型別加入List<T>集合，有什麼不同：

using System;
using System.Collections.Generic;

namespace NetCoreList
{
    //類People，是引用型別
    class People
    {
        public string Name
        {
            get;
            set;
        }
    }

    //結構體Person，是值型別
    struct Person
    {
        
 
public string Name
        {
            get;
            set;
        }
    }


    class Program
    {
        //顯示Person集合所有元素
        static void DisplayPersonCollection(List<Person> collection)
        {
            Console.WriteLine("==========PersonCollection==========");

            foreach
 
 (Person person in collection)
            {
                Console.WriteLine("Person:" + person.Name);
            }
        }

        //顯示People集合所有元素
        static void DisplayPeopleCollection(List<People> collection)
        {
            Console.WriteLine("==========PeopleCollection==========
 
");

            foreach (People people in collection)
            {
                Console.WriteLine("People:" + people?.Name);
            }
        }

        static void Main(string[] args)
        {
            //定義一個Person
            Person person = new Person()
            {
                Name = "Jack"
            };

            //定義一個People
            People people = new People()
            {
                Name = "Jack"
            };

            Console.WriteLine("person's name is:" + person.Name);
            Console.WriteLine("people's name is:" + people.Name);

            //定義Person集合personCollection
            List<Person> personCollection = new List<Person>();
            personCollection.Add(person);
            personCollection.Add(person);
            personCollection.Add(person);

            //定義People集合peopleCollection
            List<People> peopleCollection = new List<People>();
            peopleCollection.Add(people);
            peopleCollection.Add(people);
            peopleCollection.Add(people);

            People peopleNull = null;
            peopleCollection.Add(peopleNull);
            peopleCollection.Add(peopleNull);

            DisplayPersonCollection(personCollection);
            DisplayPeopleCollection(peopleCollection);

            Console.WriteLine();
            Console.WriteLine();
            Console.WriteLine();

            person.Name = "Tom";//更改person的Name
            people.Name = "Tom";//更改people的Name

            Console.WriteLine("person's name is:" + person.Name);
            Console.WriteLine("people's name is:" + people.Name);

            DisplayPersonCollection(personCollection);
            DisplayPeopleCollection(peopleCollection);

            Console.WriteLine("Press any key to end...");
            Console.ReadKey();
        }
    }
}

執行上面的程式碼，結果如下所示：

可以看到：

每次我們將值型別Person結構體的物件例項person，加入List<Person>集合personCollection時，相當於是複製了一個Person結構體物件例項，然後加入到List<Person>集合personCollection中，所以後面當我們修改物件例項person的Name屬性為"Tom"後，personCollection集合中的三個Person結構體物件例項都沒有改變，它們的Name屬性還是"Jack"。

每次我們將引用型別People類的物件例項people，加入List<People>集合peopleCollection時，實際上是將同一個People類物件例項的引用（相當於指標），加入到了List<People>集合peopleCollection中，所以實際上List<People>集合peopleCollection中儲存的都是People類物件例項的引用（相當於指標），前三個不為null的People類引用（相當於指標），指向的都是記憶體中同一個People類物件例項，所以後面當我們修改物件例項people的Name屬性為"Tom"後，peopleCollection集合中三個不為null的People類物件例項都跟著改變了，因為它們三個實際上是同一個People類物件例項。

在微軟的官方文件這個地方，有對List<T>集合型別的如下描述：

List<T> accepts null as a valid value for reference types and allows duplicate elements.

參考文獻：

List<T> Class

List<T>.Add(T) Method