建議61:避免在finally內撰寫無效代碼
在闡述建議之前,需要先提出一個問題:是否存在一種打破try-finally執行順序的情況,答案是:不存在(除非應用程序本身因為某些很少出現的特殊情況在try塊中退出)。應該始終認為finally內的代碼會在方法return之前執行,哪怕return在try塊中。
正是這點,可能會讓你寫出無效的代碼,有時候,這樣的無效代碼會是一個隱藏很深的Bug。
看下面代碼:
private static int TestIntReturnBelowFinally() { int i; try { i = 1; } finally { i = 2; Console.WriteLine("\t將int結果改為2,finally執行完畢"); } return i; }
返回值是2。
但是:
private static int TestIntReturnInTry() { int i; try { return i = 1; } finally { i = 2; Console.WriteLine("\t將int結果改為2,finally執行完畢"); } }
返回值是1。
再看下面代碼:
static User TestUserReturnInTry() { User user = new User() { Name = "Mike", BirthDay = new DateTime(2010, 1, 1) }; try { return user; } finally { user.Name = "Rose"; user.BirthDay = new DateTime(2010, 2, 2); Console.WriteLine("\t將user.Name改為Rose"); } }
user類:
class User { public string Name { get; set; } public DateTime BirthDay { get; set; } } View CodeTestUserReturnInTry方法返回的User中,Name的值已經改為Rose了。
現在來解釋為什麼上面3個函數會有3種結果。查看TestIntReturnBelowFinally的finally部分的IL代碼:
finally { IL_0004: ldc.i4.2 IL_0005: stloc.0 IL_0006: ldstr bytearray (09 00 06 5C 69 00 6E 00 74 00 D3 7E 9C 67 39 65 // ...\i.n.t..~.g9e 3A 4E 32 00 0C FF 66 00 69 00 6E 00 61 00 6C 00 // :N2...f.i.n.a.l. 6C 00 79 00 67 62 4C 88 8C 5B D5 6B ) // l.y.gbL..[.k IL_000b: call void [mscorlib]System.Console::WriteLine(string) IL_0010: endfinally } // end handler IL_0011: ldloc.0 IL_0012: ret }
“IL_0004: ldc.i4.2”首先將2壓入棧頂
“IL_0005: stloc.0”將最頂層堆棧的值,也就是2賦值給本地變量,也就是 i (index 0)
“IL_0011: ldloc.0”將本地變量 i (index 0)的值再次壓入棧
“IL_0012: ret”結束函數,同時把棧內的返回值壓入調用者的棧中。就函數將2賦值給了返回值。
看方法TestIntReturnInTry()的Debug版本的IL代碼:
.method private hidebysig static int32 TestIntReturnInTry() cil managed { // 代碼大小 27 (0x1b) .maxstack 2 .locals init ([0] int32 i, [1] int32 CS$1$0000) IL_0000: nop .try { IL_0001: nop IL_0002: ldc.i4.1 IL_0003: dup IL_0004: stloc.0 IL_0005: stloc.1 IL_0006: leave.s IL_0018 } // end .try finally { IL_0008: nop IL_0009: ldc.i4.2 IL_000a: stloc.0 IL_000b: ldstr bytearray (09 00 06 5C 69 00 6E 00 74 00 D3 7E 9C 67 39 65 // ...\i.n.t..~.g9e 3A 4E 32 00 0C FF 66 00 69 00 6E 00 61 00 6C 00 // :N2...f.i.n.a.l. 6C 00 79 00 67 62 4C 88 8C 5B D5 6B ) // l.y.gbL..[.k IL_0010: call void [mscorlib]System.Console::WriteLine(string) IL_0015: nop IL_0016: nop IL_0017: endfinally } // end handler IL_0018: nop IL_0019: ldloc.1 IL_001a: ret } // end of method Program::TestIntReturnInTry
TestIntReturnInTry在IL中創建了兩個本地變量 i 和CS$1$0000 ,i 存儲的是1,然後finally中 i 被賦值為2。調用者真正得到的是由IL創建的CS$1$0000所對應的值。用Reflector查看C#代碼:
private static int TestIntReturnInTry() { int i; int CS$1$0000; try { CS$1$0000 = i = 1; } finally { i = 2; Console.WriteLine("\t將int結果改為2,finally執行完畢"); } return CS$1$0000; }
實際上,finally中i=2沒有任何意義,所以在本函數的release版本中,IL中找不到對應的代碼:
.method private hidebysig static int32 TestIntReturnInTry() cil managed { // 代碼大小 17 (0x11) .maxstack 1 .locals init ([0] int32 CS$1$0000) .try { IL_0000: ldc.i4.1 IL_0001: stloc.0 IL_0002: leave.s IL_000f } // end .try finally { IL_0004: ldstr bytearray (09 00 06 5C 69 00 6E 00 74 00 D3 7E 9C 67 39 65 // ...\i.n.t..~.g9e 3A 4E 32 00 0C FF 66 00 69 00 6E 00 61 00 6C 00 // :N2...f.i.n.a.l. 6C 00 79 00 67 62 4C 88 8C 5B D5 6B ) // l.y.gbL..[.k IL_0009: call void [mscorlib]System.Console::WriteLine(string) IL_000e: endfinally } // end handler IL_000f: ldloc.0 IL_0010: ret } // end of method Program::TestIntReturnInTry
用Reflector查看release版本中C#代碼:
private static int TestIntReturnInTry() { int CS$1$0000; try { CS$1$0000 = 1; } finally { Console.WriteLine("\t將int結果改為2,finally執行完畢"); } return CS$1$0000; }
再解釋第三個方法TestUserReturnInTry為什麼返回的是“Rose”。Reflector查看release版本中C#代碼:
private static User TestUserReturnInTry() { User CS$1$0000; User <>g__initLocal0 = new User { Name = "Mike", BirthDay = new DateTime(0x7da, 1, 1) }; User user = <>g__initLocal0; try { CS$1$0000 = user; } finally { user.Name = "Rose"; user.BirthDay = new DateTime(0x7da, 2, 2); Console.WriteLine("\t將user.Name改為Rose"); } return CS$1$0000; }
User是引用類型, CS$1$0000 = user;說明CS$1$0000和user指向的是同一個對象,當在finally中 user.Name = "Rose"時CS$1$0000的Name也會變為“Rose”。所以返回的CS$1$0000的Name為“Rose”。
再舉一個例子:
private static User TestUserReturnInTry2() { User user = new User() { Name = "Mike", BirthDay = new DateTime(2010, 1, 1) }; try { return user; } finally { user.Name = "Rose"; user.BirthDay = new DateTime(2010, 2, 2); user = null; Console.WriteLine("\t將user置為anull"); } }
返回的結果不是null,而一個Name=“Rose”,BirthDay = new DateTime(2010, 2, 2)的User對象。Reflector查看release版本中C#代碼:
private static User TestUserReturnInTry2() { User CS$1$0000; User <>g__initLocal1 = new User { Name = "Mike", BirthDay = new DateTime(0x7da, 1, 1) }; User user = <>g__initLocal1; try { CS$1$0000 = user; } finally { user.Name = "Rose"; user.BirthDay = new DateTime(0x7da, 2, 2); user = null; Console.WriteLine("\t將user置為anull"); } return CS$1$0000; }
CS$1$0000和user指向的是同一個對象,當在finally中 user=null 時,只是user指向為null了,CS$1$0000指向的對象並沒有變。
轉自:《編寫高質量代碼改善C#程序的157個建議》陸敏技