“Conversion from type 'DBNull' to type 'Boolean' is not valid”, after checking that it's not DBNull
-
17-04-2021 - |
문제
In my ASP.Net Web-Application, I'm getting this error:
Conversion from type 'DBNull' to type 'Boolean' is not valid.
From this function:
Namespace atc
Public Class Nil
'...
Public Shared Function Bool(ByVal Item As Object) As Boolean
Return IIf(Item IsNot Nothing AndAlso Not IsDBNull(Item), CBool(Item), False)
End Function
'...
End Class
End Namespace
As you can see, I'm explicitly checking if Item
is DBNull
, and if it is then I return False
.
The error does not occur when Item
is not DBNull
, so I don't understand why this is happening.
해결책
When using IIf
then all arguments get evaluated, no matter if the condition evaluates to true or false. In your case the function will return false
if Item is null or DBNull, but CBool(Item)
will be silently executed in the background anyway and therefore throws an exception.
In VB.NET 2008 the If
keyword was added to provide a real ternary operator. Replace your IIf
function call with the following:
Public Shared Function Bool(ByVal Item As Object) As Boolean
Return If(Item IsNot Nothing AndAlso Not IsDBNull(Item), CBool(Item), False)
End Function
Excerpt from MSDN:
An
IIf
function always evaluates all three of its arguments, whereas anIf
operator that has three arguments evaluates only two of them.