IEnumerable vs List - What to Use? How do they work?

Question

I have some doubts over how Enumerators work, and LINQ. Consider these two simple selects:

List sel = (from animal in Animals 
                         


        

Answer 1

I will share one misused concept that I fell into one day:

var names = new List {"mercedes", "mazda", "bmw", "fiat", "ferrari"};

var startingWith_M = names.Where(x => x.StartsWith("m"));

var startingWith_F = names.Where(x => x.StartsWith("f"));


// updating existing list
names[0] = "ford";

// Guess what should be printed before continuing
print( startingWith_M.ToList() );
print( startingWith_F.ToList() );

Expected result

// I was expecting    
print( startingWith_M.ToList() ); // mercedes, mazda
print( startingWith_F.ToList() ); // fiat, ferrari

Actual result

// what printed actualy   
print( startingWith_M.ToList() ); // mazda
print( startingWith_F.ToList() ); // ford, fiat, ferrari

Explanation

As per other answers, the evaluation of the result was deferred until calling ToList or similar invocation methods for example ToArray.

So I can rewrite the code in this case as:

var names = new List {"mercedes", "mazda", "bmw", "fiat", "ferrari"};

// updating existing list
names[0] = "ford";

// before calling ToList directly
var startingWith_M = names.Where(x => x.StartsWith("m"));

var startingWith_F = names.Where(x => x.StartsWith("f"));

print( startingWith_M.ToList() );
print( startingWith_F.ToList() );

Play arround

https://repl.it/E8Ki/0