Question

My unsafe method accepts a collection byte[]s. All of these byte[]s are of the same size.

I need to iterate over them all, searching for certain patterns. The search is inherently reinterpret-cast style: at each offset, I need to consider a value as if it were a float, a double, a short, an int, etc. So getting a byte* for each input byte[] and incrementing it on each iteration seems like a natural approach.

Unfortunately I can’t find any way to create a collection of byte* – or, more specifically, to initialize it from a collection of arrays. Any ideas?

Here’s a somewhat contrived version of the task:

static unsafe void SearchIteration(List<byte[]> arrays, byte[] resultArr)
{
    fixed (byte* resultFix = resultArr)
    {
        byte* resultPtr = resultFix;
        byte*[] pointers = new byte*[arrays.Count];

        <some code to fix all the arrays and store the pointers in "pointers">

        int remaining = resultArr.Length;
        while (remaining > 0)
        {
            <look at resultPtr and each of the pointers and update *resultPtr>

            remaining--;
            for (int i = 0; i < pointers.Length; i++)
                pointers[i]++;
        }
    }
}

Essentially the question is how to initialize pointers with the addresses of arrays, while pinning the arrays so that GC doesn’t move them.

Answer 1

use GCHandle.Alloc() from System.Runtime.InteropServices:

var handles = new GCHandle[arrays.Count];
byte*[] pointers = new byte*[arrays.Count];
for(int i = 0; i < arrays.Count; ++i)
{
    handles[i] = GCHandle.Alloc(arrays[i], GCHandleType.Pinned);
    pointers[i] = (byte*)handles[i].AddrOfPinnedObject();
}
try
{
    /* process pointers */
}
finally
{
    for(int i = 0; i < arrays.Count; ++i)
    {
        handles[i].Free();
    }
}