wtf?

[jason]

hello everyone. can anyone tell me why this chunk of code:

if (Convert.IsDBNull(oCommand.Parameters["@userid"].Value))
{
p_nErrorCode = 10;
p_sErrorMessage = "Login failed, no matching credentials found";
goto Failed;
}
sReturn = Convert.ToString(oCommand.Parameters["@userid"].Value);

generates the following runtime error:

System.InvalidCastException: Object cannot be cast from DBNull to other
types

this seems impossible. no ErrorCode or ErrorMessage is recorded, which
would suggest that the if condition is false, and the value is not
DBNull. further evidence being that if i comment out the last line
(sReturn = Convert...) then the code generates no runtime exception.
that further suggests the if condition is false, and the value is not
DBNull. so how on earth can the last line generate the DBNull cast
exception? either it can't be NOT DBNull and then suddenly DBNull.

clearly i'm missing something. do you guys see it? do you need to see
more code?

thanks for any help,

jason

(in case anyone is wondering, this is all Failed does:)

Failed:
if (oCommand != null)
oCommand.Dispose();
if (oConn != null)
{
oConn.Close();
oConn.Dispose();
}

(and that's the end of the function. no more tampering with the
ErrorCode or ErrorMessage, or anything that would generate a DBNull
casting exception)

[Jon Skeet [C# MVP]]

jason <ia****@yahoo.com> wrote:

hello everyone. can anyone tell me why this chunk of code:

if (Convert.IsDBNull(oCommand.Parameters["@userid"].Value))
{
p_nErrorCode = 10;
p_sErrorMessage = "Login failed, no matching credentials found";
goto Failed;
}
sReturn = Convert.ToString(oCommand.Parameters["@userid"].Value);

generates the following runtime error:

System.InvalidCastException: Object cannot be cast from DBNull to other
types

this seems impossible. no ErrorCode or ErrorMessage is recorded, which
would suggest that the if condition is false, and the value is not
DBNull. further evidence being that if i comment out the last line
(sReturn = Convert...) then the code generates no runtime exception.
that further suggests the if condition is false, and the value is not
DBNull. so how on earth can the last line generate the DBNull cast
exception? either it can't be NOT DBNull and then suddenly DBNull.

clearly i'm missing something. do you guys see it? do you need to see
more code?

Well, a short but complete example would help - see
http://www.pobox.com/~skeet/csharp/complete.html for details of
what I mean by that.

Note that using "goto" is very rarely a good idea in C# - likewise
error codes and manual disposal. Exceptions usually take care of the
first two (in error cases, at least) and the "using" statement makes
the latter easier to write correctly.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

[jason]

well, a complete working example would be rather huge, having to
include the whole function, which calls many other functions. i would
have modified the example to attain completeness, as i've done in my
many other postings, except that this problem is specific to this code
context. i'm doing the same kind of test in a dozen other places, and
it works just fine. in fact -- THIS code works just fine when called
from classic ASP. it seems to only choking when called from ASP.NET,
which is equally baffling. so in short, modifying it for completeness
would likely remove whatever peculiarity about the context is causing
the error. my hope is that someone would recognize the error in the
codes present, unaltered shape, as is quite often the possible.

on that note, another synonymous shape the code can take and still
produce the error:

if (!Convert.IsDBNull(oCommand.Parameters["@userid"].Value))
sReturn = Convert.ToString(oCommand.Parameters["@userid"].Value);

this reports the DBNull cast error on the sReturn = ... line. how on
earth this is possible baffles me, simply looking at these two lines
alone!

regarding your other notes: so far as i know, .NET exceptions can't be
gracefully handled by ASP classic code. if they can, do let me know
where to learn more. otherwise, we have to manually code errors so that
ASP classic applications which consume this class library can handle
errors correctly. i'll take a look at the using statement to see if i
grock how that might improve my disposal mechanisms.

thanks for the comment,

jason