[C#6] 4-string 插值

0. 目录

C#6 新增特性目录

1. 老版本的代码

 internal class Person
 {
     public string Name { get; set; }
     public int Age { get; set; }

     public override string ToString()
     {
         return string.Format("[name={0},age={1}]", Name, Age);
     }
 }

通常我们在格式化字符串的时候会使用string的静态方法Format来进行字符串拼接，然后使用{0}...{n}来充当占位符。如果{n}过大，代码的可读性就会急剧下降，C#6引入一个新语法来简化这种书写方式。

2. $"{xxx:}"

我们之间来看新语法：

 internal class Person
 {
     public string Name { get; set; }
     public int Age { get; set; }

     public override string ToString()
     {
         return $"[name={Name},age={Age}]";
     }
 }

新语法采用 $  开头，然后把之前的{n}换成了有意义的表达式，直观且简洁，且在VS2015中会有智能提示。好了，基本用法就是这样，按老习惯，对比下IL代码吧。

老版本的IL：

 .method public hidebysig virtual instance string
         ToString() cil managed
 {
   // Code size       33 (0x21)
   .maxstack
   .locals init ([] string V_0)
   IL_0000:  nop
   IL_0001:  ldstr      "[name={0},age={1}]"
   IL_0006:  ldarg.
   IL_0007:  call       instance string csharp6.Person::get_Name()
   IL_000c:  ldarg.
   IL_000d:  call       instance int32 csharp6.Person::get_Age()
   IL_0012:  box        [mscorlib]System.Int32
   IL_0017:  call       string [mscorlib]System.String::Format(string,
                                                               object,
                                                               object)
   IL_001c:  stloc.
   IL_001d:  br.s       IL_001f
   IL_001f:  ldloc.
   IL_0020:  ret
 } // end of method Person::ToString

新语法的IL:

 .method public hidebysig virtual instance string
         ToString() cil managed
 {
   // Code size       33 (0x21)
   .maxstack
   .locals init ([] string V_0)
   IL_0000:  nop
   IL_0001:  ldstr      "[name={0},age={1}]"
   IL_0006:  ldarg.
   IL_0007:  call       instance string csharp6.Person::get_Name()
   IL_000c:  ldarg.
   IL_000d:  call       instance int32 csharp6.Person::get_Age()
   IL_0012:  box        [mscorlib]System.Int32
   IL_0017:  call       string [mscorlib]System.String::Format(string,
                                                               object,
                                                               object)
   IL_001c:  stloc.
   IL_001d:  br.s       IL_001f
   IL_001f:  ldloc.
   IL_0020:  ret
 } // end of method Person::ToString

第一眼看到新版本的IL代码，我还以为我没有重新编译我的代码。C#编译器帮我们转成了老版本的写法而已，一模一样的。。。so,这又是一个语法层面的优化。

3. Example

 //支持方法调用
 string s1 = $"{person.GetHashCode()}";
 //支持表达式
 string s2 = $"person.{nameof(person.Name)} is {person?.Name}";
 //支持格式化输出
 DateTime now = DateTime.Now;
 string s3 = $"DateTime.Now={now:yyyy-MM-dd HH:mm:ss}";
 //组合表达式和格式化输出
 string s4 = $"{person.Name,2} is {person.Age:D2} year{(person.Age == 1 ? "" : "s")} old.";
 //支持的隐式类型转换
 IFormattable s5 = $"Hello, {person.Name}";
 FormattableString s6 = $"Hello, {person.Name}"

新语法支持表达式求值，支持:格式化操作，还支持到IFormattable的隐式转换，编译结果是利用 System.Runtime.CompilerServices.FormattableStringFactory.Create 这个静态方法构造一个 FormattableString 实现的。IL如下：

 IL_0095:  stloc.s s4
 IL_0097:  ldstr      "Hello, {0}"
 IL_009c:  ldc.i4.1
 IL_009d:  newarr[mscorlib] System.Object
 IL_00a2:  dup
 IL_00a3:  ldc.i4.0
 IL_00a4:  ldloc.0
 IL_00a5:  callvirt instance string csharp6.Person::get_Name()
 IL_00aa:  stelem.ref
 IL_00ab:  call class [mscorlib]System.FormattableString[mscorlib] System.Runtime.CompilerServices.FormattableStringFactory::Create(string,object[])
 IL_00b0:  stloc.s s5
 IL_00b2:  ldstr      "Hello, {0}"
 IL_00b7:  ldc.i4.1
 IL_00b8:  newarr[mscorlib] System.Object
 IL_00bd:  dup
 IL_00be:  ldc.i4.0
 IL_00bf:  ldloc.0
 IL_00c0:  callvirt instance string csharp6.Person::get_Name()
 IL_00c5:  stelem.ref
 IL_00c6:  call class [mscorlib]System.FormattableString[mscorlib] System.Runtime.CompilerServices.FormattableStringFactory::Create(string,object[])

4. 参考

Interpolated Strings