C#•12mo ago

✅ Difference between `string[]` and `IEnumerable<string>`

in the following the code, the output seems to be the same:

using System;
using System.IO;

class Program
{
  public static void Main()
  {
    foreach (string dir in Directory.GetDirectories("Settings"))
      Console.WriteLine(dir);
    
    foreach (string dir in Directory.EnumerateDirectories("Settings"))
      Console.WriteLine(dir);
  }
}

using System;
using System.IO;

class Program
{
  public static void Main()
  {
    foreach (string dir in Directory.GetDirectories("Settings"))
      Console.WriteLine(dir);
    
    foreach (string dir in Directory.EnumerateDirectories("Settings"))
      Console.WriteLine(dir);
  }
}

i know that enums are used to define a set of exhaustive types, and are closely related to ints. but what does it mean to have IEnumerable<string> and how different is it from string[]?

9 Replies

ero•12mo ago

GetDirectories returns a complete collection. all items have been evaluated and transformed. EnumerateDirectionaries yields every result back to you one by one. you can use it if you don't want to go over every single result; for example when you're looking for just one specific directory. when you break in the loop over EnumerateDirectories, it won't continue searching for the next result. EnumerateDirectories is an iterator method

MODiX•12mo ago

ero

REPL Result: Success

const int Length = 3;

static int[] M1()
{
  var ret = new int[Length];

  for (int i = 0; i < Length; i++)
  {
    Console.WriteLine($"M1: {i}");
    ret[i] = i;
  }

  return ret;
}

static IEnumerable<int> M2()
{
  for (int i = 0; i < Length; i++)
  {
    Console.WriteLine($"M2: {i}");
    yield return i;
  }
}

foreach (var i in M1())
  break;

foreach (var i in M2())
  break;

const int Length = 3;

static int[] M1()
{
  var ret = new int[Length];

  for (int i = 0; i < Length; i++)
  {
    Console.WriteLine($"M1: {i}");
    ret[i] = i;
  }

  return ret;
}

static IEnumerable<int> M2()
{
  for (int i = 0; i < Length; i++)
  {
    Console.WriteLine($"M2: {i}");
    yield return i;
  }
}

foreach (var i in M1())
  break;

foreach (var i in M2())
  break;

Console Output

M1: 0
M1: 1
M1: 2
M2: 0

M1: 0
M1: 1
M1: 2
M2: 0

Compile: 577.671ms | Execution: 99.265ms | React with ❌ to remove this embed.

ero•12mo ago

it's a bit confusing if you don't know what's going on

tommyOP•12mo ago

what i have gathered from this is that there's a difference in the iteration: EnumerateDirectories will iterate over the directories one by one to match a condition's expression, whereas GetDirectories will iterate over all the files/directories first, provide a string[] over which we iterate

FusedQyou•12mo ago

Yes so essentially EnumerateDirectories doesn't do anything until you call ToArray/ToList, at which it will start to actually perform the logic and enumerate Meaning that you can do this multiple times and it will return the current state. This is often a pitfall for developers using IEnumerable because they do not realize that they enumerate multiple times It's a very generic way to enumerate something because it simply specifies a method to get the current item and to go to the next one, so you can wrap it in many things in .NET most collection types implement IEnumerable for this so you can foreach the collections So in the end it's sort of the same It's a complex topic

tommyOP•12mo ago

what does it mean when you say current state here?

FusedQyou•12mo ago

See https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.ienumerable-1?view=net-8.0 The only thing it does is implement GetEnumerator https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.ienumerable-1.getenumerator?view=net-8.0#system-collections-generic-ienumerable-1-getenumerator So collections implement this and have this method

tommyOP•12mo ago

okay 👍

FusedQyou•12mo ago

This returns IEnumerator, which is something else https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.ienumerator-1?view=net-8.0 And this is something that collections or things implement to returns their current state. So for example, arrays and lists just return the current index, and they will have an integer for their current position, or state Managed by Current, MoveNext() and there's also Reset() to reset iterating So when you call ToArray for example it uses these methods to create an array from your IEnumerable

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✅ Difference between `string[]` and `IEnumerable<string>`

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