LINQ to Entities 不识别方法“System.String ToString()”，因此该方法无法转换为存储表达式。

var data = DataSource.Skip(iDisplayStart).Take(iDisplayLength).Select(o => new
{
MatNR = o.MatNR,
MatDB = o.MatDB,
 CreatedOn=o.CreatedOn.ToString(),
CreatedBy = o.CreatedBy,
Id2 = o.MatNR
});

以上语句就会出现这个提示：LINQ to Entities 不识别方法“System.String ToString()”，因此该方法无法转换为存储表达式。

解决方法有两种

一、使用SqlFunctions

1、

CreatedOn = SqlFunctions.DateName("year", o.CreatedOn) + "-"
+ SqlFunctions.DateName("month", o.CreatedOn) +"-"
+ SqlFunctions.DateName("day",o.CreatedOn),

2、

CreatedOn= SqlFunctions.DateName("yyyy", o.CreatedOn)
+"-"+
SqlFunctions.StringConvert((decimal)SqlFunctions.DatePart("mm", o.CreatedOn))
+"-"+
SqlFunctions.DateName("dd",o.CreatedOn),

3、

CreatedOn = SqlFunctions.DateName("yyyy", o.CreatedOn)
+ "-" +
SqlFunctions.StringConvert((decimal)SqlFunctions.DatePart("mm", o.CreatedOn)).Trim()
+ "-" +
SqlFunctions.DateName("dd", o.CreatedOn),

注意：月份只有1位数！还不够完美。

二、Linq-to-entities 转换到Linq-to-objects

//Do it in two steps - one L2E query that does the L2E supported part, and a L2O query that does the date formatting and other stuff that is best done in .net code...

////Linq-to-entities query to get the fullname, categoryname, and date

//var query = from c in db.Contacts select new { c.FullName, c.Category.CategoryName, c.DateCreated };

////Linq-to-objects query (.AsEnumerable will separate the L2E from L2O part) that call date formatting methods and other stuff that isn't supported by L2E

//var r = from c in query.AsEnumerable() select new { c.FullName, c.CategoryName, ShortDate = c.DateCreated.ToShortDateString() };

var data = DataSource.Skip(iDisplayStart).Take(iDisplayLength).Select(o => new
{
MatNR = o.MatNR,
MatDB = o.MatDB,
CreatedOn=o.CreatedOn,
CreatedBy = o.CreatedBy,
Id2 = o.MatNR
});

var a = from c in data.AsEnumerable()
select new
{
MatNR = c.MatNR,
MatDB = c.MatDB,
//CreatedOn = Convert.ToDateTime(c.CreatedOn).ToShortDateString(),// 2014/2/28
//CreatedOn = c.CreatedOn.ToString(),//2014/2/28 15:09:40
CreatedOn =c.CreatedOn.ToString("yyyy-MM-dd"),//如果数据库里的日期字段可以为null，则CreatedOn.ToString("yyyy-MM-dd")这种方式是不可以的，如果不为null则可以；

CreatedBy = c.CreatedBy,
Id2 = c.MatNR
};