c# - Array indexer signature returns object - does it box? -


i stumbled on fact indexer this[int index] { get; } works differently array of structs list of structs. namely, indexer in case of t[] returns reference element within array whereas indexer in case of list<t> returns copy of element.

this big semantic , performance difference, , i'm glad t[] allows work around performance limitation of list<t>.

however, i'm puzzled actual implementation. code array in .net reference sources reads thus:

object ilist.this[int index] {     { return getvalue(index); }     set { setvalue(value, index); } }

where getvalue defined follows:

public unsafe object getvalue(int index) {     if (rank != 1)        throw new argumentexception(environment.getresourcestring("arg_need1darray"));     contract.endcontractblock();     typedreference elemref = new typedreference();     internalgetreference(&elemref, 1, &index);     return typedreference.internaltoobject(&elemref); }

the return type of indexer object, implying boxing take place.

so question is, can no boxing occur when access element of t[] t struct?

i assume compiler and/or clr treat array specially, , don't bother signature of indexer. correct? there fuller discussion of somewhere?

namely, indexer in case of t[] returns reference element within array

not really. it's more there isn't indexer arrays - instead element-access expression represents array access instead of element access (sections 7.6.6.1 , 7.6.6.2 of c# 5 spec respectively).

there's significant difference between these 2 - in particular, array access classified variable, whereas indexer access classified value.

it's similar difference between property , field - both have same syntax access, property invokes function member , returns value, whereas field access yields variable.

so question is, can no boxing occur when access element of t[] t struct?

if you're accessing as t[], sure. indexer looked @ used when you're viewing array ilist. if use:

ilist array = new int[2]; object x = array[0];

then yes, that'll box value... if write

int[] array = new int[2]; int x = array[0];

then won't, , won't accessing indexer code or getvalue method @ all.


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