VB Net IsDBNull issue

Question

I have an Issue with the IsDBNull method. This is my code:

   Dim a as Bool=  IIf(IsDBNull(reader("field")), _
                       False, _
                       Convert.ToBoolean(reader("field")))

The code above should return False if the column is false but I get the exception:

Impossible to cast DBNull to other types.

The Convert.ToBoolean fails

possible duplicate of VB.NET: How do I use coalesce with db column values and nullable types? Or is there a better solution? — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Dec 11, 2014 at 10:48
Disagree on that Q as a duplicate: that question is about casting nothing (null) to the right kind of null; this is about order of evaluation. — Richard
– Richard, Commented Dec 12, 2014 at 7:17

Richard · Accepted Answer · 2014-12-11 10:44:22Z

2

All arguments of Iif are evaluated when called, whether they'll be used or not.

In this respect it is not equivalent to C#'s conditional operator (which only evaluates the boolean control sub-expression and one of the other two sub-exptressions).

You need to use VB.Net's If operator which does do lazy evaluation.

answered Dec 11, 2014 at 10:44

Richard

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Anil · Accepted Answer · 2014-12-11 10:45:23Z

1

If(IsDBNull(reader("field"))) then 
return False

else 

Convert.ToBoolean(reader("field")))

end if

IIF will evaluate failed part also.

answered Dec 11, 2014 at 10:45

Anil

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