C#: should object variables be assigned to null?

Question

In C#, is it necessary to assign an object variable to null if you have finished using it, even when it will go out of scope anyway?

Answer 1

Assigning to null is generally a bad idea:

1. Conceptually, it's just pointless.
2. With many variables, you could have enough extra assignments to null that you appreciably increase the size of the method. The longer the source code for something is, the more mental effort is taken to read it (even if much of it is just stuff that can be filtered out) and the easier it is to spot a bug. Make code more verbose than necessary only when doing so makes it more understandable.
3. It is possible that your null assignment won't be optimised away. In that case it's possible that the compiled code won't do the real deallocation until it actually reaches that null assignment, whereas in most cases once the variable is going to do nothing else other than fall out of scope (and sometimes even before) it can be deallocated. You can therefore have a very minor performance impact.

The only time I would assign something to null to "clear" a variable that will no longer be used, rather than because null is actually a value I explicitly want to assign, is in one of the two possible cases:

1. It is a member of a possibly long-lived object, will no longer be used by that object, and is of considerable size. Here assigning to null is an optimisation.
2. It is a member of a possibly long-lived object, will no longer be used by that object, and has therefore been disposed to release its resources. Here assigning to null is a matter of safety as it can be easier to find a case where one accidentally uses a null object than where one accidentally uses a disposed object.

Neither of these cases apply to local variables, only to members, and both are rare.