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no compile error while type mismatch

Posted on 2001-06-18
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Last Modified: 2010-05-02
i have a function:

MyFunction (Param1 as MyEnum) as Variant

While i call the function like this:
lsReturn = MyFunction("123")

MyEnum is a long and "123" is a literal string.

How come no compile error is prompt while compile and making a EXE?

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Question by:chnelson
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LVL 9

Expert Comment

by:Valliappan AN
ID: 6201723
It is by design in MS way. Look at this link:

http://msdn.microsoft.com/library/devprods/vs6/vbasic/vbcon98/vbconpassingargumentstoprocedures.htm

There is a similar example, as you your case.

Cheers.
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LVL 15

Expert Comment

by:ameba
ID: 6201742
Note that you are passing parameter "123" ByVal.
You are not passing string variable, but calculated value.

Option Explicit
Private Enum MyEnum
    dsNoRecords = 123
    dsNothingSelected
    dsNormal
    dsModify
    dsAdd
End Enum

Private Sub Form_Click()
    Dim lsReturn As Variant, x As String
    x = "123"
    lsReturn = MyFunction(x)     ' type mismatch error
   
    lsReturn = MyFunction((x))   ' no error, you are passing value of x
   
    lsReturn = MyFunction(x * 1) ' no error, you are passing calculated value
   
End Sub

Private Function MyFunction(Param1 As MyEnum) As Variant
    Beep
End Function
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LVL 15

Expert Comment

by:ameba
ID: 6202582
If you pass parameter by value, VB compiler will insert converson function automatically.
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LVL 9

Expert Comment

by:Valliappan AN
ID: 6203081
A nice explanation by ameba. :)
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LVL 1

Author Comment

by:chnelson
ID: 6203158
ameba,

is it some way to disable that kind of auto convertion?

This kind of weak type checking is really dangerous which cause lots of run-time error before delopying the EXE to customers.
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LVL 1

Author Comment

by:chnelson
ID: 6203163
ameba,

is it some way to disable that kind of auto convertion?

This kind of weak type checking is really dangerous which cause lots of run-time error before delopying the EXE to customers.
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LVL 15

Accepted Solution

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ameba earned 40 total points
ID: 6203321
>is it some way to disable that kind of auto convertion?

Type checking is present if you pass variable ByRef.

VB will do many things 'behind the scene' to allow easy and fast programming.  If you remove 'Option Explicit' you don't have to declare variables.

Of course this can be dangerous, many VB programmers asked for 'Option Strict' or 'Option Anal Strict' to disable those 'features'...
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Author Comment

by:chnelson
ID: 6206583
oh...
i got it...
asking you for a comment ameba,
to enforce strong type checking,
is it a good way to use ByRef instead of ByVal?
the only side-effect i think is that the parameters (ByRef) may be modified accidentally...
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LVL 15

Expert Comment

by:ameba
ID: 6206660
>to enforce strong type checking,
>is it a good way to use ByRef instead of ByVal?

No, use ByVal if you need to pass parameter one-way [in]
use ByRef if you need to pass parameter two-way [in]+[out]

--
Please, maintain your old questions! You still have many opened questions.
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Author Comment

by:chnelson
ID: 6207087
thanks ameba, very clear explaination
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