Create a subarray reference in C# (using unsafe ?)

Question

Hello there,

I'm refactoring a library we currently use, and I'm faced with the following problem.

We used to have the following stuff :

class Blah
{
    float[][] data;
    public float[] GetDataReference(int index)
    {
        return data[index];
    }
}

For various reasons, I have replaced this jagged array version with a 1 dimensionnal array version, concatenating inner arrays. My question is : how can I still return a reference to a sub array of data ?

class Blah
{
    float[] data;
    int rows;

    public float[] GetDataReference(int index)
    {
        // Return a reference data from offset i to offset j;
    }
}

I was thinking that unsafe and pointers stuff may be of use, is it doable ?

Answer 1

No, you can't do this - but you should look at using ArraySegment instead.

Note that an array object consists of metadata about its length etc and then the data itself. You can't create a slice of an existing array and still have the metadata next to the data, if you see what I mean - there'd have to be an extra level of indirection (which is what ArraySegment provides).

(I'm slightly surprised that ArraySegment doesn't do more wrapping, e.g. by implementing IList<T>, but there we go. It would be easy enough to create such a structure if you wanted to.)

Answer 2

Check the following question on SO, but don't use the accepted answer (you don't want to copy after all), but rather follow the other options of using an Enumerator for the slice/subpart of the original array.

Array slices in C#