Question

I am having a problem while retrieving data from an ArrayList and displaying them into textboxs. I am getting an error: Unable to cast object of type 'Lab_9.Book' to type 'Lab_9.Magazine'. I tried use 2 foreach loops but that seems to be out of the question. How could I avoid this problem?

Problem occurs here:

    // 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;
    }

Regards.

Answer 1

The items of your pub object(ArrayList) are Object type instances for Book and Magazine types. So you need to filter using .OfType() to display each type. This method will returns only the instances of the target type(T) in ArrayList.

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

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

To combine the two foreach, I will suggest you to override ToString() or to make a base class having the string property. For using the base class, set all of values(eg title + “,” + bookNumber…) to the string property. I will show you overriding ToString().

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

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

And then you can combine the two loop.

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