c#-string 插值

Posted 2022-01-05 wx5a20cf699eb6f

1. 老版本的代码

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：

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：

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

新语法的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

1 //支持方法调用
 2 string s1 = $"person.GetHashCode()";
 3 //支持表达式
 4 string s2 = $"person.nameof(person.Name) is person?.Name";
 5 //支持格式化输出
 6 DateTime now = DateTime.Now;
 7 string s3 = $"DateTime.Now=now:yyyy-MM-dd HH:mm:ss";
 8 //组合表达式和格式化输出
 9 string s4 = $"person.Name,2 is person.Age:D2 year(person.Age == 1 ? "" : "s") old.";
10 //支持的隐式类型转换
11 IFormattable s5 = $"Hello, person.Name";
12 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：

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[])

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