What is the difference between a C# Reference and a Pointer?

Question

I do not quite understand the difference between a C# reference and a pointer. They both point to a place in memory don\'t they? The only difference I can figure out is that

Answer 1

First I think you need to define a "Pointer" in your sematics. Do you mean the pointer you can create in unsafe code with fixed? Do you mean an IntPtr that you get from maybe a native call or Marshal.AllocHGlobal? Do you mean a GCHandle? The all are essentially the same thing - a representation of a memory address where something is stored - be it a class, a number, a struct, whatever. And for the record, they certainly can be on the heap.

A pointer (all of the above versions) is a fixed item. The GC has no idea what is at that address, and therefore has no ability to manage the memory or life of the object. That means you lose all of the benefits of a garbage collected system. You must manually manage the object memory and you have the potential for leaks.

A reference on the other hand is pretty much a "managed pointer" that the GC knows about. It's still an address of an object, but now the GC knows details of the target, so it can move it around, do compactions, finalize, dispose and all of the other nice stuff a managed environment does.

The major difference, really, is in how and why you would use them. For a vast majority of cases in a managed language, you're going to use an object reference. Pointers become handy for doing interop and the rare need for really fast work.

Edit: In fact here's a good example of when you might use a "pointer" in managed code - in this case it's a GCHandle, but the exact same thing could have been done with AllocHGlobal or by using fixed on a byte array or struct. I tend to prefer the GCHandle becasue it feels more ".NET" to me.