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Integer array or array of structures - which is better?

In my application, I store data Bitmapin a two-dimensional integer array ( int[,]). To access the values ​​of R, G, and B, I use something like this:

// read:
int i = _data[x, y];
byte B = (byte)(i >> 0);
byte G = (byte)(i >> 8);
byte R = (byte)(i >> 16);
// write:
_data[x, y] = BitConverter.ToInt32(new byte[] { B, G, R, 0 }, 0);

I use whole arrays instead of the actual one System.Drawing.Bitmap, because my application runs on Windows Mobile devices, where the memory available for creating raster images is very limited.

I am interested, however, if it would be more appropriate to declare such a structure:

public struct RGB
{
    public byte R;
    public byte G;
    public byte B;
}

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class Program
{
    static void Main()
    {
        int[] i = { -1 };
        RGB rgb = i[0];
    }
}
[StructLayout( LayoutKind.Explicit)]
public struct RGB
{
    public RGB(int value) {
        this.R = this.G = this.B = 0; this.Value = value;
    }
    [FieldOffset(0)]
    public int Value;
    [FieldOffset(2)]
    public byte R;
    [FieldOffset(1)]
    public byte G;
    [FieldOffset(0)]
    public byte B;

    public static implicit operator RGB(int value) {
        return new RGB(value);
    }
    public static implicit operator int(RGB value) {
        return value.Value;
    }
}
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