convert List> to IList>

Question

I have written a method which is public List<List<object>> Fetch(string data), inside I create List<List<object>> p = new List<List<object>>();

my boss now wants to return a IList<IList<object>> instead of List<List<object>> ie
public IList<IList<object>> Fetch(string data),

so when I try do return (IList<IList<object>>) p; //throws an exception

How do I convert List<List<object>> to IList<IList<object>> and back to List<List<object>>

If you need to convert it back your design is most likely broken. — Albin Sunnanbo, Jan 25 '12 at 16:11

score 93 · Accepted Answer · answered Jan 25 '12 at 16:14

93

You can't perform that conversion via straight casting - it wouldn't be safe. Instead, you should use:

IList<IList<object>> ret = new List<IList<object>>();

Then for each "sublist" you can use:

// Or whatever
ret.Add(new List<object>());

Finally, just return ret.

You could use LINQ to perform the conversion of your existing List<List<object>> when you return it - but it would be better to just create a more appropriate type to start with, as shown above.

To understand why some of the existing answers are wrong, suppose you could do this:

IList<IList<object>> p = new List<List<object>>();

Then this would be valid:

List<List<object>> listOfLists = new List<List<object>>();
IList<IList<object>> p = listOfLists;
p.Add(new object[]);
List<object> list = p[0];

But p[0] is a reference to an object[], not a List<object>... our supposedly type-safe code doesn't look as safe any more...

Fortunately, IList<T> is invariant to prevent exactly this problem.

answered Jan 25 '12 at 16:14

Jon Skeet

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Are you hinting that List – Muhammad Hasan Khan Jan 25 '12 at 16:21
@HasanKhan: I'm saying that `object[]` implements `IList – Jon Skeet Jan 25 '12 at 16:22
4

@JonSkeet oh didn't know array also implements IList – Muhammad Hasan Khan Jan 25 '12 at 16:23
@Jon Skeet thanks for the answer and also the detailed explanation. – david Jan 27 '12 at 05:43
@JonSkeet What about `IList> listOfList = tabOfTab;` where `tabOfTab` is `double[][]` ? – Adrien Piquerez Aug 26 '16 at 13:13
@DineMartin: What about it? I'm not sure what you're looking for. – Jon Skeet Aug 26 '16 at 13:14
@JonSkeet IList is invariant but I can cast tabOfTab to an IList> – Adrien Piquerez Aug 26 '16 at 13:19
@DineMartin: That would be due to array variance, e.g. how a `string[]` can be viewed as `object[]`. – Jon Skeet Aug 26 '16 at 13:21
@JonSkeet But I cannot convert double[] to type object[] – Adrien Piquerez Aug 26 '16 at 13:33
@DineMartine: No, because double is a value type. This is all getting a bit off the point. I suggest you try to find a related question or ask a new one. – Jon Skeet Aug 26 '16 at 13:34
this becomes very weird with `IList = ImmutableArray>` – CervEd Nov 14 '22 at 20:08

Olivier Jacot-Descombes · Answer 2 · 2017-05-08T13:35:27.187

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You would have to declare your list as

IList<IList<object>> list = new List<IList<object>>(); // Works!

This works, because only the outer list is created in the first place. You can then insert individual items that are compatible with IList<object>:

list.Add(new List<object>());
list.Add(new object[10]);

edited May 08 '17 at 13:35

answered Jan 25 '12 at 16:14

Olivier Jacot-Descombes

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score 4 · Answer 3 · answered Jan 25 '12 at 16:12

4

var myOriginalList = new List<List<Object>>();
var converted = ((IList<IList<Object>>)myOriginalList.Cast<IList<Object>>().ToList()); // EDIT: Added .ToList() here

You shouldn't need to convert back- you can do just about anything on IList that you could on List.

answered Jan 25 '12 at 16:12

Chris Shain

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Why bother creating a new list though? It's simpler just to create the right type to start with. – Jon Skeet Jan 25 '12 at 16:19
Agreed 100%, and your approach explains that well. I leave this up to show that if you needed to do it (e.g. the off chance a third party library returned a `List>` that you needed to convert for another 3rd party library), you can do it this way. – Chris Shain Jan 25 '12 at 16:21

score 2 · Answer 4 · answered Jan 25 '12 at 16:15

You need to change the declaration of the result variable from List<List<object> to IList<IList<object>>

Which you can instantiate against List<IList<object>>

And each item in the result can be of type List<object>

    static IList<IList<object>> Test()
    {
        IList<IList<object>> result = new List<IList<object>>();
        var item = new List<object>() { "one", "two", "three" };
        result.Add(item);
        return result;
    }

score 1 · Answer 5 · answered Jan 25 '12 at 16:13

1

public IList<IList<object>> Fetch(string data)
{
  IList<IList<object>> p = new List<IList<object>>();

  // add your stuff here

  return p;
}

answered Jan 25 '12 at 16:13

John Ruiz

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@vidstige JonSkeet was correct for about 30 seconds.. I originally had IList> p = new List>(); which would not compile, and shouldn't. I noticed my mistake like 10 seconds after posting. TEN SECONDS. Further proof that JonSkeet is actually a bot. =) I don't know who that was talking to Hanselman the other day... – John Ruiz Jan 25 '12 at 16:43
lol thats quick alright. But I cannot see that your post was edited? Perhaps there is some 'quiet time' where you can editit it without beeing a new revision... – vidstige Jan 25 '12 at 17:47

Aliostad · Answer 6 · 2012-01-25T16:17:37.190

0

List<T> and IList<T> do not support covariance. So this does not compile:

List<List<string>> list = new List<IList<string>>(); // DOES NOT COMPILE!!!!!

edited Jan 25 '12 at 16:17

answered Jan 25 '12 at 16:11

Aliostad

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`List` is already of Type `IList` No co- (or contra-)variance is required. – AllenG Jan 25 '12 at 16:13
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@AllenG so can you explain why that snippet I have put does not compile? – Aliostad Jan 25 '12 at 16:14
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@AllenG: Yes, contravariance *is* required here, because a `List>` isn't a `List>`. Did you try to compile the code Aliostad said won't compile? It really won't. – Jon Skeet Jan 25 '12 at 16:15
I added my comment before you added your snippet. List> should be a valid return on a Method with a signature of IList>. – AllenG Jan 25 '12 at 16:22
@AllenG that still does not justify downvote. The problem OP experiences is the result of lack of covariance/contravariance in ILst and List. – Aliostad Jan 25 '12 at 16:25
5

@AllenG: No, it absolutely should not. There are too many lists here. Should List be a valid return for a method typed as returning IList? Yes. Should List be a valid return for a method typed as returning IList? **No, absolutely not**. The caller reasonably expects that they can insert a Bottle into an IList but what you have in hand is a List that cannot contain a Bottle. Therefore the conversion must be illegal. – Eric Lippert Jan 25 '12 at 16:35

convert List> to IList>

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