Cannot apply indexing with [] to an expression of the type 'System.IntPtr'

Question

Have been reading this is an "unsafe" code and "IntPtr" is normally not to function this way.

Can someone suggest an alternative or a solution.

My skills are limited in C#. Thanks for any help !!

for (num4 = 1; num4 < i; num4 += 2)
{  
    for (num = num4; num <= length; num += num5)
    {
        num2 = num + i;
        num11 = (num7 * numRef[(num2 - 1) * 8]) - (num8 * numRef[num2 * 8]);
        double num10 = (num7 * numRef[num2 * 8]) + (num8 * numRef[(num2 - 1) * 8]);
        numRef[(num2 - 1) * 8] = numRef[(num - 1) * 8] - num11;
        numRef[num2 * 8] = numRef[num * 8] - num10;
        IntPtr ptr1 = (IntPtr)(numRef + ((num - 1) * 8));
        //ptr1[0] += (IntPtr) num11;
        ptr1[0] += (IntPtr)num11;
        IntPtr ptr2 = (IntPtr)(numRef + (num * 8));
        //ptr2[0] += (IntPtr) num10;
        ptr2[0] += (IntPtr)num10;
    }

    num7 = (((num9 = num7) * num13) - (num8 * num14)) + num7;
    num8 = ((num8 * num13) + (num9 * num14)) + num8;
}

Answer 1

If you want to use pointer arithmetic in C# you will need to use the unsafe and fixed keyword, as follows:

public unsafe void Foo()
{
    int[] managedArray = new int[100000];

    // Pin the pointer on the managed heap 
    fixed (int * numRef = managedArray)
    {
        for (num4 = 1; num4 < i; num4 += 2)
        {  
            for (num = num4; num <= length; num += num5)
            {
                num2 = num + i;
                num11 = (num7 * numRef[(num2 - 1) * 8]) - (num8 * numRef[num2 * 8]);
                double num10 = (num7 * numRef[num2 * 8]) + (num8 * numRef[(num2 - 1) * 8]);

                // You can now index the pointer
                // and use pointer arithmetic 
                numRef[(num2 - 1) * 8] = numRef[(num - 1) * 8] - num11;
                numRef[num2 * 8] = numRef[num * 8] - num10;
                ... 
                int * offsetPtr = numRef + 100; 

                // Equivalent to setting numRef[105] = 5;
                offsetPtr[i+5] = 5;
            }

            num7 = (((num9 = num7) * num13) - (num8 * num14)) + num7;
            num8 = ((num8 * num13) + (num9 * num14)) + num8;
        }
    }
}