Monday, June 10, 2013

c# REF parameter - pointer reseat

After using "ref" keyword for a while, I slowly realized it's only useful if inside the method you reseat the pointer. Suppose the  caller of method m1() has a variable ovar and the object is at address 0x3. This is passed into m1 as parameter "pvar".

If m1() reassigns to pvar, then you probably should use "ref" keyword
If m1() never reassigns to pvar, then you need not (and should NOT) use "ref" keyword

These are the guidelines for the common scenario, where pvar is class-type, so pvar is a pointer-VARIABLE (not a mere address; not a pointer-object). When pvar is reassigned, the behavior depends
** ref param? ovar is also reseated to a new address 0x7
** non-ref param? ovar is unaffected.

The much less common scenario is a struct-type method parameter. I feel most developers aren't familiar, so better avoid it.
** ref, with assignment to pvar? ovar's fields are completely overwritten and replaced. Note ovar still refers to (NOT "points to") the object at 0x3.
** ref, with editing on pvar.field1? field1 of the object at 0x3 is modified.
** non-ref, with assignment to pvar? no effect on ovar, the change is local to the method
** non-ref, with editing on pvar.field1? ditto

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