C#字符串插值是如何编译的? [英] How is C# string interpolation compiled?

问题描述

我知道插值是 string.Format() 的语法糖，但是当它与字符串格式化方法一起使用时，它是否有任何特殊的行为/识别?

如果我有一个方法:

void Print(字符串格式，params object[] 参数)

以下使用插值调用它:

Print($"{foo} {bar}");

以下哪一行调用最符合字符串插值的编译结果?

Print(string.Format("{0} {1}", new[] { foo, bar }));Print("{0} {1}", new[] { foo, bar });

问题背后的原因:NLog 等日志记录框架通常会推迟字符串格式化，直到它们确定实际写入日志消息.一般来说，我更喜欢字符串插值语法，但我需要知道它是否会导致额外的性能损失.

解决方案

它以两种方式之一编译.

如果您在需要 string 的地方使用字符串插值表达式，则会将其编译为对 string.Format 的调用.

基本上，这个:

string s = $"现在是{DateTime.Now}";

变成这样:

string s = string.Format("现在是{0}", DateTime.Now);

FormattableStringFactory.Create:

public void Test(FormattableString s){}Test($"现在是{DateTime.Now}");

那里的电话变成了这样:

Test(FormattableStringFactory.Create("now is {0}", DateTime.Now));

查看它自己在尝试罗斯林.

所以本质上，在那里回答你的最后一个问题:

这个电话:

Print($"{foo} {bar}");

将被翻译成这样:

Print(string.Format("{0} {1}", foo, bar));

这将在调用 Print 之前通过 string.Format 产生格式化成本.

如果您可以添加或找到采用 FormattableString 的 Print 重载，那么您可以推迟 string.Format 的实际成本代码> 直到您确定是否需要登录.这在运行时是否有可衡量的差异很难说.

查看它自己在尝试罗斯林.

<小时>

奖金回合

不仅延迟了实际的格式设置，而且FormattableString 的ToString 方法允许您指定一个IFormatProvider.

这意味着您也可以推迟本地化转换.

public static void Print(FormattableString s){Console.WriteLine("挪威语:" + s.ToString(CultureInfo.GetCultureInfo("nb-NO")));Console.WriteLine("我们:" + s.ToString(CultureInfo.GetCultureInfo("en-US")));Console.WriteLine("瑞典语:" + s.ToString(CultureInfo.GetCultureInfo("sv-SE")));}

I know that interpolation is syntactic sugar for string.Format(), but does it have any special behavior/recognition of when it is being used with a string formatting method?

If I have a method:

void Print(string format, params object[] parameters)

And the following call to it using interpolation:

Print($"{foo} {bar}");

Which of the following calls lines is most equivalent to the compiled result of string interpolation?

Print(string.Format("{0} {1}", new[] { foo, bar }));
Print("{0} {1}", new[] { foo, bar });

Reasoning behind the question: Logging frameworks such as NLog typically defer string formatting until they have determined that a log message will actually be written. In general I prefer the string interpolation syntax, but I need to know if it may incur an extra performance penalty.

解决方案

It is compiled in one of two ways.

If you use a string interpolation expression where a string is expected, it is compiled into a call to string.Format.

Basically, this:

string s = $"now is {DateTime.Now}";

is turned into this:

string s = string.Format("now is {0}", DateTime.Now);

See it for yourself in Try Roslyn.

Nothing magical here.

Now, on the other hand, if you use it in a place where a FormattableString (a new type in .NET 4.6) is expected, it is compiled into a call to FormattableStringFactory.Create:

public void Test(FormattableString s)
{
}

Test($"now is {DateTime.Now}");

The call there is turned into this:

Test(FormattableStringFactory.Create("now is {0}", DateTime.Now));

See it for yourself in Try Roslyn.

So in essence, to answer your final question there:

This call:

Print($"{foo} {bar}");

Will be translated to this:

Print(string.Format("{0} {1}", foo, bar));

which will incur the cost of the formatting through string.Format before Print is even called.

If you could add, or find, an overload of Print that takes a FormattableString, then you could defer the actual cost of string.Format until after you've figured out if you need to log. Whether this has a measurable different in runtime is hard to say.

See it for yourself in Try Roslyn.

---

Bonus Round

Not only is the actual formatting deferred, but the ToString method of FormattableString allows you to specify a IFormatProvider.

This means that you can defer localized transformation as well.

public static void Print(FormattableString s)
{
    Console.WriteLine("norwegian: " + s.ToString(CultureInfo.GetCultureInfo("nb-NO")));
    Console.WriteLine("us: " + s.ToString(CultureInfo.GetCultureInfo("en-US")));
    Console.WriteLine("swedish: " + s.ToString(CultureInfo.GetCultureInfo("sv-SE")));
}

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