这里有一个相当简单的通用类。泛型参数被限制为引用类型。IRepository
和DbSet
也包含相同的约束。
public class Repository<TEntity> : IRepository<TEntity>
where TEntity : class, IEntity
{
protected readonly DbSet<TEntity> _dbSet;
public void Insert(TEntity entity)
{
if (entity == null)
throw new ArgumentNullException("entity", "Cannot add null entity.");
_dbSet.Add(entity);
}
}
编译后的中间语言包含box
指令。这是发布版本(尽管调试版本也包含它)。
.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<!0> class Repository`1<!TEntity>::_dbSet
IL_001e: ldarg.1
IL_001f: callvirt instance !0 class [EntityFramework]System.Data.Entity.DbSet`1<!TEntity>::Add(!0)
IL_0024: pop
IL_0025: ret
} // end of method Repository`1::Insert
更新:
使用object.Equals(entity, default(TEntity))
甚至更糟:
.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
更新2:
对于那些感兴趣的人,这里是jit编译后在调试器中显示的代码:
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。我好奇它为什么在一些泛型类中发现了装箱,现在清楚了。
object.Equals
和==
运算符都是如此“通用”,以至于它们默认会进行装箱。尝试使用EqualityComparer<TEntity>.Default.Equals(entity, default(T))
,因为它应该使用通用方法。 - Konrad KokosaTEntity
可能是值类型,并且当应用== null
(实际上是Object.ReferenceEquals
)时,需要将其装箱为object
。我怀疑当方法基于实际闭合泛型类型被JIT时,装箱和/或空检查可能会被优化,尽管现在我没有工具来检查这一点。只是一个猜测。 - Chris Sinclair