How do you handle nullable strings

[tshad]

Can you handle nullable string vb.net 2.0?

You can with Integer but I get an error when I try to do a

Dim s as Nullable (Of String)

Now strings are a nullable type but it can't handle:

fn.FormName = dbReader("FormN ame")

You get an error saying:

Unable to cast object of type 'System.DBNull' to type 'System.String' .

So do I still need to test for null and set to "" or null if this is the
case?

Thanks,

Tom

[Herfried K. Wagner [MVP]]

Re: How do you handle nullable strings

"tshad" <tshad@dslextre me.comschrieb:

Can you handle nullable string vb.net 2.0?
>
You can with Integer but I get an error when I try to do a
>
Dim s as Nullable (Of String)
>
Now strings are a nullable type but it can't handle:
>
fn.FormName = dbReader("FormN ame")
>
You get an error saying:
>
Unable to cast object of type 'System.DBNull' to type 'System.String' .

\\\
If dbReader(...) Is DBNull.Value Then
...
Else
... = DirectCast(dbRe ader(...), String)
End If
///

--
M S Herfried K. Wagner
M V P <URL:http://dotnet.mvps.org/>
V B <URL:http://dotnet.mvps.org/dotnet/faqs/>

[Cor Ligthert[MVP]]

Re: How do you handle nullable strings

Tshad,

A non initialized value in memory has not much to do with an empty field in
a database.

DBNull tells that there is actual nothing in the DataBase field,

Nothing tells that there is no refererence (and therefore as well with a
strings)

Null tells that a value field is not even been initialized to its default
(which is the standard behaviour of VB).

Nothing when initialized at values (including strings) tells that it is the
default value.

Cor

[Andrew Morton]

Re: How do you handle nullable strings

tshad wrote:

Can you handle nullable string vb.net 2.0?
>
You can with Integer but I get an error when I try to do a
>
Dim s as Nullable (Of String)
>
Now strings are a nullable type but it can't handle:
>
fn.FormName = dbReader("FormN ame")
>
You get an error saying:
>
Unable to cast object of type 'System.DBNull' to type 'System.String' .
>
So do I still need to test for null and set to "" or null if this is
the case?

Side-stepping the problem, in the SQL query you could use
coalesce(dbFiel dName, '') to get the DB to convert nulls to empty strings.

Andrew

[rowe_newsgroups]

Re: How do you handle nullable strings

On Feb 19, 5:24 pm, "tshad" <ts...@dslextre me.comwrote:

Can you handle nullable string vb.net 2.0?
>
You can with Integer but I get an error when I try to do a
>
Dim s as Nullable (Of String)
>
Now strings are a nullable type but it can't handle:
>
fn.FormName = dbReader("FormN ame")
>
You get an error saying:
>
Unable to cast object of type 'System.DBNull' to type 'System.String' .
>
So do I still need to test for null and set to "" or null if this is the
case?
>
Thanks,
>
Tom

IIRC you can use .ToString() when returning and indexed column from a
datareader.

i.e.

///////////////
fn.FormName = dbReader("FormN ame").ToString( )
//////////////

should convert that DbNull into a blank string.

Thanks,

Seth Rowe [MVP]

[Scott M.]

Re: How do you handle nullable strings

IIRC you can use .ToString() when returning and indexed column from a

datareader.
>
i.e.
>
///////////////
fn.FormName = dbReader("FormN ame").ToString( )
//////////////
>
should convert that DbNull into a blank string.
>
Thanks,
>
Seth Rowe [MVP]

I don't believe that works.

[rowe_newsgroups]

Re: How do you handle nullable strings

On Feb 20, 11:07 am, "Scott M." <s...@nospam.no spamwrote:

IIRC you can use .ToString() when returning and indexed column from a
datareader.

>

i.e.

>

///////////////
fn.FormName = dbReader("FormN ame").ToString( )
//////////////

>

should convert that DbNull into a blank string.

>

Thanks,

>

Seth Rowe [MVP]

>
I don't believe that works.

That is odd, since that's what I use in my production code.

Thanks,

Seth Rowe [MVP]

[Steve Gerrard]

Re: How do you handle nullable strings

rowe_newsgroups wrote:

On Feb 20, 11:07 am, "Scott M." <s...@nospam.no spamwrote:

>>IIRC you can use .ToString() when returning and indexed column from
>>a datareader.

>>

>>i.e.

>>

>>///////////////
>>fn.FormName = dbReader("FormN ame").ToString( )
>>//////////////

>>

>>should convert that DbNull into a blank string.

>>

>>Thanks,

>>

>>Seth Rowe [MVP]

>>
>I don't believe that works.

>
That is odd, since that's what I use in my production code.
>

Sure it works.

System.DBNull.V alue.ToString

returns a zero length string.

ToString is not a type converter, it always returns a string - "A String that
represents the current Object"