Can't create an array of list objects

Question

I have a line of code like this:

List<string>[] apples = new List<string>()[2];

Its purpose is simply to create an array of List objects. When I try to compile my code, the above line generates this error:

Cannot implicitly convert type 'string' to 'System.Collections.Generic.List[]

I haven't been able to find much on the subject of creating an array of List objects (actually only this thread), maybe because no search engines will search for brackets.

Is the only way to create a collection of Lists to put them in another list, like below?

List<List<string>> apples = new List<List<string>>(); //I've tried this and it works as expected

Thanks for any suggestions, I'm really just curious as to why the first line of code (the List[] example) doesn't work.

Haha, actually I was trying to go through the error message logically in my head before I posted this question....It didn't make a whole lot of sense to me, but I figured it was because I'm a newbie. Though I think if I asked why it's such a lousy error message, no one would have the answer :) — Brian Snow, Feb 10 '12 at 00:37

score 7 · Accepted Answer · answered Feb 10 '12 at 00:19

You can do this. The syntax would be:

List<string>[] apples = new List<string>[2];

Note that this only allocates an array of references - you'll need to actually construct the individual list elements before you use them:

List<string>[] apples = new List<string>[2];
apples[0] = new List<string>();
apples[1] = new List<string>();

Alternatively, you can use the collection initialization syntax (works well for small numbers of fixed elements), ie:

List<string>[] apples = new[] { new List<string>(), new List<string>() };

Thanks, I didn't know about the 'collection initialization syntax,' I can see how that would be useful. — Brian Snow, Feb 10 '12 at 00:40

score 6 · Answer 2 · answered Feb 10 '12 at 00:18

6

Try this:

List<string>[] apples = new List<string>[2];

You do the initialization of each list afterwards:

apples[0] = new List<string>();

answered Feb 10 '12 at 00:18

xbrady

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score 3 · Answer 3 · answered Feb 10 '12 at 00:18

3

        var listArray = new List<string>[2];
        for (var i = 0; i < listArray.Length; i++)
        {
            listArray[i] = new List<string>();
        }

answered Feb 10 '12 at 00:18

Matthias

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Can't create an array of list objects

3 Answers3