Question

我在SortedList上使用Linq时遇到了麻烦。我有一个类库和一个Test WinForms应用程序。类库代码是：

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace JTS
{
    public static class Pictures
    {
        public static class PictureManagement
        {
            public static SortedList<int, Picture> PictureList = new SortedList<int, Picture>();

            public static class Acquire
            {
                public static Picture ById(int id)
                {
                    Picture picture = null;

                    if(PictureList != null)
                    {
                        picture = (Picture)from pic in PictureList
                                          where
                                              pic.Key == id
                                          select (pic.Value as Picture);
                    }
                    return picture;
                }
            }
        }

        public class Picture : PictureBox
        {
            public int Id { get; set; }

            public class PictureProperties
            {
                public string Name { get; set; }
                public string Extension { get; set; }
                public string OriginalPath { get; set; }
                public Image OriginalPhoto { get; set; }
                public string Credits { get; set; }

                public Size DesignTimeSize { get; set; }
                public Point DesignTimeLocation { get; set; }

            }

            public PictureProperties Properties { get; set; }

            public class PictureThumbnail
            {
                public string Name { get { return "T" + Name; } }
                public Size Size { get; set; }
            }

            public PictureThumbnail Thumbnail { get; set; }
        }
    }
}

并且测试winforms应用代码是：

using JTS;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication7
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        public void test()
        {
            for(int i = 0; i < 10; i++)
            {
                JTS.Pictures.PictureManagement.PictureList.Add(i, new JTS.Pictures.Picture()
                    {
                        Id = i,
                        Properties = new JTS.Pictures.Picture.PictureProperties()
                        {
                            Name = "Travis",
                            Credits = "people in here"
                        },

                        Thumbnail = new JTS.Pictures.Picture.PictureThumbnail()
                        {
                            Size = new Size(250, 250)
                        }
                    });

                Console.WriteLine("pics added to list");

                Console.WriteLine("getting Picture by id 3");

                JTS.Pictures.Picture pic = JTS.Pictures.PictureManagement.Acquire.ById(3);

                Console.WriteLine(pic.Id);
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            test();
        }
    }
}

如果您要将其粘贴到VS中，请不要忘记添加System.Windows.Forms作为类库的参考。

我遇到的问题是当我运行Test winforms应用程序时，在FormLoad上，我得到一个例外，其中说：

未处理的类型＆＃39; System.InvalidCastException＆＃39;发生了   在JTS.dll中

附加信息：无法投射类型的对象   ＆＃39; {WhereSelectEnumerableIterator {1}} 2 [System.Int32，JTS.Pictures +照片]，JTS.Pictures +照片]＆＃39;   输入图片＆＃39;。

现在，我知道什么是无效的演员阵容。但我不知道它指的是什么。我在哪里做了无效演员？

Answer 1

根据错误消息，错误在于：

picture = (Picture)from pic in PictureList
                               where  pic.Key == id
                               select (pic.Value as Picture);

试试这个：

picture = from pic in PictureList
                      where  pic.Key == id
                      select (pic.Value as Picture)).FirstOrDefault();

或者，因为它是SortedList，为什么不简单地使用它？

var picture = PictureList[id];

Answer 2

picture = (Picture)from pic in PictureList
                                          where
                                              pic.Key == id
                                          select (pic.Value as Picture);

此查询返回IEnumerable<Picture>，而不是Picture。

试试这个：

picture = (Picture)from pic in PictureList
                                          where
                                              pic.Key == id
                                          select (pic.Value as Picture).First();

Answer 3

此查询在执行后返回IEnumerable<T>：

from pic in PictureList
where pic.Key == id
select (pic.Value as Picture)

如果您想获得单个项目，请使用Picture或First方法将其投放到单个Single。

(from pic in PictureList
where pic.Key == id
select (pic.Value as Picture)).First();

Answer 4

使用Lambda：

Picture picture = PictureList.Where(pic => pic.Key == id).FirstOrDefault().Value;

此代码替换您从PictureList集合中执行select的部分。您的代码的问题是where子句返回Picture的集合，并且您将集合分配给不是集合的Picture，因此您获得了InvalidCastException {1}}。

System.InvalidCastException。好的，哪里？

4 个答案: