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上一篇写了一下装箱拆箱的定义和IL分析,这一篇我们看下使用泛型和不使用泛型引发装箱拆箱的情况 1. 使用非泛型集合时引发的装箱和拆箱操作 看下面的一段代码: var array = new ArrayList(); array.Add(1); array.Add(2); foreach (int value in array) { Console.WriteLine(“value is {0}”,value); } 代码声明了一个ArrayList对象,向ArrayList中添加两个数字1,2;然后使用foreach将ArrayList中的元素打印到控制台。 在这个过程中会发生两次装箱操作和两次拆箱操作,在向ArrayList中添加int类型元素时会发生装箱,在使用foreach枚举ArrayList中的int类型元素时会发生拆箱操作,将object类型转换成int类型,在执行到Console.WriteLine时,还会执行两次的装箱操作;这一段代码执行了6次的装箱和拆箱操作;如果ArrayList的元素个数很多,执行装箱拆箱的操作会更多。 你可以通过使用ILSpy之类的工具查看IL代码的box,unbox指令查看装箱和拆箱的过程 2. 使用泛型集合的情况 请看如下代码: var list = new List<int>(); list.Add(1); list.Add(2); foreach (int value in list) { Console.WriteLine("value is {0}", value); } 代码和1中的代码的差别在于集合的类型使用了泛型的List,而非ArrayList;我们同样可以通过查看IL代码查看装箱拆箱的情况,上述代码只会在Console.WriteLine()方法时执行2次装箱操作,不需要拆箱操作。 可以看出泛型可以避免装箱拆箱带来的不必要的性能消耗;当然泛型的好处不止于此,泛型还可以增加程序的可读性,使程序更容易被复用等等。 本文使用的C#代码如下: using System; using System.Collections; using System.Collections.Generic; namespace boxOrUnbox { class Program { static void Main(string[] args) { //do nothing } static void Box() { object objValue = 9; } static void Unbox() { object objValue = 4; int value = (int)objValue; } static void LookatArrayList() { var array = new ArrayList(); array.Add(1); array.Add(2); foreach (int value in array) { Console.WriteLine("value is {0}", value); } } static void LookatGenericList() { var list = new List<int>(); list.Add(1); list.Add(2); foreach (int value in list) { Console.WriteLine("value is {0}", value); } } } } C#的IL代码如下: .class private auto ansi beforefieldinit boxOrUnbox.Program extends [mscorlib]System.Object { // Methods .method private hidebysig static void Main ( string[] args ) cil managed { // Method begins at RVA 0x2050 // Code size 2 (0x2) .maxstack 8 .entrypoint IL_0000: nop IL_0001: ret } // end of method Program::Main .method private hidebysig static void Box () cil managed { // Method begins at RVA 0x2054 // Code size 10 (0xa) .maxstack 1 .locals init ( [0] object objValue ) IL_0000: nop IL_0001: ldc.i4.s 9 IL_0003: box [mscorlib]System.Int32 IL_0008: stloc.0 IL_0009: ret } // end of method Program::Box .method private hidebysig static void Unbox () cil managed { // Method begins at RVA 0x206c // Code size 16 (0x10) .maxstack 1 .locals init ( [0] object objValue, [1] int32 'value' ) IL_0000: nop IL_0001: ldc.i4.4 IL_0002: box [mscorlib]System.Int32 IL_0007: stloc.0 IL_0008: ldloc.0 IL_0009: unbox.any [mscorlib]System.Int32 IL_000e: stloc.1 IL_000f: ret } // end of method Program::Unbox .method private hidebysig static void LookatArrayList () cil managed { // Method begins at RVA 0x2088 // Code size 114 (0x72) .maxstack 2 .locals init ( [0] class [mscorlib]System.Collections.ArrayList 'array', [1] int32 'value', [2] class [mscorlib]System.Collections.IEnumerator CS$5$0000, [3] bool CS$4$0001, [4] class [mscorlib]System.IDisposable CS$0$0002 ) IL_0000: nop IL_0001: newobj instance void [mscorlib]System.Collections.ArrayList::.ctor() IL_0006: stloc.0 IL_0007: ldloc.0 IL_0008: ldc.i4.1 IL_0009: box [mscorlib]System.Int32 IL_000e: callvirt instance int32 [mscorlib]System.Collections.ArrayList::Add(object) IL_0013: pop IL_0014: ldloc.0 IL_0015: ldc.i4.2 IL_0016: box [mscorlib]System.Int32 IL_001b: callvirt instance int32 [mscorlib]System.Collections.ArrayList::Add(object) IL_0020: pop IL_0021: nop IL_0022: ldloc.0 IL_0023: callvirt instance class [mscorlib]System.Collections.IEnumerator [mscorlib]System.Collections.ArrayList::GetEnumerator() IL_0028: stloc.2 .try { IL_0029: br.s IL_004a // loop start (head: IL_004a) IL_002b: ldloc.2 IL_002c: callvirt instance object [mscorlib]System.Collections.IEnumerator::get_Current() IL_0031: unbox.any [mscorlib]System.Int32 IL_0036: stloc.1 IL_0037: nop IL_0038: ldstr "value is {0}" IL_003d: ldloc.1 IL_003e: box [mscorlib]System.Int32 IL_0043: call void [mscorlib]System.Console::WriteLine(string, object) IL_0048: nop IL_0049: nop IL_004a: ldloc.2 IL_004b: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext() IL_0050: stloc.3 IL_0051: ldloc.3 IL_0052: brtrue.s IL_002b // end loop IL_0054: leave.s IL_0070 } // end .try finally { IL_0056: ldloc.2 IL_0057: isinst [mscorlib]System.IDisposable IL_005c: stloc.s CS$0$0002 IL_005e: ldloc.s CS$0$0002 IL_0060: ldnull IL_0061: ceq IL_0063: stloc.3 IL_0064: ldloc.3 IL_0065: brtrue.s IL_006f IL_0067: ldloc.s CS$0$0002 IL_0069: callvirt instance void [mscorlib]System.IDisposable::Dispose() IL_006e: nop IL_006f: endfinally } // end handler IL_0070: nop IL_0071: ret } // end of method Program::LookatArrayList .method private hidebysig static void LookatGenericList () cil managed { // Method begins at RVA 0x2118 // Code size 90 (0x5a) .maxstack 2 .locals init ( [0] class [mscorlib]System.Collections.Generic.List`1<int32> list, [1] int32 'value', [2] valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32> CS$5$0000, [3] bool CS$4$0001 ) IL_0000: nop IL_0001: newobj instance void class [mscorlib]System.Collections.Generic.List`1<int32>::.ctor() IL_0006: stloc.0 IL_0007: ldloc.0 IL_0008: ldc.i4.1 IL_0009: callvirt instance void class [mscorlib]System.Collections.Generic.List`1<int32>::Add(!0) IL_000e: nop IL_000f: ldloc.0 IL_0010: ldc.i4.2 IL_0011: callvirt instance void class [mscorlib]System.Collections.Generic.List`1<int32>::Add(!0) IL_0016: nop IL_0017: nop IL_0018: ldloc.0 IL_0019: callvirt instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<!0> class [mscorlib]System.Collections.Generic.List`1<int32>::GetEnumerator() IL_001e: stloc.2 .try { IL_001f: br.s IL_003c // loop start (head: IL_003c) IL_0021: ldloca.s CS$5$0000 IL_0023: call instance !0 valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>::get_Current() IL_0028: stloc.1 IL_0029: nop IL_002a: ldstr "value is {0}" IL_002f: ldloc.1 IL_0030: box [mscorlib]System.Int32 IL_0035: call void [mscorlib]System.Console::WriteLine(string, object) IL_003a: nop IL_003b: nop IL_003c: ldloca.s CS$5$0000 IL_003e: call instance bool valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32>::MoveNext() IL_0043: stloc.3 IL_0044: ldloc.3 IL_0045: brtrue.s IL_0021 // end loop IL_0047: leave.s IL_0058 } // end .try finally { IL_0049: ldloca.s CS$5$0000 IL_004b: constrained. valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<int32> IL_0051: callvirt instance void [mscorlib]System.IDisposable::Dispose() IL_0056: nop IL_0057: endfinally } // end handler IL_0058: nop IL_0059: ret } // end of method Program::LookatGenericList .method public hidebysig specialname rtspecialname instance void .ctor () cil managed { // Method begins at RVA 0x2190 // Code size 7 (0x7) .maxstack 8 IL_0000: ldarg.0 IL_0001: call instance void [mscorlib]System.Object::.ctor() IL_0006: ret } // end of method Program::.ctor } // end of class boxOrUnbox.Program
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