How can I quickly up-cast object[,] into double[,]?

Question

I using Microsoft.Office.Interop.Excel I get returned a 2D array of type object[,] which contains double for elements. Note that the i

Answer 1

I wouldn't say that there is one way that's faster than another so long as you don't do anything stupid. I'd say that, if you can, cast them when you access them rather than up front. Of course this depends on how you intend to access them. If you're going to index into the array more than once then the cost of unboxing might start to get too much. If you're only scanning the array once, then cast as you go.

Answer 2

object[,] src = new object[2, 3];

// Initialize src with test doubles.
src[0, 0] = 1.0;
src[0, 1] = 2.0;
src[0, 2] = 3.0;
src[1, 0] = 4.0;
src[1, 1] = 5.0;
src[1, 2] = 6.0;

double[,] dst = new double[src.GetLength(0), src.GetLength(1)];
Array.Copy(src, dst, src.Length);

Answer 3

Array.Copy(src, dst, src.Length);

This code will get error if any of the value in src is null. Since in the above code src have some defined value it works fine. If the value of src is dynamically set and unfortunately if any of the value is null, the above code will not work. Value wont be copied successfully.

Answer 4

This should work in most cases, but may throw an exception if you don't assign a conversion delegate.

public static TResult[,] Convert<TSource, TResult>(
    this TSource[,] array, Func<TSource, TResult> conversion = null) {

        if(array == null) throw new ArgumentNullException("array");

        if (conversion == null) {
            var resultType = typeof(TResult);
            conversion = source => (TResult)System.Convert.ChangeType(source, resultType);
        }

        var width = array.GetLength(1);
        var height = array.GetLength(0);
        var result = new TResult[height, width]; 

        for (int i = 0; i < height; ++i)
            for (int j = 0; j < width; ++j)
                result[i, j] = conversion(array[i, j]);

        return result;
    }

Answer 5

There are a couple problems here.

First, since double is not a reference type it must be boxed to be stored in an object[], so the only way to get to the values is to unbox the values into a double[] (copy copy).

The other problem is that in C#, arrays are covariant but not contravariant, you can assign an array to a reference of a more-derived type, not not to one of a less-derived type.

string[] strings = new string[10];
object[] objects = strings; // OK, covariant
string[] strings2 = objects; // not OK, contravariant
objects[0] = 10; // Compiles fine, runtime ArrayTypeMismatchException!