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 pointers are not as clever, cannot point to anything on the heap, are exempt from garbage collection, and can only reference structs or base types.

One of the reasons I ask is that there is a perception that people need to understand pointers well (from C, I guess) to be a good programmer. A lot of people who learn higher level languages miss this out and therefore have this weakness.

I just don't get what is so complex about a pointer? It is basically just a reference to a place in memory is it not? It can return its location and interact with the object in that location directly?

Have I missed a massive point?

Answer 1

There is a slight, yet extremely important, distinction between a pointer and a reference. A pointer points to a place in memory while a reference points to an object in memory. Pointers are not "type safe" in the sense that you cannot guarantee the correctness of the memory they point at.

Take for example the following code

int* p1 = GetAPointer();

This is type safe in the sense that GetAPointer must return a type compatible with int*. Yet there is still no guarantee that *p1 will actually point to an int. It could be a char, double or just a pointer into random memory.

A reference however points to a specific object. Objects can be moved around in memory but the reference cannot be invalidated (unless you use unsafe code). References are much safer in this respect than pointers.

string str = GetAString();

In this case str has one of two state 1) it points to no object and hence is null or 2) it points to a valid string. That's it. The CLR guarantees this to be the case. It cannot and will not for a pointer.