Why can .NET not have memory leaks?

Ignoring unsafe code, .NET cannot have memory leaks. I've read this endlessly from many experts and I believe it. However, I do not understand why this is so.

It is my understanding that the framework itself is written in C++ and C++ is susceptible to memory leaks.

Is the underlying framework so well-written, that it absolutely does not have any possibility of internal memory leaks?
Is there something within the framework's code that self-manages and even cures its own would-be memory leaks?
Is the answer something else that I haven't considered?

Solution

There are already some good answers here, but I want to address one additional point. Let's look very carefully again at your specific question:

It is my understanding that the framework itself is written in C++ and C++ is susceptible to memory leaks.

Is the underlying framework so well-written, that it absolutely does not have any possibility of internal memory leaks?
Is there something within the framework's code that self-manages and even cures its own would-be memory leaks?
Is the answer something else that I haven't considered?

The key here is to distinguish between your code and their code. The .Net framework (and Java, Go, python, and other garbage-collected languages) promise that if you rely on their code, your code will not leak memory... at least in the traditional sense. You might find yourself in situations where some objects are not freed as you expect, but these cases are subtly different from traditional memory leaks because the objects are still reachable in your program.

You are confused because you correctly understand that this is not the same thing as saying any program you create can't possibly have a traditional memory leak at all. There could still be a bug in their code that leaks memory.

So now you have to ask yourself: would you rather trust your code, or their code? Keep in mind here that their code is not only tested by the original developers (just like yours, right?), it's also battle-hardened from daily use by thousands (perhaps millions) of other programmers like yourself. Any significant memory leak issues would be among the first things identified and corrected. Again, I'm not saying it's not possible. It's just that it's generally a better idea to trust their code than it is your own... at least in this respect.

Therefore the correct answer here is that it's a variant of your first suggestion:

Is the underlying framework so well-written, that it absolutely does not have any possibility of internal memory leaks?

It's not that there's no possibility, but that it's much safer than managing it yourself. I'm certainly not aware of any known leaks in the framework.