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A.8 Dynamic memory allocation

Visual Studio .NET 2003

Except for the stackalloc operator, C# provides no predefined constructs for managing non-garbage collected memory. Such services are typically provided by supporting class libraries or imported directly from the underlying operating system. For example, the Memory class below illustrates how the heap functions of an underlying operating system might be accessed from C#:

using System;
using System.Runtime.InteropServices;
public unsafe class Memory
{
   // Handle for the process heap. This handle is used in all calls to the
   // HeapXXX APIs in the methods below.
   static int ph = GetProcessHeap();
   // Private instance constructor to prevent instantiation.
   private Memory() {}
   // Allocates a memory block of the given size. The allocated memory is
   // automatically initialized to zero.
   public static void* Alloc(int size) {
      void* result = HeapAlloc(ph, HEAP_ZERO_MEMORY, size);
      if (result == null) throw new OutOfMemoryException();
      return result;
   }
   // Copies count bytes from src to dst. The source and destination
   // blocks are permitted to overlap.
   public static void Copy(void* src, void* dst, int count) {
      byte* ps = (byte*)src;
      byte* pd = (byte*)dst;
      if (ps > pd) {
         for (; count != 0; count--) *pd++ = *ps++;
      }
      else if (ps < pd) {
         for (ps += count, pd += count; count != 0; count--) *--pd = *--ps;
      }
   }
   // Frees a memory block.
   public static void Free(void* block) {
      if (!HeapFree(ph, 0, block)) throw new InvalidOperationException();
   }
   // Re-allocates a memory block. If the reallocation request is for a
   // larger size, the additional region of memory is automatically
   // initialized to zero.
   public static void* ReAlloc(void* block, int size) {
      void* result = HeapReAlloc(ph, HEAP_ZERO_MEMORY, block, size);
      if (result == null) throw new OutOfMemoryException();
      return result;
   }
   // Returns the size of a memory block.
   public static int SizeOf(void* block) {
      int result = HeapSize(ph, 0, block);
      if (result == -1) throw new InvalidOperationException();
      return result;
   }
   // Heap API flags
   const int HEAP_ZERO_MEMORY = 0x00000008;
   // Heap API functions
   [DllImport("kernel32")]
   static extern int GetProcessHeap();
   [DllImport("kernel32")]
   static extern void* HeapAlloc(int hHeap, int flags, int size);
   [DllImport("kernel32")]
   static extern bool HeapFree(int hHeap, int flags, void* block);
   [DllImport("kernel32")]
   static extern void* HeapReAlloc(int hHeap, int flags,
      void* block, int size);
   [DllImport("kernel32")]
   static extern int HeapSize(int hHeap, int flags, void* block);
}

An example that uses the Memory class is given below:

class Test
{
   static void Main() {
      unsafe {
         byte* buffer = (byte*)Memory.Alloc(256);
         try {
            for (int i = 0; i < 256; i++) buffer[i] = (byte)i;
            byte[] array = new byte[256];
            fixed (byte* p = array) Memory.Copy(buffer, p, 256); 
         }
         finally {
            Memory.Free(buffer);
         }
         for (int i = 0; i < 256; i++) Console.WriteLine(array[i]);
      }
   }
}

The example allocates 256 bytes of memory through Memory.Alloc and initializes the memory block with values increasing from 0 to 255. It then allocates a 256 element byte array and uses Memory.Copy to copy the contents of the memory block into the byte array. Finally, the memory block is freed using Memory.Free and the contents of the byte array are output on the console.

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