using System;

public class Program

{

    private static bool IsNullOrDBNull(object value)

    {

        return value == null || value == DBNull.Value;

    }

​

    public static string DBNullToString(object value, string defaultValue)

    {

        if (IsNullOrDBNull(value))

        {

            return defaultValue;

        }

        return (string)value; //this is the bug

        //return value.ToString(); //this is the fix

    }   

    public static void Main()

    {

        Console.WriteLine(DBNullToString(DateTime.Now, String.Empty));

    }

}