类库探源——System.String

string str1 = "3"+"b"; 这句话设计到1个String对象

using System;
class App
{
    static void Main()
    {        
                // 这段代码会创建3个 string 对象
        string str = "a";
        str += "1";
        str += "3";        
        Console.WriteLine(str.Length);
    }
}

/*
.method private hidebysig static void  Main() cil managed
{
  .entrypoint
  // 代码大小       32 (0x20)
  .maxstack  2
  .locals init (string V_0)
  IL_0000:  nop
  IL_0001:  ldstr      "a"
  IL_0006:  stloc.0
  IL_0007:  ldloc.0
  IL_0008:  ldstr      "1"
  IL_000d:  call       string [mscorlib]System.String::Concat(string,
                                                              string)
  IL_0012:  stloc.0
  IL_0013:  ldloc.0
  IL_0014:  ldstr      "3"
  IL_0019:  call       string [mscorlib]System.String::Concat(string,
                                                              string)
  IL_001e:  stloc.0
  IL_001f:  ret
} // end of method App::Main

*/

上面的 ldstr  指令是向堆上创建字符串对象的指令