Cast from type 'DBNull' to type 'String' is not valid. #2

i have an login page which use e-mail address and password to login , once the system comfirm this is valid login it get customer details

and in my login page i have the following code

Dim loginsystem As onlineshop.Customers = New onlineshop.Customers()
Dim customerId As String = loginsystem.customer_login(login.email_address, login.password)

If customerId <> "" Then
Dim customerDetails As onlineshop.customer_Detail = loginsystem.Get_Customer_Details(customerid)

and at the onlineshop.customer_detail function i have the following

Public Function Get_customer_details(ByVal CUSTOMERID As string) As Customer_detail

and i get the error message:Cast from type 'DBNull' to type 'String' is not valid.
i turn the debug and trace show stop compile with following code in the login page

Dim customerDetails As onlineshop.customer_Detail = loginsystem.Get_Customer_Details(customerId)

what cause this problem ??? it is because it can not convert the small customerid in login page to the big CUSTOMERID in the get_customer_details function or other reasons?????
0
vito_huang
3/18/2005 11:22:26 PM
asp.net.getting-started 91979 articles. 4 followers. Follow

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try this:


Dim customerDetails As onlineshop.customer_Detail = loginsystem.Get_Customer_Details(Convert.ToString(customerId))
0
adwins04
3/19/2005 12:16:52 AM
still not working  

i don't under stand why do i have to convert to string since i already dim customerid as string before.
0
vito_huang
3/19/2005 11:22:45 AM
There is coming null data (DBNull type in .NET represents this) somewhere in that function where you get the error, and it is coming as a result of SQL query (in the resultset), which is tried to convert to string but it can't be done. That's why you get the exception.

You can manage this by using Convert.IsDBNull to check for null value (returns a boolean). If it is null, use String.Empty as a default value, instead of trying converting to string in that case.
Thanks,

Teemu Keiski
Finland, EU
0
joteke
3/19/2005 11:29:30 AM
sit works now, i used convert.lsdbnull to check the value before i convert it to string,thanks for the tip

i interesting in how did you determine this error is in the coming null data from database as a result of sql query???? it that because the DBnull data type??or other factors????
0
vito_huang
3/19/2005 3:19:29 PM
Hello,

yes, that's the place where one would see DBNulls. They won't appear elsewhere and the error you got is very, very common in this situation.
Thanks,

Teemu Keiski
Finland, EU
0
joteke
3/20/2005 5:35:31 AM
Reply:

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