Unable to cast object of type 'System.DBNull' to type 'System.String`
You can use C#'s null coalescing operator
return accountNumber ?? string.Empty;
With a simple generic function you can make this very easy. Just do this:
return ConvertFromDBVal<string>(accountNumber);
using the function:
public static T ConvertFromDBVal<T>(object obj)
{
if (obj == null || obj == DBNull.Value)
{
return default(T); // returns the default value for the type
}
else
{
return (T)obj;
}
}
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()
ExecuteScalar will return
- null if there is no result set
- otherwise the first column of the first row of the resultset, which may be DBNull.
If you know that the first column of the resultset is a string, then to cover all bases you need to check for both null and DBNull. Something like:
object accountNumber = ...ExecuteScalar(...);
return (accountNumber == null) ? String.Empty : accountNumber.ToString();
The above code relies on the fact that DBNull.ToString returns an empty string.
If accountNumber was another type (say integer), then you'd need to be more explicit:
object accountNumber = ...ExecuteScalar(...);
return (accountNumber == null || Convert.IsDBNull(accountNumber) ?
(int) accountNumber : 0;
If you know for sure that your resultset will always have at least one row (e.g. SELECT COUNT(*)...), then you can skip the check for null.
In your case the error message "Unable to cast object of type ‘System.DBNull’ to type ‘System.String`" indicates that the first column of your result set is a DBNUll value. This is from the cast to string on the first line:
string accountNumber = (string) ... ExecuteScalar(...);
Marc_s's comment that you don't need to check for DBNull.Value is wrong.