Bitwise AND in an If/Else block

[Dave Kreskowiak]

treddie wrote:

After testing, the first example works, the second does not

Oh, it works, just not the way to think it does. The result of (ValueA And MaskA) results in an integer value, not True/False. That integer is compared to zero. If the result of the expression is non-zero, True is returned. The Not operator will invert that to False. If the result of the expression is zero, False is returned, then inverted to True by the Not operator. Withouting seeing the values of ValueA and Mask, it's pretty much all we can tell you.

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Dave Kreskowiak

[Lost User]

treddie wrote:

After testing, the first example works, the second does not.

You need to show us the values of ValueA and MaskA, so we can try to figure out why.

Use the best guess

[TnTinMn]

Elimate the confusion and set "Option Strict On"; it is off in the default configuration. This will disallow the implied conversion from integer to boolean. see: Visual Studio compiler options, Part 2: Option Strict Also, review Operator Precedence in Visual Basic

[treddie]

The Not operator will invert that to False. But that is not what happens. Here are some actual tests I performed:

The value and the mask in these examples are:
FileAttributes Value that contains a bit field that indicates whether a resource is a folder or a file.
Folder = 8208D, File = 8224D

Mask		Bit mask that tests for the presence of a folder (16D).

Option Strict is NOT set.
_____________________________________________________________________________________________________

GOAL: Run code on "True" condition (Resource is a folder):
If (FileAttributes And FILE_ATTRIBUTE_DIRECTORY) Then
'Execute code.
End If

When FileAttributes = 8208D (Folder):
	8208D (Folder)    =    		2	0	1	0	H
					0010	0000	0001	0000	B
	16D (Mask)        =	AND	0000	0000	0001	0000	B
                                            \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
	Result			        0000	0000	0001	0000	B    <>	0, ("True"), Code executes as it should.


When FileAttributes = 8224D (File):
	8224D (File)    =    		2	0	2	0	H
					0010	0000	0010	0000	B
	16D (Mask)      =	AND	0000	0000	0001	0000	B
                                            \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
	Result			        0000	0000	0000	0000	B     =	0, ("False"), Code does not execute.
											      This is correct.

_____________________________________________________________________________________________________
GOAL: Run code on "False" condition (Resource is a file):
If Not (FileAttributes And FILE_ATTRIBUTE_DIRECTORY) Then
'Execute code.
End If

When FileAttributes = 8208D (Folder):
	8208D (Folder)    =    		2	0	1	0	H
					0010	0000	0001	0000	B
	16D (Mask)        =	AND	0000	0000	0001	0000	B
                                            \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
	Result			        0000	0000	0001	0000	B    <> 0, ("True"), Code WOULD  execute, if left as is.
                                            \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
				NOT	1111	1111	1110	1111	B    <>	0, ("True"), Code executes when it SHOULD NOT.


When FileAttributes = 8224D (File):
	8224D (File)    =    		2	0	2	0	H
					0010	0000	0010	0000	B
	16D (Mask)      =	AND	0000	0000	0001	0000	B
                                            \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
	Result			        0000	0000	0000	0000	B    =  0, ("False"), Code does not execute, if left as is.
                                            \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

[Dave Kreskowiak]

OK. You should be testing the value of the bitwise operation against the value of the mask, NOT depending on an implied true or false.

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Dave Kreskowiak

[Lost User]

That is the difference between a logical and a bitwise expression. In your expressions you cannot use NOT to invert from non-zero (implied TRUE) to FALSE. I think as Dave suggests you need something like:

If (FileAttributes And FILE_ATTRIBUTE_DIRECTORY) Then
' it is a directory.
Else
' it is NOT a directory.
End If

or even

If (FileAttributes And FILE_ATTRIBUTE_DIRECTORY) = FILE_ATTRIBUTE_DIRECTORY Then
' it is a directory.
Else
' it is NOT a directory.
End If

Use the best guess

[treddie]

That is the conclusion I came to in an earlier post. But I needed to understand why, and now I think I get the behavior. It's kind of like that old saying, "You can prove a positive, but you can't prove a negative." At least not without that extra Else clause thrown in.

[treddie]

Good advice.

[Lost User]

treddie wrote:

It's kind of like that old saying, "You can prove a positive, but you can't prove a negative." At least not without that extra Else clause thrown in.

Well that's not quite the issue. The issue is that you are using a bitwise operation whose result will be some number of 1 bits, including zero. If you than apply the NOT operator (which is also a bitwise operator) to that result you will get the bitwise complement. Neither of these is a boolean expression so their results are neither TRUE nor FALSE, they are just some number of bits from 0 to hex FF etc. If the result has any bits set then it is generally assumed to be TRUE in an IF clause; if it has no bits set then it is assumed to be FALSE. But applying the NOT operator to a non-zero result, can still give a non-zero answer. If you want strict boolean tests then you must include a comparison operator in the expression, as I did in the second example of my previous answer.

Use the best guess

[treddie]

Makes total sense.