10

我有这样的事情：

For i = 1 To 4
        Dim x As Boolean
        If i < 3 Then x = True
        Console.WriteLine(x)
Next

给我：真真真真

和这个

For i = 1 To 4
        Dim x As Boolean = False
        If i < 3 Then x = True
        Console.WriteLine(x)
Next

给我：真真假假

他们不是都应该给我第二个结果吗？MSDN 文章说 Boolean 的默认值为 False。

这是一个错误还是我错过了什么？

4

2 回答 2

13

感谢您提出非常有趣的问题。我拿了示例代码：

Module Module1

    Sub Main()
        Test1()
        Test2()
        Console.ReadKey()
    End Sub

    Public Sub Test1()
        For i = 1 To 4
            Dim x As Boolean
            If i < 3 Then x = True
            Console.WriteLine(x)
        Next
    End Sub

    Public Sub Test2()
        For i = 1 To 4
            Dim x As Boolean = False
            If i < 3 Then x = True
            Console.WriteLine(x)
        Next
    End Sub
End Module

并通过 ILSpy 查看了生成的 IL：

测试1()

    .method public static 
        void Test1 () cil managed 
    {
        // Method begins at RVA 0x2120
        // Code size 33 (0x21)
        .maxstack 2
        .locals init (
            [0] int32 i,
            [1] bool x,
            [2] bool VB$CG$t_bool$S0,
            [3] int32 VB$CG$t_i4$S0
        )

        IL_0000: nop
        IL_0001: ldc.i4.1                   
        IL_0002: stloc.0                        // put 1 on top of stack
        // loop start (head: IL_0003)
            IL_0003: ldloc.0                    // load i on top of stack 
            IL_0004: ldc.i4.3                   // load 3 on top of stack
            IL_0005: clt                        // compare if 0 is less than 3
            IL_0007: stloc.2                 
            IL_0008: ldloc.2
            IL_0009: brfalse.s IL_000d          // if i >= 3, jump to IL_000d

            IL_000b: ldc.i4.1                   // load true onto stack
            IL_000c: stloc.1                    // set x = true

            IL_000d: ldloc.1                    // load x onto stack
            IL_000e: call void [mscorlib]System.Console::WriteLine(bool) // print x
            IL_0013: nop
            IL_0014: nop
            IL_0015: ldloc.0                    // load i onto stack
            IL_0016: ldc.i4.1                   // load 1 onto stack
            IL_0017: add.ovf                    // add i + 1
            IL_0018: stloc.0                    // set i = i + 1
            IL_0019: ldloc.0                    // load i
            IL_001a: ldc.i4.4                   // load 4
            IL_001b: stloc.3                    // store 4 in iterator variable
            IL_001c: ldloc.3                    // load 4 from iterator variable
            IL_001d: ble.s IL_0003              // if i <= 4, go to beginning of loop
        // end loop
        IL_001f: nop
        IL_0020: ret
    } // end of method Module1::Test1

测试2() 

        .method public static 
        void Test2 () cil managed 
    {
        // Method begins at RVA 0x2150
        // Code size 35 (0x23)
        .maxstack 2
        .locals init (
            [0] int32 i,
            [1] bool x,
            [2] bool VB$CG$t_bool$S0,
            [3] int32 VB$CG$t_i4$S0
        )

        IL_0000: nop
        IL_0001: ldc.i4.1                   // load 1 onto stack
        IL_0002: stloc.0                    // set i = 1
        // loop start (head: IL_0003)       
            IL_0003: ldc.i4.0               // load 0 onto stack
            IL_0004: stloc.1                // set x = false
            IL_0005: ldloc.0                // load i onto stack
            IL_0006: ldc.i4.3               // load 3 onto stack
            IL_0007: clt                    // compare i to 3
            IL_0009: stloc.2                // store result in iterator variable
            IL_000a: ldloc.2                // load iterator variable
            IL_000b: brfalse.s IL_000f      // if i >= 3, jump to IL_00f

            IL_000d: ldc.i4.1               // load 1 onto stack
            IL_000e: stloc.1                // set x = true

            IL_000f: ldloc.1                // load x onto stack
            IL_0010: call void [mscorlib]System.Console::WriteLine(bool)    // print x
            IL_0015: nop            
            IL_0016: nop
            IL_0017: ldloc.0                // load i onto stack
            IL_0018: ldc.i4.1               // load 1 onto stack
            IL_0019: add.ovf                // add i + 1
            IL_001a: stloc.0                // set i = i + 1
            IL_001b: ldloc.0                // load i onto stack
            IL_001c: ldc.i4.4               // load 4 onto stack
            IL_001d: stloc.3                // store 4 into iterator variable
            IL_001e: ldloc.3                // load 4 from iterator variable
            IL_001f: ble.s IL_0003          // if i <= 4, go to beginning of loop
        // end loop
        IL_0021: nop
        IL_0022: ret
    } // end of method Module1::Test2

这告诉我们的是，即使您在循环 ( Dim x as Boolean) 和 ( Dim x as boolean = False) 中声明 x，变量的实际声明仍发生在循环之外。

因为您在 in 的循环中为变量提供了默认值Test2，所以它实际上是False在每次迭代开始时将值设置为。因为您没有在循环中为 x 赋值，Test1并且它只在循环外声明一次，所以它保留了它在前一次迭代中的值。

于 2012-11-26T01:29:12.553 回答
4

变量 x 不会在循环的每次迭代中初始化 - 只有第一次。示例 1 与

Dim x As Boolean
For i = 1 To 4
        If i < 3 Then x = True
        Console.WriteLine(x)
Next

当您以这种方式看到它时，为什么最终得到 True True True True 是有道理的，因为您在第一次通过循环时将其设置为 True，并且没有任何东西将其设置为 False。

示例 2 与

Dim x As Boolean
For i = 1 To 4
        x = False
        If i < 3 Then x = True
        Console.WriteLine(x)
Next

当然不直观，但这就是编译器正在做的事情

于 2012-11-26T01:27:41.937 回答