c# - LINQ to SQL 更新中的(C# 变量的)默认值问题

Question

我有以下代码用于使用 LINQ to SQL 更新帐户表。 AccountNumber 是主键列。唯一需要更新的值是 AccountType；但是持续时间也更新为零(int 的默认值)。我们怎样才能避免这种不必要的覆盖？

注意:我使用的是Attach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:

1. Can LINQ-to-SQL omit unspecified columns on insert so a database default value is used?

2. How can I bind an Enum to a DbType of bit or int?

3. Linq to SQL: Why am I getting IDENTITY_INSERT errors?

4. LINQ to SQL: Updating without Refresh when “UpdateCheck = Never”

Answer 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();