Convert an array of pointers to an array of IntPtr

Question

I'm stuck on a seemingly trivial task and need your help.

I need to write a method with the following signature:

System.Array ToIntPtrArray(System.Array a)

where an actual argument can be an array of any pointer type (e.g. int*[], long**[], void*[,]) and returning an array of the same shape with elements of type System.IntPtr having the same numeric values as elements of an input array. The problem is that I do not understand how to extract numeric values of pointers if I do not know their types beforehand.

---

For example, if I knew beforehand that my argument is always of type void*[], I could write the method as follows:

unsafe IntPtr[] ToIntPtrArray(void*[] a)
{
    var result = new IntPtr[a.Length];
    for (int i = 0; i < a.Length; i++)
        result[i] = (IntPtr) a[i];

    return result;
}

But the problem is it could be not void*[], but void**[] or anything else, and the method should be able to handle all cases.

Answer 1

The short answer is, this cannot be done directly. The reasons are that if you pass your conversion function any of the conventional index-capable containers (System.Array, Collections.IList, ArrayList, etc.) performing the index operations will attempt to cast the result to System.Object. Pointers in C# do not derive from Object, so this will result in an SystemNotSupported or similar exception.

There are two reasonable workarounds:

1. Convert the pointer arrays to void pointer arrays before calling the method.
2. Convert the pointer arrays to void pointer pointers before calling the method.

The first one is rather cumbersome, as it requires duplicating the entire contents of the array with a for loop. The second option requires passing in the length of the array as it is no longer wrapped with a managed System.Array object.

Sample Code

Method:

    unsafe Array ToIntPtrArray(void** a, int count)
    {
        IntPtr[] intPtrArray = new IntPtr[count];

        for (int n = 0; n < count; n++)
            intPtrArray[n] = new IntPtr(a[n]);

        return intPtrArray;
    }

Sample Usage (integer pointer array):

int*[] intPtrArray;

// Code that initializes the values of intPtrArray

fixed(int** ptr = &intPtrArray[0])
{
   Array result = ToIntPtrArray((void**)ptr, intPtrArray.Length);
}

Sample Usage (void pointer pointer array):

void**[] voidPtrPtrArray;

// Code that initializes the values of voidPtrPtrArray

fixed(void*** ptr = &voidPtrPtrArray[0])
{
    Array result = ToIntPtrArray((void**)ptr, voidPtrPtrArray.Length);
}

Sample Usage (multidimensional int pointer array):

int*[,] int2dArray;

// Code that initializes the values of int2dArray

fixed(int** ptr = &int2dArray[0,0])
{
    Array result = ToIntPtrArray((void**)ptr, TotalSize(int2dArray));
    Array reshaped = ReshapeArray(result,int2dArray);
}

Where TotalSize and ReshapeArray are helper functions that are written to deal with multi-dimensional arrays. For tips on how to accomplish this see: Programatically Declare Array of Arbitrary Rank.

Answer 2

This is a rather difficult problem. Creating an array of the proper shape isn't too bad.

unsafe System.Array ToIntPtrArray(System.Array a)
{
    int[] lengths = new int[a.Rank];
    int[] lowerBounds = new int[a.Rank];
    for (int i = 0; i < a.Rank; ++i)
    {
        lengths[i] = a.GetLength(i);
        lowerBounds[i] = a.GetLowerBound(i);
    }
    Array newArray = Array.CreateInstance(typeof (IntPtr), lengths, lowerBounds);

    // The hard part is iterating over the array.
    // Multiplying the lengths will give you the total number of items.
    // Then we go from 0 to n-1, and create the indexes
    // This loop could be combined with the loop above.
    int numItems = 1;
    for (int i = 0; i < a.Rank; ++i)
    {
        numItems *= lengths[i];
    }

    int[] indexes = new int[a.Rank];
    for (int i = 0; i < numItems; ++i)
    {
        int work = i;
        int inc = 1;
        for (int r = a.Rank-1; r >= 0; --r)
        {
            int ix = work%lengths[r];
            indexes[r] = lowerBounds[r] + ix;
            work -= (ix*inc);
            inc *= lengths[r];
        }

        object obj = a.GetValue(indexes);
        // somehow create an IntPtr from a boxed pointer
        var myPtr = new IntPtr((long) obj);
        newArray.SetValue(myPtr, indexes);
    }
    return newArray;
}

That creates an array of the right type and shape (dimensions and length), but it has a problem. The GetValue method, which you use to get an item from the array, returns an object. And you can't cast a pointer type to an object. No way, no how. So you can't get the value from the array! If you call GetValue on an array of long*, for example, you'll get "type not supported."

I think you need some way to copy that oddly-shaped array to a one-dimensional array of int* (or any other pointer type). Then you could directly index the temporary array and get the values to populate your IntPtr array.

It's an interesting chicken-and-egg problem. If you pass it as a System.Array, then you can't get items from it because there's no conversion path from object to int* (or any other pointer type). But if you pass it as a pointer type (i.e. int**), then you can't get the shape of the thing.

I suppose you could write it as:

unsafe System.Array ToIntPtrArray(System.Array a, void** aAsPtr)

You then have the System.Array metadata and the actual data in a form that you can use.

Answer 3

Even though the question has been answered well, I feel the need to leave a note/warning to future readers.

The CLR is designed to keep you safe, or at least as safe as possible. It accomplishes this with (among other things) type safety and abstracting memory operations. While you can turn some of these protections off with the unsafe block, some protections are hardcoded into the compiler/runtime. In order to circumvent this additional hurdle, one must resort to some hackey, and possibly slow, code. While these methods work, experience has taught me that doing such things leads to problems down the road, whether it be a change in the runtime, a future programmer needing to modify that segment of code, or you yourself needing to do something else with those pointers later on in the code.

At this point, I would seriously consider a helper dll written in Managed C++ to handle the "raw pointer" side of things. My reasoning is that by using Unsafe, you're already throwing out many protections the CLR offers. You may find it easier to work unencumbered by any additional, baked in protections. Not to mention you can use pointers to their full extent, and then cast them back to intptr when you're done. If nothing else, you may want to look at implementing ToIntPtrArray in C++. Still pass it pointers on the C# side, but escape the CLR's oversight.

Note that I'm not saying that every time you use "unsafe" you should bust out the C++. Quite contrary - unsafe will allow you to do quite a bit - but in this instance, a C++ helper is probably something to consider.

Disclaimer: I have not done a whole lot with managed C++. It could be that I am totally wrong and the CLR would still monitor the pointers. If some more experienced soul could comment and tell me either way, It'd be much appreciated.