学步园

问题:因为每次更新的时候只是某个类的一部分,但是这个类的属性比较多.
更新函数如下

 

static void updateRe(log n)
        {
            using (DataClasses1DataContext dc = new DataClasses1DataContext())
            {
                using (StreamWriter sw=new StreamWriter("t.log"))
                {
                    dc.Log = sw;
                    dc.log.Attach(n);
                    dc.Refresh(RefreshMode.KeepCurrentValues,n);

                    dc.SubmitChanges();
                }
            }
        }

 

            updateRe(n1);
        }

static void Main(string[] args)
        {
            log n1 = new log() ;
            n1.logId = 1;
            n1.logMessage = "xxxx";

 

 

这时候产生的sql如下
SELECT [t0].[logId], [t0].[logMessage], [t0].[x]
FROM [dbo].[log] AS [t0]
WHERE [t0].[logId] = @p0
-- @p0: Input Int (Size = 0; Prec = 0; Scale = 0) [1]

UPDATE [dbo].[log]
SET [logMessage] = @p1, [x] = @p2
WHERE [logId] = @p0
-- @p0: Input Int (Size = 0; Prec = 0; Scale = 0) [1]
-- @p1: Input VarChar (Size = 4; Prec = 0; Scale = 0) [xxxx]
-- @p2: Input NChar (Size = 10; Prec = 0; Scale = 0) [Null]

 

问题就是这样做会将你没有符过值的都更新为Null,我继续做实验,将main函数改为如下

 

 

 

static void Main(string[] args)
        {
            DataClasses1DataContext dc = new DataClasses1DataContext();
            log n1 = (from x in dc.log
                      select x).SingleOrDefault(c => c.logId == 1);
            n1.logMessage = "xxxy";

            updateRe(n1);
        }

 

会引发"已尝试 Attach 或 Add 实体，该实体不是新实体，可能是从其他 DataContext 中加载来的。不支持这种操作。"异常,没找到把n1从它的DataContext脱离的办法.所以我使用如下的解决方案:
在log的部分类中书写克隆方法:

 

public partial class log
    {
        public log Clone()
        {
            log l = new log();
            l.logId = this.logId;
            l.logMessage = this.logMessage;
            l.x = this.x;
            return l;
        }
   }
static void Main(string[] args)
        {
            DataClasses1DataContext dc = new DataClasses1DataContext();
            log n1 = ((from x in dc.log
                      select x).SingleOrDefault(c => c.logId == 1)).Clone();
            n1.logMessage = "xxxy";

            updateRe(n1);
}

 

 

生成的sql语句如下
SELECT [t0].[logId], [t0].[logMessage], [t0].[x]
FROM [dbo].[log] AS [t0]
WHERE [t0].[logId] = @p0
-- @p0: Input Int (Size = 0; Prec = 0; Scale = 0) [1]

UPDATE [dbo].[log]
SET [logMessage] = @p1
WHERE [logId] = @p0
-- @p0: Input Int (Size = 0; Prec = 0; Scale = 0) [1]
-- @p1: Input VarChar (Size = 4; Prec = 0; Scale = 0) [xxxy]

感觉这个Clone()方法也不是好的解决方案,大家有什么好的想法么?

 ps:

可以使用反射代码自动拷贝dbml中的相应属性

class Utility
    {
        public static T Clone<T>(T source) where T: new() 
        {
            T t = new T();
            var ps = source.GetType().GetProperties().Where(p => p.GetCustomAttributes(false)
               .Where(a => a is System.Data.Linq.Mapping.ColumnAttribute).Count() != 0);
            foreach (var item in ps)
            {
                t.GetType().GetProperty(item.Name).SetValue(t, item.GetValue(source, null), null);
            }
            return t;
        }
    }

 

网友：@Gray Zhang 的这个方法我觉得不错，整理如下：

 

static void Update(int id, Action<log> updater)
        {
            using (DataClasses1DataContext ctx = new DataClasses1DataContext())
            {
                ctx.Log = Console.Out;
                log entity = ctx.log.First(c => c.logId == id);
                //执行updater
                updater(entity);
                ctx.SubmitChanges();
            }
        }
        public static void set(log l)
        {
            l.logMessage = "xxtx";
        }
        static void Main(string[] args)
        {
            Update(1, set);
        }