DataRow的泛型扩展方法

最近发现以前写的下面两个方法都有错误，default(string) 是null ，null is string 返回false

 static class Extend

    {
        const string def = "dd";
        public static T GetValue<T>(this DataRow dr, string name)
        {
            try
            {
                if (dr[name] == DBNull.Value)
                {
                    object o = def;
                    return (T)o;
                }
                else
                {
                    return (T)dr[name];
                }
            }
            catch (ArgumentException ex)
            {
                T t = default(T);
                object obj;
                if (t is ValueType)
                {
                    obj = t;
                }
                else if (t is string)
                {
                    obj = def;
                }
                else
                {
                    obj = t;
                }
                return (T)obj;
            }

            //if (dr[name] == DBNull.Value)
            //{
            //    T item = default(T);
            //    object obj;
            //    if (item is ValueType)
            //    {
            //        obj = 0;

            //    }
            //    else
            //    {
            //        obj = "";
            //    }
            //    return (T)obj;
            //}
            //else
            //{
            //    T res = (T)dr[name];
            //    return res;
            //}
        }

    }

public static class MyExtend
    {

        public static T GetValue<T>(this DataRow dr, string name)
        {
            try
            {
                if (dr[name] == DBNull.Value)
                {                    
                    return GetDefault<T>();
                }
                else
                {
                    return (T)dr[name];
                }
            }
            catch (ArgumentException ex)
            {
                return GetDefault<T>();
            }
        }

        static T GetDefault<T>()
        {
            T t = default(T);
            object obj;
            if (t is string)
            {
                obj = "";
            }
            else
            {
                obj = t;
            }
            return (T)obj;
        }
    }