c# - Why does this happen IEnumerable versus List -

i have following bl method

public static void somemethod (list<someclass> group) {     ienumerable<someclass> groupwithfalse =(from someclass gr in group                                             gr.someprop== false                                             select gr);     foreach (someclass grfalse in groupwithfalse)     {       grfalse.save();     }      if (groupwithfalse.any())     {       // stuff     } } 

the mock implementation dl saving (which cannot changed used in lot of unit tests) is:

public void save() {    group.someprop = true; } 

if try unit test last statement of flow e.g if (groupwithfalse.any()) statement fails, apparently there no more elements property set false. if change code in business logic :

public static void somemethod (list<someclass> group) {     list<someclass> groupwithfalse = new list<someclass>();     foreach (var g in group)     {       if (g.someprop == false)           groupwithfalse.add(g);     }      foreach (someclass grfalse in groupwithfalse)     {       grfalse.save();     }      if (groupwithfalse.any())     {       // stuff     } } 

the conditional statement if (groupwithfalse.any()) not fail in unit tests. why happen? thanks

running linq query not store results.
instead, re-execute query each time enumerate it.

after calling save(), query empty, since none of items meet where clause.

change

var unsaved = group.where(g => !g.someprop).tolist(); 

calling tolist() store results in list<t>; avoids re-executing query.