Empty field in Access

[Joachim]

Hi

I am a beginner in VB.NET, and have a problem with empty field in Access

I have transfered a worksheet in Excel to Access table. Some of the cels are empty
I use VB.NET program to acces this Access table (product.mdb)

One of the statements is to validate whether the field empty or not
My statement is
If Trim(dr_Product ("bt_m3event ")) = "" Or Trim(dr_Product ("bt_m3event ")) = " " The
........
End I

dr_Product ==> is OleDbDataReade
bt_m3event ==>is the field nam

The bt_m3event is empty
When this statement is processed, I got a message like below

An unhandled exception of type 'System.Invalid CastException' occurred in microsoft.visua lbasic.dl
Additional information: Cast from type 'DBNull' to type 'String' is not valid

I have tried so many times and many ways, but it didn't work
Can anybody tell me how to validate an empty field

Thanks in advance

Joachim

[Armin Zingler]

Re: Empty field in Access

"Joachim" <anonymous@disc ussions.microso ft.com> schrieb[color=blue]
> Hi,
>
> I am a beginner in VB.NET, and have a problem with empty field in
> Access.
>
> I have transfered a worksheet in Excel to Access table. Some of the
> cels are empty. I use VB.NET program to acces this Access table
> (product.mdb).
>
> One of the statements is to validate whether the field empty or
> not. My statement is :
> If Trim(dr_Product ("bt_m3event ")) = "" Or
> Trim(dr_Product ("bt_m3event ")) = " " Then
> .........
> End If
>
> dr_Product ==> is OleDbDataReader
> bt_m3event ==>is the field name
>
> The bt_m3event is empty.
> When this statement is processed, I got a message like below :
>
> An unhandled exception of type 'System.Invalid CastException' occurred
> in microsoft.visua lbasic.dll Additional information: Cast from type
> 'DBNull' to type 'String' is not valid.
>
> I have tried so many times and many ways, but it didn't work.
> Can anybody tell me how to validate an empty field ?[/color]


First, you should enable Option Strict. Your code can not be compiled here
because the type of the DataReader's Item property is Object. You can not
compare every object to a string and you can't pass it to the Trim function.

As the exception message says, the field contains a Null value. The only
instance of Null in the Framework is DBNull.Value:

If dr_Product("bt_ m3event") Is DBNull.Value Then
msgbox "Null found"
else
dim FieldContent as string
FieldContent = dr_Product("bt_ m3event").ToStr ing
end if

For all other date types, you must use type casting:

If dr_Product("Any IntegerField") Is DBNull.Value Then
msgbox "Null found"
else
dim FieldContent as Integer
FieldContent = Directcast(dr_P roduct("bt_m3ev ent"), Integer)
end if


--
Armin

How to quote and why:

http://www.plig.net/nnq/nquote.html

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[Herfried K. Wagner [MVP]]

Re: Empty field in Access

* "=?Utf-8?B?Sm9hY2hpbQ= =?=" <anonymous@disc ussions.microso ft.com> scripsit:[color=blue]
> I have transfered a worksheet in Excel to Access table. Some of the cels are empty.
> I use VB.NET program to acces this Access table (product.mdb).
>
> One of the statements is to validate whether the field empty or not.
> My statement is :
> If Trim(dr_Product ("bt_m3event ")) = "" Or Trim(dr_Product ("bt_m3event ")) = " " Then[/color]

'If dr_Product("bt_ m3event") Is DBNull.Value Then...'. The right part of
the 'Or' IMO doesn't make sense at all because 'Trim' will remove the spaces.

--
Herfried K. Wagner [MVP]
<URL:http://dotnet.mvps.org/>

[Joachim]

RE: Empty field in Access

Armin and Herfried
Thanks to you. It works
But I have another problem. I have posted it in the other thread
I think you can help me out.