为什么我的索引器不起作用？

Question

  

索引器允许对类或结构的实例进行索引   阵列。索引器类似于属性，除了它们的访问器   参数。

我有这样的代码

class StudentMemento
    {
        Student student;
        public Student this[int index]
        {
            get { return student; }
            set { student = new Student { time = DateTime.Now }; }
        }
    }
    class Client
    {
        static void Main()
        {
            StudentMemento s = new StudentMemento();
            Student s1 = s[1];
            Student s2 = s[2];
            Student s3 = s[1];
            Console.Read();
        }
    }

根据msdn中的文档，我应该在以下成员s1，s2中获取Student的实例，因为我在Indexer中返回Student对象，但是我得到一个空引用。任何人都可以帮助我理解，为什么会这样。谢谢

Answer 1

在

null

s.student将是student。 StudentMemento s = new StudentMemento(); s[1] = null; Student s1 = s[1]; Student s2 = s[2]; Student s3 = s[1]; 字段仅在索引器设置器中分配，因此您需要在调用getter之前调用它，例如。

var folderPicker = new Windows.Storage.Pickers.FolderPicker();
folderPicker.SuggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.Desktop;
folderPicker.FileTypeFilter.Add("*");
folderPicker.ViewMode = Windows.Storage.Pickers.PickerViewMode.List;
Windows.Storage.StorageFolder folder = await folderPicker.PickSingleFolderAsync();

if (folder != null)
{
    Windows.Storage.AccessCache.StorageApplicationPermissions.FutureAccessList.AddOrReplace("GameFilesToken", folder);
    System.Diagnostics.Debug.WriteLine(folder.Path);

    await FileManagementHelper.getFolders(folder.Path);
}

public static async Task<Queue<string>> getFolders(String TopLevel)
{
    var Folder = await Windows.Storage.AccessCache.StorageApplicationPermissions.FutureAccessList.GetFolderAsync("GameFilesToken");

    var queryResults = Folder.CreateFolderQuery();
    var folderList = await queryResults.GetFoldersAsync();

    return new Queue<string>();
}