cast problem

vickyj · September 6th, 2004, 03:55 AM

I write following code

dim chk as string
Dim maxid as integer

cmdmax = New SqlCommand("select max(Reply_ID) from Reply where Q_ID='" & Request("Topic") & "' and Tech_ID='" & Request("Tech") & "' ", conPubs)

        chk = cmdmax.ExecuteScalar
        If (chk = "NULL") Then
            maxid = 0
        Else
            maxid = CInt("chk")
        End If
        maxid = maxid + 1

It gives the following error
"Cast from type 'DBNull' to type 'String' is not valid."
What is the problem.

vicky

Jeff Mason · September 6th, 2004, 09:40 AM

There are several things wrong with your code.

If no rows meet your selection criteria when the query executes, the database will return a NULL value. This is not the string "NULL", but rather a special (object) value represented by the constant system.dbnull.value.

The ExecuteScalar method returns a value of type object which you must cast to the appropriate type.

The CInt() function converts its argument to an integer. You are attempting to convert the string "chk" to an integer, which won't work and will generate an invalid cast exception.

Jeff Mason
Custom Apps, Inc.
www.custom-apps.com

vickyj · September 8th, 2004, 12:08 AM

Now what is the solution for this problem.

vicky

Santhi · September 8th, 2004, 01:03 AM

Typecast the return value of ExecuteScalar method to the appropriate type

vickyj · September 8th, 2004, 05:32 AM

But how to Typecast the returned value,is there any function for this.
pls solve this with a ex.

vicky

Santhi · September 8th, 2004, 06:16 AM

chk is string right so use CStr(cmdmax.ExecuteScalar)..
as u have done for integer conversion

Jeff Mason · September 8th, 2004, 06:21 AM

Quote:

quote:Originally posted by vickyj
Now what is the solution for this problem.

If you cannot work out the "solution" on your own with the information that was given, you are going to have a lot of trouble down the line, since it seems there are many fundamental things about programming in VB.NET you don't appear to understand. You really shouldn't have to be given the answer.

But, I don't want to be accused of being unhelpful, so:

Code:

dim chk as object
...
chk = cmdmax.executescalar
If chk = system.dbnull.value Then
    maxid = 0
Else
    maxid = CInt(chk)
EndIf
...

The 'typecast' function in VB.NET is either CTYPE or DIRECTCAST depending on the situation. DIRECTCAST doesn't really apply here, so the type conversion function CTYPE could be used in place of the CInt, (they're equivalent) as:

Code:

    maxid = CType(chk, Integer)

Alternatively, you could also use the IsDBNull function to test for the NULL value, as:

Code:

If IsDBNull(chk) Then
    maxid = 0
Else
    maxid = CInt(chk)
EndIf

Good luck. You are probably going to need it.

Jeff Mason
Custom Apps, Inc.
www.custom-apps.com