LINQ to SQL 更新中的默认值（C# 变量）问题答案

【问题标题】：Default Values (of C# variables) Issue in LINQ to SQL UpdateLINQ to SQL 更新中的默认值（C# 变量）问题
【发布时间】：2012-06-25 09:39:02
【问题描述】：

我有以下代码用于使用 LINQ to SQL 更新 Account 表。 AccountNumber 是主键列。唯一需要更新的值是 AccountType；但是 Duration 也会更新为零（int 的默认值）。我们如何避免这种不必要的覆盖？

注意：我使用的是附加方法

注意：我理解这种行为的原因。 “DataContext 无法区分分配值为零的字段和仅未分配的字段。”。我正在寻找解决方案来克服这个问题。

[global::System.Data.Linq.Mapping.ColumnAttribute(Storage="_Duration", DbType="Int NOT NULL"
public int Duration

表数据

表结构：

 CREATE TABLE [dbo].[Account](
[AccountNumber] [int] NOT NULL,
[AccountType] [nchar](10) NOT NULL,
[Duration] [int] NOT NULL,
[DepositedAmount] [int] NULL,
 CONSTRAINT [PK_Account] PRIMARY KEY CLUSTERED 
 (
[AccountNumber] ASC
 )WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
 ) ON [PRIMARY]

代码

    public void UpdateAccount()
    {
        RepositoryLayer.Account acc1 = new RepositoryLayer.Account();
        acc1.AccountNumber = 4;
        acc1.AccountType = "Verify";

        accountRepository.UpdateChangesByAttachAndRefresh(acc1);
        accountRepository.SubmitChanges();

    }


    public virtual void UpdateChangesByAttachAndRefresh(T entity)
    {

        //Can GetOriginalEntityState cause any bug? Is it unnecessary?           
        if (GetTable().GetOriginalEntityState(entity) == null)
        {
            //If it is not already attached

            Context.GetTable<T>().Attach(entity);
            Context.Refresh(System.Data.Linq.RefreshMode.KeepCurrentValues, entity);


        }

    }

生成的 SQL

UPDATE [dbo].[Account]
SET [AccountType] = @p3, [Duration] = @p4
WHERE ([AccountNumber] = @p0) 
AND ([AccountType] = @p1) 
AND ([Duration] = @p2) 
AND ([DepositedAmount] IS NULL)

-- @p0: Input Int (Size = -1; Prec = 0; Scale = 0) [4]
-- @p1: Input NChar (Size = 10; Prec = 0; Scale = 0) [TEST      ]
-- @p2: Input Int (Size = -1; Prec = 0; Scale = 0) [10]
-- @p3: Input NChar (Size = 10; Prec = 0; Scale = 0) [Verify]
-- @p4: Input Int (Size = -1; Prec = 0; Scale = 0) [0]

-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 4.0.30319.1

阅读：

【问题讨论】：

标签： c# .net sql-server linq linq-to-sql

【解决方案1】：

应该这样做：

int number = 4;
var acc1 = new accountRepository.Accounts.Where(a => a.Number == number).FirstOrDefault();

if (acc1 == null)
{
    // Not found by ID, create new
    acc1 = new RepositoryLayer.Account();
    acc1.Number = number;
    accountRepository.Accounts.AddObject(acc1);
}

acc1.AccountType = "Verify";

accountRepository.SubmitChanges();

【讨论】：

谢谢。我们不能在使用“附加”和刷新方法时实现它吗？
@Lijo：Attach 过去只是给我带来了问题。使用上述方法而不是;p
@leppie 你能列出这些问题或提供参考吗？

【解决方案2】：

我通过按照LINQ to SQL: Updating without Refresh when “UpdateCheck = Never”中的答案更改更新方法解决了这个问题

Duration 列的 UpdateCheck 设置为从不

    public void UpdateAccount()
    {
        //Used value from previous select
        DateTime previousDateTime = new DateTime(2012, 6, 26, 11, 14, 15, 327);
        int prevDuration = 0;

        RepositoryLayer.Account accEntity = new RepositoryLayer.Account();
        accEntity.AccountNumber = 1; //Primary Key
        accEntity.ModifiedTime = previousDateTime; //Concurrency column
        //accEntity.Duration = prevDuration;

        accountRepository.UpdateChangesByAttach(accEntity);

        //Values to be modified after Attach
        accEntity.AccountType = "WIN-WIN";
        accEntity.ModifiedTime = DateTime.Now;

        try
        {
            accountRepository.SubmitChanges();
        }
        catch(System.Data.Linq.ChangeConflictException e)
        {
            throw new Exception(e.Message);
        }

    }


   public virtual void UpdateChangesByAttach(T entity)
    {

        if (Context.GetTable<T>().GetOriginalEntityState(entity) == null)
        {
            //If it is not already attached
            Context.GetTable<T>().Attach(entity);
        }

    }

生成的 SQL

UPDATE [dbo].[Account]
SET [AccountType] = @p2, [ModifiedTime] = @p3
WHERE ([AccountNumber] = @p0) 
      AND ([ModifiedTime] = @p1)

-- @p0: Input Int (Size = -1; Prec = 0; Scale = 0) [1]
-- @p1: Input DateTime (Size = -1; Prec = 0; Scale = 0) [6/26/2012 11:14:15 AM]
-- @p2: Input NChar (Size = 10; Prec = 0; Scale = 0) [WIN-WIN]
-- @p3: Input DateTime (Size = -1; Prec = 0; Scale = 0) [6/26/2012 11:16:29 AM]

【讨论】：