I have something like this:
For i = 1 To 4
Dim x As Boolean
If i < 3 Then x = True
Console.WriteLine(x)
Next
Gives me: True True True True
and this
For i = 1 To 4
Dim x As Boolean = False
If i < 3 Then x = True
Console.WriteLine(x)
Next
Gives me: True True False False
Aren't they both supposed to give me the second result? The MSDN article says that The default value of Boolean is False.
Is it a bug or am I missing something?
The default value of Boolean is False .
Use the create() or update() methods to specify the value of the new property as its default value, that is, false for boolean or 0 for integer.
A Boolean value can never be null ( Nothing ), the values that are possible are True and False .
Boolean variables are stored as 16-bit (2-byte) numbers, but they can only be True or False.
Thank you for the very interesting question. I took the sample code:
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
And took a look at the generated IL via ILSpy:
Test1()
.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
Test2()
.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
What this tells us is that even though you are declaring x in the loop (Dim x as Boolean
) and (Dim x as boolean = False
), the actual declaration of the variable happens outside the loop.
Because you give your variable a default value inside the loop in Test2
, it is actually setting the value to False
at the beginning of each iteration. Because you do not assign a value to x in the Test1
loop and it only gets declared once outside of the loop, it retains the value it had in the previous iteration.
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