Dynamically setting value on array of pointers to integers

Question

I have a multidimentional array of pointers to integer (of unknown rank) being passed into my function as such:

    public unsafe static void MyMethod(Array source, ...)
    {
         //...
    }

Multidimensional arrays of pointers are being constructed outside of the method and being passed in. Here's an example:

int*[,,,] testArray = new int*[10,10,5,5];

MyMethod(testArray);

How can I set a value at an runtime-computed index in the array? Array.SetValue(...) works perfectly fine for non-pointer arrays, but refuses to work for my int* array. Using reflector, I see SetValue reduces down to calling InternalSetValue which takes an object for the value but it's marked as extern and I can't see the implementation. I took a shot in the dark and tried passing in boxed pointer, but no luck.

dtb · Accepted Answer · 2009-12-07 19:07:03Z

4

This works:

unsafe static void MyMethod(int** array)
{
    array[10] = (int*)0xdeadbeef;
}

private static unsafe void Main()
{
    int*[, , ,] array = new int*[10, 10, 10, 10];

    fixed (int** ptr = array)
    {
        MyMethod(ptr);
    }

    int* x = array[0, 0, 1, 0]; // == 0xdeadbeef
}

Does that help?

Question to the experts: Is it wrong to assume that the array is allocated consecutively in memory?

edited Dec 7, 2009 at 19:07

answered Dec 7, 2009 at 18:53

dtb

218k37 gold badges413 silver badges438 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

dtb Over a year ago

@Maximilian: Thanks. Is this documented somewhere? Or is it an implementation detail if the CLR?

Pavel Minaev Over a year ago

Yes, this is specified by Ecma-335 (CLI spec): "Array elements shall be laid out within the array object in row-major order (i.e., the elements associated with the rightmost array dimension shall be laid out contiguously from lowest to highest index)."

Tim Robinson · Accepted Answer · 2009-12-07 18:44:27Z

1

This doesn't work because it's not possible to box a pointer in .NET, so you can never call the Array.SetValue and pass an int*.

Can you declare MyMethod to accept int*[,,,] instead?

Edit: for further reading, an interesting recent post from Eric Lippert.

answered Dec 7, 2009 at 18:44

Tim Robinson

54.9k12 gold badges127 silver badges141 bronze badges

3 Comments

Robert Venables Over a year ago

The requirement for the method is that it works with an array of any dimension. Also, see: msdn.microsoft.com/en-us/library/…

Tim Robinson Over a year ago

OK, but Array.SetValue doesn't know how to unbox a pointer boxed using Pointer.Box: I get "System.NotSupportedException: Type is not supported."

Robert Venables Over a year ago

I found that out (see above) - It's actually Array.InternalGetReference (also marked as extern) throwing that exception.

Collectives™ on Stack Overflow

Dynamically setting value on array of pointers to integers

2 Answers 2

2 Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related