为什么'box'指令是针对泛型发出的？

这是一个相当简单的泛型类.通用参数被约束为引用类型.IRepository并且DbSet还包含相同的约束.

public class Repository : IRepository
    where TEntity : class, IEntity
{
    protected readonly DbSet _dbSet;
    public void Insert(TEntity entity)
    {
        if (entity == null) 
        throw new ArgumentNullException("entity", "Cannot add null entity.");
        _dbSet.Add(entity);
    }
}

编译的IL包含box指令.这是发布版本(虽然调试版本也包含它).

.method public hidebysig newslot virtual final 
    instance void  Insert(!TEntity entity) cil managed
{
  // Code size       38 (0x26)
  .maxstack  8
  IL_0000:  ldarg.1
  >>>IL_0001:  box        !TEntity
  IL_0006:  brtrue.s   IL_0018
  IL_0008:  ldstr      "entity"
  IL_000d:  ldstr      "Cannot add null entity."
  IL_0012:  newobj     instance void [mscorlib]System.ArgumentNullException::.ctor(string,
                                           string)
  IL_0017:  throw
  IL_0018:  ldarg.0
  IL_0019:  ldfld      class [EntityFramework]System.Data.Entity.DbSet`1 class Repository`1::_dbSet
  IL_001e:  ldarg.1
  IL_001f:  callvirt   instance !0 class [EntityFramework]System.Data.Entity.DbSet`1::Add(!0)
  IL_0024:  pop
  IL_0025:  ret
} // end of method Repository`1::Insert

更新:

有了object.Equals(entity, default(TEntity))它看起来更糟糕:

  .maxstack  2
  .locals init ([0] !TEntity CS$0$0000)
  IL_0000:  ldarg.1
  >>>IL_0001:  box        !TEntity
  IL_0006:  ldloca.s   CS$0$0000
  IL_0008:  initobj    !TEntity
  IL_000e:  ldloc.0
  >>>IL_000f:  box        !TEntity
  IL_0014:  call       bool [mscorlib]System.Object::Equals(object,
                                object)
  IL_0019:  brfalse.s  IL_002b

UPDATE2:

对于那些感兴趣的人,这里是调试器中显示的jit编译的代码:

0cd5af28 55              push    ebp
0cd5af29 8bec            mov     ebp,esp
0cd5af2b 83ec18          sub     esp,18h
0cd5af2e 33c0            xor     eax,eax
0cd5af30 8945f0          mov     dword ptr [ebp-10h],eax
0cd5af33 8945ec          mov     dword ptr [ebp-14h],eax
0cd5af36 8945e8          mov     dword ptr [ebp-18h],eax
0cd5af39 894df8          mov     dword ptr [ebp-8],ecx
    //entity reference to [ebp-0Ch]
0cd5af3c 8955f4          mov     dword ptr [ebp-0Ch],edx
    //some debugger checks
0cd5af3f 833d9424760300  cmp     dword ptr ds:[3762494h],0
0cd5af46 7405            je      0cd5af4d  Branch
0cd5af48 e8e1cac25a      call    clr!JIT_DbgIsJustMyCode (67987a2e)
0cd5af4d c745fc00000000  mov     dword ptr [ebp-4],0
0cd5af54 90              nop

    //comparison or entity ref with  zero
0cd5af55 837df400        cmp     dword ptr [ebp-0Ch],0
0cd5af59 0f95c0          setne   al
0cd5af5c 0fb6c0          movzx   eax,al
0cd5af5f 8945fc          mov     dword ptr [ebp-4],eax
0cd5af62 837dfc00        cmp     dword ptr [ebp-4],0
    //if not zero, jump further
0cd5af66 7542            jne     0cd5afaa  Branch
    //throwing exception here      

这个问题的原因实际上是NDepend警告使用装箱/拆箱.我很好奇为什么它在一些通用类中发现拳击,现在很清楚.