从ArrayList检索对象-强制转换为类型？

Question

I'm taking a basic course in C# programming, and this is a part of our assignment. Very new to programming so I feel more than a bit lost with this.

The assignment is to add an ArrayList and insert strings from a file to this, which I hopefully have done with the code below:

• Read () is a method in another class ( FileReader ) that reads the files from "info.txt" and returns an ArrayList .

• The ArrayList items are supposed to store the object items, although I'm not quite sure why I need two arrays?

The problem I have is: when you retrieve the "items" from the array, they have to be cast to a type, string , (if I understand it correctly, otherwise they are returned as objects ?). How do I do that?

Can you cast the entire ArrayList ?

public PriceFlux ()  //Constructor
{
    ArrayList items;          
    items = new ArrayList();
    FileReader infoFile = new FileReader("info.txt"); 
    items = infoFile.Read();   
}

The files from the info.txt looks approximately like this:

G&34&Kellogs K frukostflingor&Sverige&29.50&5/11/2005&29/10/2005&29/10/2006

Here is the FileReader Read() method:

public ArrayList Read ()
{
    ArrayList fileContent = new ArrayList ();
    try
    {                               
        while (line != null)
        {   
            fileContent.Add (line);
            line = reader.ReadLine ();
        }
        reader.Close ();
    } 
    catch
    {
        Console.WriteLine ("Couldn´t read from file.");
    }
    return fileContent;
}

Very grateful for suggestions on how to solve this.

Answer 1

You can use linq to do this easily:

This will cast all items to string and return an IEnumerable<string> . It will fail if any items can't be cast to a string :

items.Cast<string>();

This will cast all items that can be to a string and skip over any that can't:

items.OfType<string>();

Answer 2

You can access single elements in the ArrayList doing a cast, for example...

string s = myArrayList[100] as string;
myArrayList.Remove("hello");
myArrayList[100] = "ciao"; // you don't need a cast here.

You can also iterate through all elements without a cast...

foreach (string s in myArrayList)
    Console.WriteLine(s);

You can also use CopyTo method to copy all items in a string array...

string[] strings = new string[myArrayList.Count];
myArrayList.CopyTo(strings);

You can create another List<string> with all items in the ArrayList . Since ArrayList implements IEnumerable you can call the List<string> constructor.

List<string> mylist = new List<string>(myArrayList);

But this doesn't makes much sense... why you don't just use List<string> directly? Using directly a List<string> seems more useful to me, and is faster. ArrayList still exists mostly for compatibility purposes since generics were introduced in version 2 of the language.

I just noticed that there may be an error in your code:

    while (line != null)
    {   
        fileContent.Add (line);
        line = reader.ReadLine ();
    }

Should be instead

    for (;;)
    {   
        string line = reader.ReadLine();
        if (line == null)
            break;
        fileContent.Add(line);
    }

Answer 3

在使用每个元素之前，您必须对其进行单独转换。