Empty field in Access

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  • Start date Start date
G

Guest

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.InvalidCastException' occurred in microsoft.visualbasic.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
 
Joachim said:
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.InvalidCastException' occurred
in microsoft.visualbasic.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 ?
Click to expand...


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").ToString
end if

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

If dr_Product("AnyIntegerField") Is DBNull.Value Then
msgbox "Null found"
else
dim FieldContent as Integer
FieldContent = Directcast(dr_Product("bt_m3event"), Integer)
end if


--
Armin

How to quote and why:
http://www.plig.net/nnq/nquote.html
http://www.netmeister.org/news/learn2quote.html
 
* "=?Utf-8?B?Sm9hY2hpbQ==?= said:
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
Click to expand...

'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.
 
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.
 
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