How can I convert a list<> to a multi-dimensional array?

Question

I have the following method signature:

public void MyFunction(Object[,] obj)

I create this object:

List<List<Object>> obj = new List<List<Object>>;

Is there an easy way I can convert this to an Object[,]?

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UPDATE:

The fact is I like to use Lists because I can easily add a new item. Is there a way I can declare my List<> object to fit this need? I know the number of columns in my Object[,] but not the number of rows.

Answer 1

No. In fact, these aren't necessarily compatible arrays.

[,] defines a multidimensional array. List<List<T>> would correspond more to a jagged array ( object[][] ).

The problem is that, with your original object, each List<object> contained in the list of lists can have a different number of objects. You would need to make a multidimensional array of the largest length of the internal list, and pad with null values or something along those lines to make it match.

Answer 2

You're not going to get a very simple solution for this (i.e. a few lines). LINQ/the Enumerable class isn't going to help you in this case (though it could if you wanted a jagged array, i.e. Object[][]). Plain nested iteration is probably the best solution in this case.

public static T[,] To2dArray(this List<List<T>> list)
{
    if (list.Count == 0 || list[0].Count == 0)
        throw new ArgumentException("The list must have non-zero dimensions.");

    var result = new T[list.Count, list[0].Count];
    for(int i = 0; i < list.Count; i++)
    {
        for(int j = 0; j < list[i].Count; j++)
        {
            if (list[i].Count != list[0].Count)
                throw new InvalidOperationException("The list cannot contain elements (lists) of different sizes.");
            result[i, j] = list[i][j];
        }
    }

    return result;
}

I've included a bit of error handling in the function just because it might cause some confusing errors if you used it on a non-square nested list.

This method of course assumes that each List<T> contained as an element of the parent List is of the same length. (Otherwise you really need to be using a jagged array.)