[C#6] 4-string 插值

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0. 目錄

C#6 新增特性目錄

1. 老版本的代碼

 1 internal class Person 2 { 3     public string Name { get; set; } 4     public int Age { get; set; } 5  6     public override string ToString() 7     { 8         return string.Format("[name={0},age={1}]", Name, Age); 9     }10 }

通常我們在格式化字串的時候會使用string的靜態方法Format來進行字串拼接，然後使用{0}...{n}來充當預留位置。如果{n}過大，代碼的可讀性就會急劇下降，C#6引入一個新文法來簡化這種書寫方式。

2. $"{xxx:}"

我們之間來看新文法：

 1 internal class Person 2 { 3     public string Name { get; set; } 4     public int Age { get; set; } 5  6     public override string ToString() 7     { 8         return $"[name={Name},age={Age}]"; 9     }10 }

新文法採用 $  開頭，然後把之前的{n}換成了有意義的運算式，直觀且簡潔，且在VS2015中會有智能提示。好了，基本用法就是這樣，按老習慣，對比下IL代碼吧。

老版本的IL：

 1 .method public hidebysig virtual instance string  2         ToString() cil managed 3 { 4   // Code size       33 (0x21) 5   .maxstack  3 6   .locals init ([0] string V_0) 7   IL_0000:  nop 8   IL_0001:  ldstr      "[name={0},age={1}]" 9   IL_0006:  ldarg.010   IL_0007:  call       instance string csharp6.Person::get_Name()11   IL_000c:  ldarg.012   IL_000d:  call       instance int32 csharp6.Person::get_Age()13   IL_0012:  box        [mscorlib]System.Int3214   IL_0017:  call       string [mscorlib]System.String::Format(string,15                                                               object,16                                                               object)17   IL_001c:  stloc.018   IL_001d:  br.s       IL_001f19   IL_001f:  ldloc.020   IL_0020:  ret21 } // end of method Person::ToString

新文法的IL:

 1 .method public hidebysig virtual instance string  2         ToString() cil managed 3 { 4   // Code size       33 (0x21) 5   .maxstack  3 6   .locals init ([0] string V_0) 7   IL_0000:  nop 8   IL_0001:  ldstr      "[name={0},age={1}]" 9   IL_0006:  ldarg.010   IL_0007:  call       instance string csharp6.Person::get_Name()11   IL_000c:  ldarg.012   IL_000d:  call       instance int32 csharp6.Person::get_Age()13   IL_0012:  box        [mscorlib]System.Int3214   IL_0017:  call       string [mscorlib]System.String::Format(string,15                                                               object,16                                                               object)17   IL_001c:  stloc.018   IL_001d:  br.s       IL_001f19   IL_001f:  ldloc.020   IL_0020:  ret21 } // end of method Person::ToString

第一眼看到新版本的IL代碼，我還以為我沒有重新編譯My Code。C#編譯器幫我們轉成了老版本的寫法而已，一模一樣的。。。so,這又是一個文法層面的最佳化。

3. Example

 1 //支援方法調用 2 string s1 = $"{person.GetHashCode()}"; 3 //支援運算式 4 string s2 = $"person.{nameof(person.Name)} is {person?.Name}"; 5 //支援格式化輸出 6 DateTime now = DateTime.Now; 7 string s3 = $"DateTime.Now={now:yyyy-MM-dd HH:mm:ss}"; 8 //組合運算式和格式化輸出 9 string s4 = $"{person.Name,2} is {person.Age:D2} year{(person.Age == 1 ? "" : "s")} old.";10 //支援的隱式類型轉換11 IFormattable s5 = $"Hello, {person.Name}";12 FormattableString s6 = $"Hello, {person.Name}"

新文法支援運算式求值，支援:格式化操作，還支援到IFormattable的隱式轉換，編譯結果是利用 System.Runtime.CompilerServices.FormattableStringFactory.Create 這個靜態方法構造一個 FormattableString 實現的。IL如下：

 1 IL_0095:  stloc.s s4 2 IL_0097:  ldstr      "Hello, {0}" 3 IL_009c:  ldc.i4.1 4 IL_009d:  newarr[mscorlib] System.Object 5 IL_00a2:  dup 6 IL_00a3:  ldc.i4.0 7 IL_00a4:  ldloc.0 8 IL_00a5:  callvirt instance string csharp6.Person::get_Name() 9 IL_00aa:  stelem.ref10 IL_00ab:  call class [mscorlib]System.FormattableString[mscorlib] System.Runtime.CompilerServices.FormattableStringFactory::Create(string,object[])13 IL_00b0:  stloc.s s514 IL_00b2:  ldstr      "Hello, {0}"15 IL_00b7:  ldc.i4.116 IL_00b8:  newarr[mscorlib] System.Object17 IL_00bd:  dup18 IL_00be:  ldc.i4.019 IL_00bf:  ldloc.020 IL_00c0:  callvirt instance string csharp6.Person::get_Name()21 IL_00c5:  stelem.ref22 IL_00c6:  call class [mscorlib]System.FormattableString[mscorlib] System.Runtime.CompilerServices.FormattableStringFactory::Create(string,object[])

4. 參考

Interpolated Strings

 

[C#6] 4-string 插值