CODEWITHSUNDEEP
c#arrayliststreamreader
James Dawson
James Dawson

Reputation: 5409

Adding ArrayList to ArrayList

I'm trying to have nested ArrayLists, and tried to add a new ArrayList as an element of an existing one, but it doesn't seem to work. Here's the code:

    private void readDataFile()
    {
        // Open stream reader
        StreamReader reader = new StreamReader(dataFileLoc);

        string line = string.Empty;
        line = reader.ReadLine();
        myArgus.Add(new ArrayList().Add(line));

        while ((line = reader.ReadLine()) != null)
        {
            // TODO
        }

        // Close the reader
        reader.Close();
    }

If I add a breakpoint and look at the data in myArgus (the already existing ArrayList), the first element is just 0. It should be an ArrayList which has the first element as 10016 (that's the first line of the text file).

What am I doing wrong here?

Thanks!

Edit: It's worth noting that we are required to use an ArrayList.

Upvotes: 0

Views: 3625

Answers (3)

Alexei Levenkov
Alexei Levenkov

Reputation: 100547

ArrayList.Add does not return ArrayList, but simply int.

So you want to add elements in 2 steps.

int index = myArgus.Add(new ArrayList());
((ArrayList)myArgus[index]).Add(line);

Note: Use strongly typed collections (List<string>) to avoid this type of errors.

Upvotes: 3

Sofian Hnaide
Sofian Hnaide

Reputation: 2364

You can technically still do it in one step

myArgus.Add(new ArrayList() { line } );

Upvotes: 2

Heinzi
Heinzi

Reputation: 172260

This line:

myArgus.Add(new ArrayList().Add(line));

adds 0 as the first element to myArgus. Why? Because the return value of ArrayList.Add is defined as follows:

Return Value
Type: System.Int32
The ArrayList index at which the value has been added.

Since your new ArrayList() does not contain any items, line is added at index 0.

What you probably meant to write was:

var subList = new ArrayList();
subList.Add(line);
myArgus.Add(subList);

In fact, what you should do is to use typed lists:

List<List<string>> myArgus;

This helps catch this kind of error, because the compiler complains if an int is added instead of the expected List<string>.

Upvotes: 3

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