How to use interlocked operations against memory-mapped files in .Net - .net-4.0

Question

Is there any way to use the Interlocked.CompareExchange(); and Interlocked.Increment(); methods against values stored in a memory-mapped file?
I'd like to implement a multi-threaded service that will store its data in a memory-mapped file, but since it's multi-threaded I need to prevent conflicting writes, therefore I wonder about the Interlocked operations rather than using explicit locks.
I know it's possible with native code, but can it be done in managed code on .NET 4.0?

Answer 1

OK, this is how you do it! We had to figure this out, and I figured we could give some back to stackoverflow!
class Program
{
internal static class Win32Stuff
{
[DllImport("kernel32.dll", SetLastError = true)]
unsafe public static extern int InterlockedIncrement(int* lpAddend);
}
private static MemoryMappedFile _mmf;
private static MemoryMappedViewStream _mmvs;
unsafe static void Main(string[] args)
{
const int INT_OFFSET = 8;
_mmf = MemoryMappedFile.CreateOrOpen("SomeName", 1024);
// start at offset 8 (just for example)
_mmvs = _mmf.CreateViewStream(INT_OFFSET, 4);
// Gets the pointer to the MMF - we dont have to worry about it moving because its in shared memory
var ptr = _mmvs.SafeMemoryMappedViewHandle.DangerousGetHandle();
// Its important to add the increment, because even though the view says it starts at an offset of 8, we found its actually the entire memory mapped file
var result = Win32Stuff.InterlockedIncrement((int*)(ptr + INT_OFFSET));
}
}
This does work, and works across multiple processes! Always enjoy a good challenge!

Answer 2

TravisWhidden, actually you can use Interlocked.Increment Static method as dan-gph said, you just have to be careful with pointer casting and operator priority, plus parenthesis usage, in facts...
You'll cast a memory pointer (plus the desired offset), into a pointer to an int variable, then you'll use that pointer as a variable. Then you'll have to use it as a variable reference.
Below you'll find the corresponding snippet of yours using .net library instead of external static import.
P&L
class Program
{
private static MemoryMappedFile _mmf;
private static MemoryMappedViewStream _mmvs;
static void Main(string[] args)
{
const int INT_OFFSET = 8;
_mmf = MemoryMappedFile.CreateOrOpen("SomeName", 1024);
_mmvs = _mmf.CreateViewStream(INT_OFFSET, 4);
unsafe
{
IntPtr ptr = _mmvs.SafeMemoryMappedViewHandle.DangerousGetHandle();
Interlocked.Increment(ref (*((int*)(ptr + INT_OFFSET)))
}
}
}