查詢Linq-to-SQL更新值[pre-SubmitChanges（）]

Question

我有一種情況，我想與已更新但尚未使用'SubmitChanges（）提交回數據庫的Linq-to-Sql托管數據（VS2008 / .NET Framework 3.5 / Sql Server 2005 Express）進行交互。 '。

我在下面給出了一個簡單的測試用例，其中我更改了一個簡單的“塊”對象的color屬性。 我枚舉了該表（塊9和10最初的Color值為空），更改了值，並枚舉了結果，一切似乎都很好。 但是，當我嘗試查詢（.Count）“塊”對象時，有些奇怪（或我的理解上的空白）在下面的兩個粗體輸出語句中突出顯示。

語句LinqtoSqlDBDataContext.Blocks.Count（Block => Block.Color == null）導致值'2'...這很奇怪，因為前面的枚舉表明所有Block都分配了顏色。 該語句是否傳回數據庫（實際上“ 2”將是正確的值，因為尚未提交任何內容）？ db跟蹤可以確認SELECT語句。

第二條語句LinqtoSqlDBDataContext.Blocks.ToList（）。Count（Block => Block.Color == null）返回正確的值'0'，但是我不確定'ToList（）'為什么會影響結果，所以。 您將注意到，該語句還生成了完全不同的SQL語句（沒有要引導的WHERE子句）。

如果未注釋SubmitChanges（）行，則LinqtoSqlDBDataContext.Blocks.Count（Block => Block.Color == null）會返回正確的值。 但是，我無法始終預測更改的數量，因此我想盡可能地輕描淡寫地返回更新數據庫的次數。

所以我想我的問題是...根據我在這里看到的內容，與預先提交的數據進行交互是否安全/有效/推薦？ 我已經進行了廣泛搜索，但沒有找到關於此場景的太多信息（如果有的話）。 顯然有什么東西讓我迷失了嗎？ 任何想法都深表感謝。

摘要 ：預先提交的Linq-to-Sql數據的枚舉似乎返回與查詢預先提交的Linq-to-Sql數據不同的結果。

DDL

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[Block](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [Color] [varchar](50) NULL,
 CONSTRAINT [PK_Table1] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO
SET ANSI_PADDING OFF

碼

using System;
using System.Collections.Generic;co
using System.Linq;
using System.Text;
using System.Data.Linq;

namespace LinqToSql1
{
    class Program
    {
        static void Main(string[] args)
        {

            LinqtoSqlDBDataContext LinqtoSqlDBDataContext = new LinqtoSqlDBDataContext();
            LinqtoSqlDBDataContext.Log = Console.Out;


            Console.WriteLine("Enumerating Blocks\n==================");
            foreach(Block Block in LinqtoSqlDBDataContext.Blocks)
            {
                Console.WriteLine("Block {0} Color is '{1}'", Block.ID, Block.Color);
            }
            Console.WriteLine();


            Console.WriteLine("Updating Blocks\n===============");
            foreach(Block Block in LinqtoSqlDBDataContext.Blocks.Where(b => b.Color == null))
            {
                Console.WriteLine("Block {0} Color was '{1}'", Block.ID, Block.Color);
                Block.Color = "Gray";
                Console.WriteLine("Block {0} Color is '{1}'", Block.ID, Block.Color);
            }
            Console.WriteLine();


            Console.WriteLine("Enumerating Blocks\n==================");
            foreach(Block Block in LinqtoSqlDBDataContext.Blocks)
            {
                Console.WriteLine("Block {0} Color is '{1}'", Block.ID, Block.Color);
            }
            Console.WriteLine();


            // Console.WriteLine("Submitting Changes\n==================");
            // LinqtoSqlDBDataContext.SubmitChanges();


            Console.WriteLine("Counting Blocks\n===============");
            Console.WriteLine("LinqtoSqlDBDataContext.Blocks.Count(Block => Block.Color == null) is {0}\n", LinqtoSqlDBDataContext.Blocks.Count(Block => Block.Color == null));
            Console.WriteLine("LinqtoSqlDBDataContext.Blocks.ToList().Count(Block => Block.Color == null) is {0}\n", LinqtoSqlDBDataContext.Blocks.ToList().Count(Block => Block.Color == null));


            LinqtoSqlDBDataContext.Dispose();


            Console.WriteLine("Press [Enter] to continue");
            Console.ReadLine();
        }
    }
}

輸出量

Enumerating Blocks
==================
SELECT [t0].[ID], [t0].[Color]
FROM [dbo].[Block] AS [t0]
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.30729.1

Block 1 Color is 'Red'
Block 2 Color is 'Green'
Block 3 Color is 'Yellow'
Block 4 Color is 'Black'
Block 5 Color is 'Orange'
Block 6 Color is 'Purple'
Block 7 Color is 'Blue'
Block 8 Color is 'White'
Block 9 Color is ''
Block 10 Color is ''

Updating Blocks
===============
SELECT [t0].[ID], [t0].[Color]
FROM [dbo].[Block] AS [t0]
WHERE [t0].[Color] IS NULL
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.30729.1

Block 9 Color was ''
Block 9 Color is 'Gray'
Block 10 Color was ''
Block 10 Color is 'Gray'

Enumerating Blocks
==================
SELECT [t0].[ID], [t0].[Color]
FROM [dbo].[Block] AS [t0]
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.30729.1

Block 1 Color is 'Red'
Block 2 Color is 'Green'
Block 3 Color is 'Yellow'
Block 4 Color is 'Black'
Block 5 Color is 'Orange'
Block 6 Color is 'Purple'
Block 7 Color is 'Blue'
Block 8 Color is 'White'
Block 9 Color is 'Gray'
Block 10 Color is 'Gray'

Counting Blocks
===============
SELECT COUNT(*) AS [value]
FROM [dbo].[Block] AS [t0]
WHERE [t0].[Color] IS NULL
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.30729.1

LinqtoSqlDBDataContext.Blocks.Count(Block => Block.Color == null) is 2

SELECT [t0].[ID], [t0].[Color]
FROM [dbo].[Block] AS [t0]
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.30729.1

LinqtoSqlDBDataContext.Blocks.ToList().Count(Block => Block.Color == null) is 0

Press [Enter] to continue

Answer 1

如果要保留這些值，則可能必須先序列化結果，然后再進行修改。 例如

Console.WriteLine("Updating Blocks\n===============");
var blocksToUpdate = LinqtoSqlDBDataContext.Blocks.Where(b => b.Color == null).ToArray();
foreach( var block in blocksToUpdate )
{
 // do something...
}

這樣，對象將保留其狀態，盡管這可能會導致問題/需要您以不同的方式進行提交更改。