Where did variable = null as “object destroying” come from?

Question

Working on a number of legacy systems written in various versions of .NET, across many different companies, I keep finding examples of the following pattern:

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Answer 1

It's FUDcargo cult programming (thanks to Daniel Earwicker) by developers who are used to "free" resources, bad GC implementations and bad API.

Some GCs didn't cope well with circular references. To get rid of them, you had to break the cycle "somewhere". Where? Well, if in doubt, then everywhere. Do that for a year and it's moved into your fingertips.

Also setting the field to null gives you the idea of "doing something" because as developers, we always fear "to forget something".

Lastly, we have APIs which must be closed explicitly because there is no real language support to say "close this when I'm done with it" and let the computer figure it out just like with GC. So you have an API where you have to call cleanup code and API where you don't. This sucks and encourages patterns like the above.