C#6 新增特性目錄
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符串的時候會使用string的靜態方法Format來進行字符串拼接，然後使用{0}...{n}來充當占位符。如果{n}過大，代碼的可讀性就會急劇下降，C#6引入一個新語法來簡化這種書寫方式。
我們之間來看新語法：
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 }
新語法采用 $ 開頭，然後把之前的{n}換成了有意義的表達式，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.0
10 IL_0007: call instance string csharp6.Person::get_Name()
11 IL_000c: ldarg.0
12 IL_000d: call instance int32 csharp6.Person::get_Age()
13 IL_0012: box [mscorlib]System.Int32
14 IL_0017: call string [mscorlib]System.String::Format(string,
15 object,
16 object)
17 IL_001c: stloc.0
18 IL_001d: br.s IL_001f
19 IL_001f: ldloc.0
20 IL_0020: ret
21 } // end of method Person::ToString
新語法的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.0
10 IL_0007: call instance string csharp6.Person::get_Name()
11 IL_000c: ldarg.0
12 IL_000d: call instance int32 csharp6.Person::get_Age()
13 IL_0012: box [mscorlib]System.Int32
14 IL_0017: call string [mscorlib]System.String::Format(string,
15 object,
16 object)
17 IL_001c: stloc.0
18 IL_001d: br.s IL_001f
19 IL_001f: ldloc.0
20 IL_0020: ret
21 } // 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我的代碼。C#編譯器幫我們轉成了老版本的寫法而已，一模一樣的。。。so,這又是一個語法層面的優化。
1 //支持方法調用
2 string s1 = $"{person.GetHashCode()}";
3 //支持表達式
4 string s2 = $"person.{nameof(person.Name)} is {person?.Name}";
5 //支持格式化輸出
6 DateTime now = DateTime.Now;
7 string s3 = $"DateTime.Now={now:yyyy-MM-dd HH:mm:ss}";
8 //組合表達式和格式化輸出
9 string s4 = $"{person.Name,2} is {person.Age:D2} year{(person.Age == 1 ? "" : "s")} old.";
10 //支持的隱式類型轉換
11 IFormattable s5 = $"Hello, {person.Name}";
12 FormattableString s6 = $"Hello, {person.Name}"
新語法支持表達式求值，支持:格式化操作，還支持到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.ref
10 IL_00ab: call class [mscorlib]System.FormattableString[mscorlib] System.Runtime.CompilerServices.FormattableStringFactory::Create(string,object[])
13 IL_00b0: stloc.s s5
14 IL_00b2: ldstr "Hello, {0}"
15 IL_00b7: ldc.i4.1
16 IL_00b8: newarr[mscorlib] System.Object
17 IL_00bd: dup
18 IL_00be: ldc.i4.0
19 IL_00bf: ldloc.0
20 IL_00c0: callvirt instance string csharp6.Person::get_Name()
21 IL_00c5: stelem.ref
22 IL_00c6: call class [mscorlib]System.FormattableString[mscorlib] System.Runtime.CompilerServices.FormattableStringFactory::Create(string,object[])
Interpolated Strings
