NULL Value - Specified cast is not valid

[fniles]

I am accessing MS Access db with tblA whose column UNABLE can have NULL
value. When I access UNABLE whose value is null, I got an error "
Run-time exception thrown : System.InvalidC astException - Specified cast is
not valid." either when I do
IsDBNull(dr.Get String(0))
or
dr.GetString(0)
How can I fix this problem ?

Thanks.

Dim sUnable as string
Dim sql As String
Dim dr As OleDb.OleDbData Reader
Dim cmd As New OleDb.OleDbComm and

sql = "select UNABLE from tblA where ACCOUNT = 'xyz'"
With cmd
.Connection = g_ConnectionDem oOLE
.CommandText = sql
dr = .ExecuteReader( )
End With
Do While dr.Read
If bErr Then
sUnable = ""
Else
If IsDBNull(dr.Get String(0)) Then sUnable = "" Else sUnable =
dr.GetString(0) --> ERROR HERE
End If
If bErr Then sUnable = ""
Loop

[Jim Wooley]

> I am accessing MS Access db with tblA whose column UNABLE can have

NULL
value. When I access UNABLE whose value is null, I got an error "
Run-time exception thrown : System.InvalidC astException - Specified
cast is
not valid." either when I do
IsDBNull(dr.Get String(0))
or
dr.GetString(0)
How can I fix this problem ?
Thanks.

You need to check the IsDBNull on the column, not the result of the column.
Thus you should change it to:

If dr.IsDBNull(0) then MyVal = dr.GetSTring(0)

Jim Wooley
http://devauthority.com/blogs/jwooley/default.aspx

[Göran Andersson]

That is because you are calling the GetString method before checking if
the value is null. Use the IsDBNull method:

If dr.IsDBNull(0) Then sUnable = String.Empty Else sUnable = dr.GetString(0)

fniles wrote:

I am accessing MS Access db with tblA whose column UNABLE can have NULL
value. When I access UNABLE whose value is null, I got an error "
Run-time exception thrown : System.InvalidC astException - Specified cast is
not valid." either when I do
IsDBNull(dr.Get String(0))
or
dr.GetString(0)
How can I fix this problem ?

Thanks.

Dim sUnable as string
Dim sql As String
Dim dr As OleDb.OleDbData Reader
Dim cmd As New OleDb.OleDbComm and

sql = "select UNABLE from tblA where ACCOUNT = 'xyz'"
With cmd
.Connection = g_ConnectionDem oOLE
.CommandText = sql
dr = .ExecuteReader( )
End With
Do While dr.Read
If bErr Then
sUnable = ""
Else
If IsDBNull(dr.Get String(0)) Then sUnable = "" Else sUnable =
dr.GetString(0) --> ERROR HERE
End If
If bErr Then sUnable = ""
Loop

[Jay B. Harlow [MVP - Outlook]]

fniles,
As the other suggest I normally use dr.IsDBNull(0) also.

However the "problem" you are having is that dr.GetString is attempting to
return a String, when you have a Null. You then attempt to pass this String
to the IsDBNull function. You need to pass an Object to the IsDBNull
function, you can use need to use dr.GetValue to retrieve this object then
use the IsDBNull function:

If IsDBNull(dr.Get Value(0)) Then

However as I stated, I simply use dr.IsDBNull instead...

| If dr.IsDBNull(0) Then sUnable = "" Else sUnable =
| dr.GetString(0)

As it avoids "getting" the value twice...

--
Hope this helps
Jay B. Harlow [MVP - Outlook]
..NET Application Architect, Enthusiast, & Evangelist
T.S. Bradley - http://www.tsbradley.net
"fniles" <fn****@pfmail. com> wrote in message
news:%2******** ********@TK2MSF TNGP04.phx.gbl. ..
|I am accessing MS Access db with tblA whose column UNABLE can have NULL
| value. When I access UNABLE whose value is null, I got an error "
| Run-time exception thrown : System.InvalidC astException - Specified cast
is
| not valid." either when I do
| IsDBNull(dr.Get String(0))
| or
| dr.GetString(0)
| How can I fix this problem ?
|
| Thanks.
|
| Dim sUnable as string
| Dim sql As String
| Dim dr As OleDb.OleDbData Reader
| Dim cmd As New OleDb.OleDbComm and
|
| sql = "select UNABLE from tblA where ACCOUNT = 'xyz'"
| With cmd
| .Connection = g_ConnectionDem oOLE
| .CommandText = sql
| dr = .ExecuteReader( )
| End With
| Do While dr.Read
| If bErr Then
| sUnable = ""
| Else
| If IsDBNull(dr.Get String(0)) Then sUnable = "" Else sUnable =
| dr.GetString(0) --> ERROR HERE
| End If
| If bErr Then sUnable = ""
| Loop
|
|
|
|