C# Cookbook

16.7. Creating and Using an Array of Pointersto Unknown Types

Problem

You need to create and operate on elements of an array that holds objects of unknown types.

Solution

The solution is to create an array of void pointers so that we do not need to know at design time what type(s) we will be pointing to:

unsafe
{
    long x = 10;
    long y = 20;
    long z = 1;

    void*[] arrayOfPtrs = new void*[3];
    arrayOfPtrs[0] = &X;
    arrayOfPtrs[1] = &Y;
    arrayOfPtrs[2] = &Z;

    Console.WriteLine(*((long*)arrayOfPtrs[0]));
    Console.WriteLine(*((long*)arrayOfPtrs[1]));
    Console.WriteLine(*((long*)arrayOfPtrs[2]));
}

This code creates an array, arrayOfPtrs, that will contain three void pointers. The pointers that are saved to this array are pointers to the three variables x, y, and z of type long. It is a simple matter to change the long data type to something different such as a byte or char. However, when the pointers in this array are used, they must be cast back to their original type. This cast is shown in the last three lines, where each pointer in the array is being dereferenced and displayed. If you do the wrong cast, you get undefined results, but the next example helps address this.

The following code creates an array of two void pointers and points the first pointer at a NewBrush structure and the second at an integer type variable:

unsafe { NewBrush theNewBrush1 = new NewBrush( ); int* theInt = stackalloc int[1]; void*[] arrayOfPtrs = new void*[2]; arrayOfPtrs[0] = &theNewBrush1; arrayOfPtrs[1] = theInt; ...