字符串连接及$

• 一、+号
• 二、String.Format
• 三、$号
• 四、运行结果

一、+号

常见的用string的+用于字符连接时：

using System;
namespace 输出打印方式
{
    class Program
    {
        static void Main(string[] args)
        {
            int age = 18;
            string Prefix = "\"dp\" is ";
            string test1 = Prefix + age;
            Console.WriteLine(test1);
            Console.ReadKey();
        }
    }
}

扩展：String加号的重载？
首先查看一下String类：
其中只有关于（!=和==）的重载如下并无+的重载。

namespace System
{
    [ComVisible(true)]
    [DefaultMember("Chars")]
    public sealed class String : IComparable, ICloneable, IConvertible, IComparable<String>, IEnumerable<char>, IEnumerable, IEquatable<String>
    {
        [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
        public static bool operator ==(String a, String b);
        [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries")]
        public static bool operator !=(String a, String b);
    }

通过ILDASM工具转换成中间代码：

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // 代码大小       37 (0x25)
  .maxstack  2
  .locals init ([0] int32 age,
           [1] string Prefix,
           [2] string test1)
  IL_0000:  nop
  IL_0001:  ldc.i4.s   18
  IL_0003:  stloc.0
  IL_0004:  ldstr      "\"dp\" is "
  IL_0009:  stloc.1
  IL_000a:  ldloc.1
  IL_000b:  ldloc.0
  IL_000c:  box        [mscorlib]System.Int32
  IL_0011:  call       string [mscorlib]System.String::Concat(object,   //关键语句
                                                              object)
  IL_0016:  stloc.2
  IL_0017:  ldloc.2
  IL_0018:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_001d:  nop
  IL_001e:  call       valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey()
  IL_0023:  pop
  IL_0024:  ret
} // end of method Program::Main

可以看见，默认的直接就把String的+转换成的String.Concat函数来执行。

二、String.Format

代码如下：

using System;
namespace 输出打印方式
{
    class Program
    {
        static void Main(string[] args)
        {
            int age = 18;
            string Prefix = "\"dp\" is ";
            string test2 = string.Format("{0}{1}",Prefix,age);
            Console.WriteLine(test2);
            Console.ReadKey();
        }
    }
}

三、$号

代码如下：

using System;
namespace 输出打印方式
{
    class Program
    {
        static void Main(string[] args)
        {
            int age = 18;
            string Prefix = "\"dp\" is ";
            string test3 = $"{Prefix}{age}";
            Console.WriteLine(test3);
            Console.ReadKey();
        }
    }
}

再来看一下IL转的中间代码：

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // 代码大小       42 (0x2a)
  .maxstack  3
  .locals init ([0] int32 age,
           [1] string Prefix,
           [2] string test3)
  IL_0000:  nop
  IL_0001:  ldc.i4.s   18
  IL_0003:  stloc.0
  IL_0004:  ldstr      "\"dp\" is "
  IL_0009:  stloc.1
  IL_000a:  ldstr      "{0}{1}"
  IL_000f:  ldloc.1
  IL_0010:  ldloc.0
  IL_0011:  box        [mscorlib]System.Int32
  IL_0016:  call       string [mscorlib]System.String::Format(string,//关键代码
                                                              object,
                                                              object)
  IL_001b:  stloc.2
  IL_001c:  ldloc.2
  IL_001d:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_0022:  nop
  IL_0023:  call       valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey()
  IL_0028:  pop
  IL_0029:  ret
} // end of method Program::Main

也可以轻易发现，$起始也是默认转换成的Format的形式进行连接的。

四、运行结果

一切皆从真实心中作。