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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