使用 foreach 循环显示对象

Question

我在从 ArrayList 检索数据并将它们显示到文本框中时遇到问题。我收到一个错误：Unable to cast object of type 'Lab_9.Book' to type 'Lab_9.Magazine'。我尝试使用 2 个 foreach 循环，但这似乎是不可能的。我怎样才能避免这个问题？

问题出现在这里：

    // Displaying all Book objects from pubs ArrayList.
    foreach (Book list in pubs)
    {
        bookNumber++; // Count books from the begining.

        // Displaying and formating the output 
        // in txtBookList textbox.
        bookList.txtBookList.Text +=
            "=== Book " + bookNumber + " ===" + Environment.NewLine +
            list.Title + Environment.NewLine +
            list.getAuthorName() + Environment.NewLine +
            list.PublisherName + Environment.NewLine +
            "$" + list.Price + Environment.NewLine
            + Environment.NewLine;
    }

问候。

Answer 1

这将允许您只有一个循环，但它并不漂亮

foreach (object publication in pubs)
{
   var book = publication as Book;
   var magazine = publication as Magazine;
   if (book != null) {
     //it's a book, do your thing
   } else if (magazine != null) {
     //it's a magazine, do your thing
   } else {
    throw new InvalidOperationException(publication.GetType().Name + " is not a book or magazine: ");
   }
}

你真的想定义一个接口来封装所有发布的公共属性和方法，而不是继承。

public interface IPublication
{
  string Title {get;set;}
  float Price {get;set;}
  // etc.
}

接下来让你的类实现接口

public class Book : IPublication
{
  public string Title {get;set;}
  public float Price {get;set;}
  //the rest of the book implementation
}

public class Magazine: IPublication

{
  public string Title {get;set;}
  public float Price {get;set;}
  //the rest of the magazine implementation
}

此时您已经获得了更多的灵活性，但出于您的目的，您可以保持其余代码不变，并且只使用一个更简洁、更符合 jwJung 解决方案的循环

foreach (var publication in pubs.OfType<IPublication>())
{
  // publication is either a book or magazine, we don't care we are just interested in the common properties
  Console.WriteLine("{0} costs {1}",  publication.Title, publication.Price);
}

Answer 2

您的 pub 对象 (ArrayList) 的项目是 Book 和 Magazine 类型的 Object 类型实例。因此，您需要使用 .OfType() 进行过滤以显示每种类型。此方法将仅返回 ArrayList 中目标类型 (T) 的实例。

foreach (Book list in pubs.OfType<Book>())
{
}

foreach (Magazine list in pubs.OfType<Magazine>())
{
}

要结合这两个 foreach，我建议你重写 ToString() 或创建一个具有字符串属性的基类。要使用基类，请将所有值（例如 title + "," + bookNumber...) 设置为字符串属性。我将向您展示重写 ToString()。

internal class Book
{
    public override string ToString()
    {
        return "=== Book " + bookNumber + " ===" + Environment.NewLine +....;
    }
}

internal class Magazine
{
    public override string ToString()
    {
        return "=== Publication: " + bookNumber + ....;
    }
}

然后你可以将两个循环结合起来。

foreach (string item in pub)
{
    Console.WriteLine(item.ToString());
}

Answer 3

编辑：我重新阅读了您的问题和您的评论（对此感到抱歉），我认为您正在寻找这个：

foreach (var list in pubs)
{
    if(list is Book)
    {
        Book tmp = (Book)list;
        // Print tmp.<values> 
    }

    if(list is Magazine)
    { 
        Magazine tmp = (Magazine)list;
        // Print tmp.<values>         
    }
}