c# - 使用 foreach 循环显示对象

Question

我在从 ArrayList 检索数据并将其显示到文本框中时遇到问题。我收到错误:无法将“Lab_9.Book”类型的对象转换为“Lab_9.Magazine”类型。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？

问题发生在这里:

    // 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

这将允许你只有一个循环，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: ");
   }
}

您真正想要定义一个接口(interface)来封装所有发布的公共(public)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(interface)

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大的灵 active ，但出于您的目的，您可以保持其余代码不变，并仅使用一个更干净的循环，并且与 jwJung 的解决方案更加内联

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);
}