LINQ to SQL 更新中的(C# 变量的)默认值问题

问题描述

I have the following code for updating Account table with LINQ to SQL. AccountNumber is the primary key column. The only value which need to be updated is AccountType; however the Duration also gets updated with zero (default value for int). How can we avoid this unnecessary overwrite?

Note: I am using Attach method

Note: I understand the reason for this behavior. "The DataContext cannot distinguish between a field with an assigned value of zero and one that is merely unassigned.". I am looking for a solution to overcome this.

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

Table DATA

Table Structure:

 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]

CODE

    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);


        }

    }

Generated 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

READING:

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"

I solved this issue by changing the update approach by following the answer in LINQ to SQL: Updating without Refresh when "UpdateCheck = Never"

UpdateCheck is set as Never for the Duration column

    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);
        }

    }

Generated 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]

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