[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  3
  .locals init ([0] string V_0)
  IL_0000:  nop
  IL_0001:  ldstr      "[name={0},age={1}]"
  IL_0006:  ldarg.0
  IL_0007:  call       instance string csharp6.Person::get_Name()
  IL_000c:  ldarg.0
  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.0
  IL_001d:  br.s       IL_001f
  IL_001f:  ldloc.0
  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  3
  .locals init ([0] string V_0)
  IL_0000:  nop
  IL_0001:  ldstr      "[name={0},age={1}]"
  IL_0006:  ldarg.0
  IL_0007:  call       instance string csharp6.Person::get_Name()
  IL_000c:  ldarg.0
  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.0
  IL_001d:  br.s       IL_001f
  IL_001f:  ldloc.0
  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