C# linq order by 和其他带有 foreach 的语句，是否存在性能差异？

Question

我经常使用 Entity Framework 、集合进行编程，还使用 ​​where-conditions 和 order by actions。我一直在问自己这个问题，但一直没弄明白。

假设我有以下两段代码；

示例 1:

// An unsorted string array.
string[] letters = { "d", "c", "a", "b" };
// Use LINQ query syntax to sort the array alphabetically.
var sorted = from letter in letters
         orderby letter
         select letter;

// Loop with the foreach keyword.
foreach (string value in sorted)
{
    Console.WriteLine(value);
}

示例 2:

// An unsorted string array.
string[] letters = { "d", "c", "a", "b" };

// Loop with the foreach keyword.
foreach (string val in letters.OrderBy(l => l))
{
    console.writeline(val)
}

第一个示例首先对结果集进行排序，然后遍历集合，在我们要对其进行迭代时，第二个示例具有排序依据。现在真正的问题我一直想知道在哪里.. 性能有什么区别(如果有的话)？这两种方法中的一种比另一种更好吗？与 where 条件(带连接的简单条件和复杂条件)相同，有什么显着差异吗？

Answer 1

两者没有区别..

代码-

static void Main(string[] args)
{
    string[] letters = { "d", "c", "a", "b" };
    // Use LINQ query syntax to sort the array alphabetically.
    var sorted = from letter in letters
                    orderby letter
                    select letter;

    // Loop with the foreach keyword.
    foreach (string value in sorted)
    {
        Console.WriteLine(value);
    }

    foreach (string val in letters.OrderBy(letter => letter))
    {
        Console.WriteLine(val);
    }
}

生成的代码-

private static void Main(string[] args)
{
  string[] strArray1 = new string[4]
  {
    "d",
    "c",
    "a",
    "b"
  };
  string[] strArray2 = strArray1;
  if (Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate2 == null)
  {
    // ISSUE: method pointer
    Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate2 = new Func<string, string>((object) null, __methodptr(\u003CMain\u003Eb__0));
  }
  Func<string, string> keySelector1 = Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate2;
  foreach (string str in (IEnumerable<string>) Enumerable.OrderBy<string, string>((IEnumerable<string>) strArray2, keySelector1))
    Console.WriteLine(str);
  string[] strArray3 = strArray1;
  if (Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate3 == null)
  {
    // ISSUE: method pointer
    Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate3 = new Func<string, string>((object) null, __methodptr(\u003CMain\u003Eb__1));
  }
  Func<string, string> keySelector2 = Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate3;
  foreach (string str in (IEnumerable<string>) Enumerable.OrderBy<string, string>((IEnumerable<string>) strArray3, keySelector2))
    Console.WriteLine(str);
}

[CompilerGenerated]
private static string \u003CMain\u003Eb__0(string letter)
{
  return letter;
}

[CompilerGenerated]
private static string \u003CMain\u003Eb__1(string letter)
{
  return letter;
}

编辑: 这是一个有趣的变体，我很想尝试。在上面的例子中，对于查询表达式，编译器足够聪明，可以优化 Select 。但是，如果在第二个变体中，我们采用在显式 Select 上。这并不奇怪，但编译器仍然没有优化显式 .Select(与查询表达式中的显式 Select 相比——编译器的要求)。

代码-

    foreach (string val in letters.OrderBy(letter => letter).Select(letter => letter))
    {
        Console.WriteLine(val);
    }

生成的代码-

  string[] strArray4 = strArray1;
  if (Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate6 == null)
  {
    // ISSUE: method pointer
    Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate6 = new Func<string, string>((object) null, __methodptr(\u003CMain\u003Eb__2));
  }
  Func<string, string> keySelector3 = Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate6;
  IOrderedEnumerable<string> orderedEnumerable = Enumerable.OrderBy<string, string>((IEnumerable<string>) strArray4, keySelector3);
  if (Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate7 == null)
  {
    // ISSUE: method pointer
    Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate7 = new Func<string, string>((object) null, __methodptr(\u003CMain\u003Eb__3));
  }
  Func<string, string> selector = Program.CS\u0024\u003C\u003E9__CachedAnonymousMethodDelegate7;
  foreach (string str in Enumerable.Select<string, string>((IEnumerable<string>) orderedEnumerable, selector))
    Console.WriteLine(str);