How do I correctly check DBNull in VB?

Question

Why does the following code:

  A = not IsDBNull(CurRow("BuyBook")) AndAlso CType(CurRow("BuyBook"), string) = "Yes"

results in the following error:

 Conversion from type 'DBNull' to type 'String' is not valid.

When AndAlso is supposed to short-circuit according to this article:

http://support.microsoft.com/kb/817250

Can you try this `A = (not IsDBNull(CurRow("BuyBook"))) AndAlso CType(CurRow("BuyBook"), string) = "Yes"` — Amar Palsapure, Jan 13 '12 at 07:17
I've even broken it into two statements and tried. I think IsDBNull is broken. — merlin2011, Jan 13 '12 at 07:25
Go through [this](http://stackoverflow.com/questions/221582/most-efficient-way-to-check-for-dbnull-and-then-assign-to-a-variable) post. — Amar Palsapure, Jan 13 '12 at 07:33
Is there any other code related to this operation. if yes then post it. — Harsh, Jan 13 '12 at 07:46
What type is CurRow by the way? anIDataReader/SqlDataReader? — Pondidum, Jan 13 '12 at 07:49
Try NOT (CurRow("BuyBook")) is System.Dbnull.Value, it should work — Harsh, Jan 13 '12 at 08:00

score 11 · Accepted Answer · 2012-01-13T08:21:51.607

You are correct. AndAlso is short circuiting.

However, the error comes by calling CurRow("GuyBook") (verify this in a debugger to make sure I'm not a liar or making some crazy assumptions or just misremembering* ;-). Before you ask for a value, you need to ask the DataRow if it has a value. That is, use:

CurRow.IsNull("BuyBook")

Happy coding.

*One should just be able to compare with DBNull.Value or use IsDBNull. However, I am fairly certain that I ran into a row before that threw these exceptions instead of returning a DBNull object. Start by finding out -- in the Immediate Window of the Debugger -- exactly which expression throws the exception.

Pondidum · Answer 2 · 2012-01-13T07:51:54.137

6

Have you tried comparing like this:

If CurRow("BuyBook") Is DBNull.Value Then
     '...
End If

edited Jan 13 '12 at 07:51

answered Jan 13 '12 at 07:40

Pondidum

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Yes, I get an exception saying that you can't compare an Object and DBNull type. – merlin2011 Jan 13 '12 at 07:41
Updated to changed the comparison done to match how its done in some of our code. – Pondidum Jan 13 '12 at 07:52

Harsh · Answer 3 · 2012-01-13T07:58:57.423

1

Instead of not IsDBNull(CurRow("BuyBook")), use NOT (CurRow("BuyBook")) is System.Dbnull.Value). Try this:

A = (NOT (CurRow("BuyBook")) is System.Dbnull.Value) AndAlso CType(CurRow("BuyBook"), string) = "Yes"

OR

A = not string.IsNullOrEmpty(CurRow("BuyBook")) AndAlso CType(CurRow("BuyBook"), string) = "Yes"

edited Jan 13 '12 at 07:58

answered Jan 13 '12 at 07:29

Harsh

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I guess you missed this `I've even broken it into two statements and tried`. – Amar Palsapure Jan 13 '12 at 07:35

score 0 · Answer 4 · edited Jan 13 '12 at 23:26

0

If dt.Rows(0)("BuyBook") = DBNull.Value Then
End If

Hope it helps.

edited Jan 13 '12 at 23:26

DaveShaw

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answered Jan 13 '12 at 08:01

Neha

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score -1 · Answer 5 · answered Jan 13 '12 at 07:59

-1

In vb.net I usually do something like this:

A = (CurRow("BuyBook") & "" = "Yes")

Old trick from vb6 era. If CurRow("BuyBook") is null, vb.net will consider it as empty string when concatenating it with another string.

answered Jan 13 '12 at 07:59

Endy Tjahjono

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Doesn't this rely upon Option Strict being off? I would prefer just using the ToString method directly `CurRow("BuyBook").ToString = "Yes"` if using this solution. – jmoreno May 24 '15 at 04:45

How do I correctly check DBNull in VB?

5 Answers5