How do I handle Conversion from type 'DBNull' to type 'String' is not valid

Question

Hi Guys,

I need some expect advice on how to handle the following:- I have a data field misc_text_2 that is of type varchar(25) and allows NULL. Now if I use the following syntax

<asp:Label ID="lblPrinter" runat="server"  Text='<%# iif(eval("misc_text_2") is dbnull.value, "", iif(eval("misc_text_2") like "NA", "None", iif(eval("misc_text_2") like "KP1", "Kitchen Printer 1", iif(eval("misc_text_2") like "KP2", "Kitchen Printer 2", iif(eval("misc_text_2") like "KP3", "Kitchen Printer 3", iif(eval("misc_text_2") like "BP1", "Bar Printer 1", iif(eval("misc_text_2") like "BP2", "Bar Printer 2", iif(eval("misc_text_2") like "BP3", "Bar Printer 3", Eval("misc_text_2")))))))))%>'></asp:Label>

I keep on getting an error Exception Details: System.InvalidCastException: Conversion from type 'DBNull' to type 'String' is not valid.

I know I'm missing something, but what...

Thanks in advance

Answer 1

You could in your sql query use isNull(misc_text_2, '') to return en empty string instead of DBNull.

Answer 2

You must explicitly check for DBNull.Value and do the conversion yourself.

In other words, build a method that will do the conversion for you, taking DBNull.Value into account.

Answer 3

Not answering your question, but: You should really create a code behind method that does the conversion. That will make the code easier to understand and debug, and will make it possible to reuse the code.

Answer 4

Since we have a legacy database that was set up for MS-Dynamics (Solomon), our method of handling nulls is to convert them to null strings either in the ASP or VB.NET code. i.e.

Trim$(misc_text_2 & " ")

Gets rid of the problem for any version of VB.