How do foreach loops work in C#? -

which types of classes can use foreach loops?

actually, strictly speaking, need use foreach public getenumerator() method returns bool movenext() method , ? current {get;} property. however, common meaning of "something implements ienumerable/ienumerable<t>, returning ienumerator/ienumerator<t>.

by implication, includes implements icollection/icollection<t>, such collection<t>, list<t>, arrays (t[]), etc. standard "collection of data" support foreach.

for proof of first point, following works fine:

using system; class foo {     public int current { get; private set; }     private int step;     public bool movenext() {         if (step >= 5) return false;         current = step++;         return true;     } } class bar {     public foo getenumerator() { return new foo(); } } static class program {     static void main() {         bar bar = new bar();         foreach (int item in bar) {             console.writeline(item);         }     } } 

how work?

a foreach loop foreach(int in obj) {...} kinda equates to:

var tmp = obj.getenumerator(); int i; // c# 4.0 while(tmp.movenext()) {     int i; // c# 5.0     = tmp.current;     {...} // code } 

however, there variations. example, enumerator (tmp) supports idisposable, used (similar using).

note difference in placement of declaration "int i" inside (c# 5.0) vs. outside (up c# 4.0) loop. it's important if use i in anonymous method/lambda inside code-block. story ;-p