is there any possibility to assign a value to a variable inside an IF condition in VB.NET?
Something like that:
Dim customer As Customer = Nothing
If IsNothing(customer = GetCustomer(id)) Then
Return False
End If
Thank you
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7 Answers
Sorry, no. On the other hand, it would get really confusing, since VB.NET uses the same operator for both assignment and equality.
If a = b Then 'wait, is this an assignment or comparison?!
Instead, just set the variable and compare:
Dim customer As Customer = Nothing
customer = GetCustomer(id)
If IsNothing(customer) Then
Return False
End If
1 Comment
Pedro Moreira
A bit more confusing, yes, but also it would help to keep the code more DRY.
No, I'm fairly sure this is not possible - but be thankful!
This is a "feature" of C-based languages that I would never encourage the use of, because it is probably misused or misinterpreted more often than not.
I think the best approach is to try and express "one thought per line", and resist coding that combines two operations in the same line. Combining an assignment and a comparison in this way usually doesn't achieve much other than making the code more difficult to understand.
Comments
VB doesn't do that very well, especially since assignment and comparison both use the = operator. It'd get really confusing. And since VB to some degree is designed to be newbie-friendly (The B in "BASIC" actually stands for "Beginner's"), expressions with multiple side effects are not something the language generally likes to do.
If it's essential that you be able to do it all in one line, then you could make the GetCustomer method assign to a ByRef parameter, and return a boolean saying whether the assigned value was null. But really, it's better to just assign and then compare to null.
3 Comments
Torben
Thank you, so I just assign and compare :)
cHao
Meh. As a programming expert, i recognize that readability > concision. :) IMO VB actually had half a good idea here. Just happened to be a result of other bad ideas.
There is no builtin support for doing this, but you could use this workaround.
Public Function Assign(Of T)(ByRef destination As T, ByVal value As T) As T
destination = value
Return destination
End Function
And then it could be used like this.
Dim customer As Customer = Nothing
If IsNothing(Assign(customer, GetCustomer(id))) Then
Return False
End If
Comments
Thinking out loud because you didn't show Customer or GetCustomer...
Dim myCust As New Customer
If myCust.GetCustomer(someID) Then
End If
Class Customer
Private _customerID As Integer
Const noCustomer As Integer = -1
Public Sub New()
Me._customerID = noCustomer 'no customer
End Sub
Public Function GetCustomer(ByVal id As Integer) As Boolean
'perform required action to get customer
'if successful then set Me._customerID to ID else set it to no customer value
Return Me.HaveCustomer
End Function
Public Function HaveCustomer() As Boolean
If Me._customerID = noCustomer Then Return False Else Return True
End Function
End Class
Comments
Yes, it is possible to assign a value to a variable inside an IF condition in VB.NET. There is even builtin support for doing this, albeit to a limited extend:
If Interlocked.Exchange(customer, GetCustomer(id)) Is Nothing Then
Return False
End If
where the methods within the Interlocked class are intended to be used within a multi-threading environment. By design, the return value of Exchange() is the old value, rather than the new one. Thus, to obtain the rather cryptic equivalent result:
If (customer = Interlocked.Exchange(customer, GetCustomer(id)) And customer Is Nothing Then
return false
End If
Comments
All of the people stating that this is an issue as VB uses the "same operator for both assignment and equality".. I would argue that this is not a good reason as they could simply make slightly different syntax for it... for example: Dim[Name = Expression]
If a = Dim[qwe = 1 + 2] Then
'qwe = 3
End If
'...
a = 3
If Dim[qwe = a = 1 + 2] Then
'qwe = True
End If
Comments
