数据库查询的时候报错，“string”必须是具有公共的无参数构造函数的非抽象类型，才能用作泛型类型或方法
代码:

public List<string> QueryAuthor(string strID)

        {

            string sql = "SELECT au_lname FROM authors WHERE au_id = @au_id";

 

            DbParameter param= dbUtility.CreateDbParameter("@au_id", strID);



            List<DbParameter> lstParam = new List<DbParameter>();



            lstParam.Add(param);



            List<string> result = new List<string>();



          result = dbUtility.QueryForList<string>(sql, lstParam, CommandType.Text);



            return result;

        }

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public List<T> QueryForList<T>(string sql, IList<DbParameter> parameters, CommandType commandType) where T : new()

        {

            DataTable data = ExecuteDataTable(sql, parameters, commandType);

            return EntityReader.GetEntities<T>(data);

        }

-------------------------------------------------------------------------------

public static List<T> GetEntities<T>(DataTable dataTable) where T : new()

        {

            if (dataTable == null)

            {

                throw new ArgumentNullException("dataTable");

            }

            //如果T的类型满足以下条件：字符串、ValueType或者是Nullable<ValueType>

            if (typeof(T) == typeof(string) || typeof(T) == typeof(byte[]) || typeof(T).IsValueType)

            {

                return GetSimpleEntities<T>(dataTable);

            }

            else

            {

                return GetComplexEntities<T>(dataTable);

            }