Select case versus If statement

[gonzalioz]

Hi all,

I am not really a fan of if statements. I think they should only be used when comparing different values like this:

Dim status as boolean = false
Dim number as integer = 5

if status.equals(true) then

elseif number = 5 then

else

end if

However, people say that if should also be used if you don't have more then 2 options.

For example:

Dim status as boolean = false

if status.equals(true) then
msgbox("true")
else
msgbox("false")
end if

I would rather use:

Select case status
case true: msgbox("true")
case else: msgbox("false")

or msgbox on a different line whatever you want....

What is this rule about that select case should only be used if there more then 2-3 options. Is it wrong to use a select case instead?

[dee-u]

It may boil down to one's preference, with many conditions SELECT CASE tends to be more readable (to me) and in cases of two conditions then IF ELSE is fine.

[JuggaloBrotha]

I tend to use Select Case a lot when it comes to this stuff, but when it's only two items and if the first condition is false, the second condition is assumed to be true then I'll use an
If Then
Else
End If

structure

[NickThissen]

In my opinion, using If/Else is much more intuitive. You can read it almost as normal english:

If (some condition is true) Then
   do this
Else
   do that
End If

The same cannot be said for select case as easily:

Select Case (some condition)
   Case True
      do this
   Case False
      do that
End Select

It's just personal preference though... I wouldn't be surprised even if it compiles to the same code in the end (but it probably doens't).

[ntg]

To find a performance difference between two If statements and the corresponding Select statement, you'd probably need to use a multimedia timer or perform both operations several million times in a row. The difference in performance is negligible so I would say it is a matter of personal preference.

Having said that, there really is a difference between If and Select. The Select statement is normally translated to the switch IL instruction which is very efficient. So for the code:

Public Enum testEnum
    enum1 = 0
    enum2
    enum3
End Enum
 
Public Sub test()
    Dim s As testEnum = testEnum.enum1
    Select Case s
        Case testEnum.enum1
            s = testEnum.enum2
        Case testEnum.enum2
            s = testEnum.enum3
        Case testEnum.enum3
            s = testEnum.enum1
    End Select
End Sub

the IL for sub test() is:

.method public static void  test() cil managed
{
  // Code size       33 (0x21)
  .maxstack  1
  .locals init ([0] valuetype recursor.Module1/testEnum s,
           [1] valuetype recursor.Module1/testEnum VB$t_i4$L0)
  IL_0000:  ldc.i4.0
  IL_0001:  stloc.0
  IL_0002:  ldloc.0
  IL_0003:  switch     ( 
                        IL_0016,
                        IL_001a,
                        IL_001e)
  IL_0014:  br.s       IL_0020
  IL_0016:  ldc.i4.1
  IL_0017:  stloc.0
  IL_0018:  br.s       IL_0020
  IL_001a:  ldc.i4.2
  IL_001b:  stloc.0
  IL_001c:  br.s       IL_0020
  IL_001e:  ldc.i4.0
  IL_001f:  stloc.0
  IL_0020:  ret
} // end of method Module1::test

However, if you use an expression which is not a boolean or an enumeration, the compiler will generate a series of If statements that will be exactly the same as you were coding a series of If statements yourself. Likewise, if you arbitrarily change the values of your enumerations, the switch statement will be altered. So for the code:

Public Enum testEnum
    enum1 = 0
    enum2 = 2   '<<< Specifically assign value.
    enum3 = 5   '<<< Specifically assign value.
End Enum
 
Public Sub test()
    Dim s As testEnum = testEnum.enum1
    Select Case s
        Case testEnum.enum1
            s = testEnum.enum2
        Case testEnum.enum2
            s = testEnum.enum3
        Case testEnum.enum3
            s = testEnum.enum1
    End Select
End Sub

the following IL is generated:

.method public static void  test() cil managed
{
  // Code size       45 (0x2d)
  .maxstack  1
  .locals init ([0] valuetype recursor.Module1/testEnum s,
           [1] valuetype recursor.Module1/testEnum VB$t_i4$L0)
  IL_0000:  ldc.i4.0
  IL_0001:  stloc.0
  IL_0002:  ldloc.0
  IL_0003:  switch     ( 
                        IL_0022,
                        IL_002c,
                        IL_0026,
                        IL_002c,
                        IL_002c,
                        IL_002a)
  IL_0020:  br.s       IL_002c
  IL_0022:  ldc.i4.2
  IL_0023:  stloc.0
  IL_0024:  br.s       IL_002c
  IL_0026:  ldc.i4.5
  IL_0027:  stloc.0
  IL_0028:  br.s       IL_002c
  IL_002a:  ldc.i4.0
  IL_002b:  stloc.0
  IL_002c:  ret
} // end of method Module1::test

Note that the switch statement switches for six cases because the compiler knows that the enumeration starts at 0 and ends at 5.

[chris128]

In my opinion, using If/Else is much more intuitive. You can read it almost as normal english

Same here, I find Select Case statements horrible to look at and read for some reason..

[minitech]

I like using Select Case like this when I want specific conditions.

Select Case True
Case j = 5 AndAlso i = 6
'code...
Case j = 6 AndAlso i = 5
End Select

Especially with long AndAlso lines, it can be very hard to read when using If.

[stanav]

I tend to use Select Case in this situation:

        Select Case x
            Case 0 To 5, 9, 12 To 20

            Case Else

        End Select

Using If, that would be very hard to read

        If (x >= 0 AndAlso x <= 5) OrElse x = 9 OrElse (x >= 12 AndAlso x <= 20) Then

        Else

        End If

[gep13]

Hey,

As has already been mentioned, it comes down to personal preference.

If I have two conditions, i.e. true or false, then to me, this is a natural fit for If/Else. However, if there are a number of conditions that need to be checked, then I would default to a Select Statement.

Gary

[Pradeep1210]

I usually prefer one liners where ever possible. I feel it easier to maintain it that way as I have lesser code to deal with. Don't know about performance implications though.

So Instead of this:

Dim status as boolean = false

if status.equals(true) then
msgbox("true")
else
msgbox("false")
end if

or this:

Select case status
case true: msgbox("true")
case else: msgbox("false")

or msgbox on a different line whatever you want....

I would prefer:

MsgBox(If(Status, "True", "False"))
'or
If Status Then MsgBox("True") Else MsgBox("False")
'etc.

[JuggaloBrotha]

I usually prefer one liners where ever possible. I feel it easier to maintain it that way as I have lesser code to deal with. Don't know about performance implications though.

So Instead of this:

Dim status as boolean = false

if status.equals(true) then
msgbox("true")
else
msgbox("false")
end if

or this:

Select case status
case true: msgbox("true")
case else: msgbox("false")

or msgbox on a different line whatever you want....

I would prefer:

MsgBox(If(Status, "True", "False"))
'or
If Status Then MsgBox("True") Else MsgBox("False")
'etc.

Why not:

Messagebox.Show(Status.ToString)

?

[gep13]

Hey,

In fairness to Pradeep, I think he was just trying to give a simple example of the technique that he uses.

Gary

[Pradeep1210]

Why not:

Messagebox.Show(Status.ToString)

?

Yes, that too is ok as long as we need to show "True" and "False" only in the messagebox. I was just trying to get the equivalent of OP's code.

[Pradeep1210]

Hey,

In fairness to Pradeep, I think he was just trying to give a simple example of the technique that he uses.

Gary

Yes exactly

[minitech]

If() new in VS 2010 or something?

[JuggaloBrotha]

If() new in VS 2010 or something?

VS 2008 actually

[Pradeep1210]

If() new in VS 2010 or something?

Yes.

Then new If() operator behaves like the short-circuited If operators (actually it is ?: in other languages). It evaluates only that part that is required.
VB was lacking this single If() operator that other languages have.
VB already had IIf() function. But it evaluates both the true as well as false conditions. So it was slower (as well as unfit for some cases) than the new If() operator

[gonzalioz]

I would prefer:

MsgBox(If(Status, "True", "False"))
'or
If Status Then MsgBox("True") Else MsgBox("False")
'etc.

@Pradeep

A bit off topic but.. How do you make your code colored?

[gep13]

Hey,

I believe I am correct in saying that this is done using VBCODE tags, rather than standard CODE tags:

MsgBox(If(Status, "True", "False"))
'or
If Status Then MsgBox("True") Else MsgBox("False")
'etc.

MsgBox(If(Status, "True", "False"))
'or
If Status Then MsgBox("True") Else MsgBox("False")
'etc.

VBCODE tags, actually render a HIGHLIGHT tag, which you can provide an argument to, i.e. HIGHLIGHT=vb, or HIGHLIGHT=vb.net

Gary

[Pradeep1210]

Yep. either select your code and click the VBCode button on the toolbar, that will automatically put the tags for you.
Or you may manually put your code between [HIGHLIGHT=vb.net] [/HIGHLIGHT] tags.

[doctrin13th]

I like using Select Case like this when I want specific conditions.

Select Case True
Case j = 5 AndAlso i = 6
'code...
Case j = 6 AndAlso i = 5
End Select

Especially with long AndAlso lines, it can be very hard to read when using If.

What is the difference in using Case j=5 And i=6 with Case j=5 AndAlso i=6?

I tend to use Select Case in this situation:

        Select Case x
            Case 0 To 5, 9, 12 To 20

            Case Else

        End Select

Using If, that would be very hard to read

        If (x >= 0 AndAlso x <= 5) OrElse x = 9 OrElse (x >= 12 AndAlso x <= 20) Then

        Else

        End If

Also here, what is the difference between And and AndAlso, and OR and OrElse?

Confused...

[akhileshbc]

What is the difference in using Case j=5 And i=6 with Case j=5 AndAlso i=6?



Also here, what is the difference between And and AndAlso, and OR and OrElse?

Confused...

Check this: http://social.msdn.microsoft.com/for...a-3815db83e2b3 ....

[gep13]

There is some more discussion on this here:

http://visualbasic.about.com/od/quic...vbnetlogop.htm

As well as the MSDN documentation here:

And
AndAlso
Or
OrElse

Gary

[cicatrix]

Also, about performance.
Even though the difference is minimal, the code will be faster if you provide the expression which is more likely to appear first. For example (applies to both If and Select Case):

If moreprobablecondition Then
   ...
Else
   ...
End If

Or

Select Case expression
     Case moreprobablevalue ' Should come first
     Case Else
End Select