Two Equal signs in one expression question

[tedsmith]

For some time I have been pondering how one particular line of some code works (contributed some time ago by our beloved guru strongm).
This is used to check if a computer is connected to your network (pinging) and works very well.

If strComputer > "" Then
        Set objWMIService = GetObject("winmgmts:" & 

"{impersonationLevel=impersonate}!\\.\root\cimv2")
        Set objPing = objWMIService.ExecQuery("Select * From Win32_PingStatus 

Where Address = '" & strComputer & "' and StatusCode=0")
        'pings computer
        For Each objStatus In objPing
            IsReachable = objStatus.StatusCode = 0
        Next
End If

(IsReachable = True if computer is connected)

The line in my question is "IsReachable = objStatus.StatusCode = 0" where you have 2 equals sign in the one statement.

So it seems the code line is equivalent to
If objStatus.StatusCode = 0 then
IsReachable = True
Else
IsReachable = False
End IF

Similarly
a=0=0: print a gives true
a=1=0: print a gives true

I would be grateful for an explanation on how the 2 equal signs work to create a boolean result to put me out of my misery!

 

[gmmastros]

a=0=0: print a gives true

In this case, VB will evaluate the (0=0) part, which results in TRUE, and then assigns this to the a variable.

a=1=0: print a gives true

In this case, VB will evaluate the (1=0) part, which results in TRUE, and then assigns this to the a variable.

Personally, I don't like to see code like this because it can be a little misleading. Don't get me wrong, I have similar code in my project, but when I use this, I usually put parenthesis around the right side expression.

Instead of:
[tt]IsReachable = objStatus.StatusCode = 0[/tt]

I would write it as:

IsReachable = [!]([/!]objStatus.StatusCode = 0[!])[/!]

It will compile the same, but for me, it's more readable.

Make sense?

-George
Microsoft SQL Server MVP
My Blogs
SQLCop
"The great things about standards is that there are so many to choose from." - Fortune Cookie Wisdom

 

[jges]

The first equal sign is an assignment operator, the second is a test for equivalence. In a different language it might look like:

IsReachable = objStatus.StatusCode == 0

 

[Andrzejek]

VB will evaluate the (1=0) part, which results in TRUE[/quote gmmastros]
Hmmm....

Dim bln As Boolean

bln = (1 = 0)

MsgBox bln

Message box shows FALSE....

Have fun.

---- Andy

 

[strongm]

>a=1=0: print a gives true

Shouldn't do

>Message box shows FALSE....

As it should do

 

[tedsmith]

Yes I made a typo error - should have been
Similarly
a=0=0: print a gives true
a=1=0: print a gives true

I can see that it is doing what you all say but my question was how?
Having 2 equals like that I would have though should produce a syntax error.
I wondered if it was some accidental thing or was it planned?

 

[strongm]

>was it planned?

Planned, ted. It's to do with the VB expression evaluator and the rules it follows

 

[Hypetia]

It is interesting to see that, not only two, VB can support many = operators in an expression, although you rarely find such applications.

See the examples.
[tt]
a = 0 = 1 = 2 = 3
MsgBox a 'gives False
a = 3 = 2 = 1 = 0
MsgBox a 'gives True
[/tt]
As both expression contain the same operator (=), VB starts to evaluate them from Left to Right.

First expression:
a = 0 = 1 = 2 = 3 = (((0 = 1) = 2) = 3) = ((0 = 2) = 3) = (0 = 3) = False

Second expression:
a = 3 = 2 = 1 = 0 = (((3 = 2) = 1) = 0) = ((0 = 1) = 0) = (0 = 0) = True

A practical example of such expression is checking for a leap year.
[tt]IsLeap = (Year Mod 400 = 0) Or ((Not Year Mod 100 = 0) And (Year Mod 4 = 0))[/tt]