MYSQL connection error through dotnet

DBNull is a special type: it represents an "empty value" in database nullable column. When you issue a SELECT request against the database any matching rows that contain an SQL NULL value is returned as an instance of the DBNull Class[^]

Although it represents a NULL value in the DB, it isn't the same as a null value in your coding language - you can't directly cast it to any other type.
I'd suggest writing a generic conversion method similar to this:

public static T ConvertDBValue<T>(object o)
    {
    if (o == null || o == DBNull.Value)
        {
        return default(T);
        }
    else
        {
        return (T)o;
        }
    }

And call that for each DB value you process.