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

Question

I got the above error in my app. Here is the original code

public string GetCustomerNumber(Guid id)
{
     string accountNumber = 
          (string)DBSqlHel         


        

Answer 1

Convert it Like

string s = System.DBNull.value.ToString();

Answer 2

A shorter form can be used:

return (accountNumber == DBNull.Value) ? string.Empty : accountNumber.ToString()

EDIT: Haven't paid attention to ExecuteScalar. It does really return null if the field is absent in the return result. So use instead:

return (accountNumber == null) ? string.Empty : accountNumber.ToString() 

Answer 3

Since I got an instance which isn't null and if I compared to DBNULL I got Operator '==' cannot be applied to operands of type 'string' and 'system.dbnull' exeption, and if I tried to change to compare to NULL, it simply didn't work ( since DBNull is an object) even that's the accepted answer.

I decided to simply use the 'is' keyword. So the result is very readable:

data = (item is DBNull) ? String.Empty : item

Answer 4

There is another way to workaround this issue. How about modify your store procedure? by using ISNULL(your field, "") sql function , you can return empty string if the return value is null.

Then you have your clean code as original version.

Answer 5

I suppose you can do it like this:

string accountNumber = DBSqlHelperFactory.ExecuteScalar(...) as string;

If accountNumber is null it means it was DBNull not string :)

Answer 6

You can use C#'s null coalescing operator

return accountNumber ?? string.Empty;