C# 将Func用作select与内联select时sql之间的差异

我试图理解为什么这两条语句的sql不同（区别似乎在于决定是在select语句中添加SignedXml还是在use case语句中添加SignedXml。但是为什么呢？）

这会产生如下结果：

exec sp_executesql N'SELECT 
[Extent1].[ID] AS [ID], 
[Extent1].[Name] AS [Name], 
[Extent1].[Created] AS [Created], 
[Extent1].[FolderID] AS [FolderID], 
[Extent1].[Locked] AS [Locked], 
[Extent1].[Modified] AS [Modified], 
[Extent1].[OwnerID] AS [OwnerID], 
CASE WHEN ([Extent1].[SignedXml] IS NOT NULL) THEN cast(1 as bit) WHEN ([Extent1].[SignedXml] IS NULL) THEN cast(0 as bit) END AS [C1]
FROM [dbo].[NativeAnalyses] AS [Extent1]
WHERE ([Extent1].[OwnerID] = @p__linq__0) AND ([Extent1].[FolderID] = @p__linq__1)',N'@p__linq__0 uniqueidentifier,@p__linq__1 

exec sp_executesql N'SELECT 
[Extent1].[ID] AS [ID], 
[Extent1].[Name] AS [Name], 
[Extent1].[ModelXml] AS [ModelXml], 
[Extent1].[Created] AS [Created], 
[Extent1].[Modified] AS [Modified], 
[Extent1].[OwnerID] AS [OwnerID], 
[Extent1].[SignedXml] AS [SignedXml], 
[Extent1].[FolderID] AS [FolderID], 
[Extent1].[Locked] AS [Locked]
FROM [dbo].[NativeAnalyses] AS [Extent1]
WHERE ([Extent1].[OwnerID] = @p__linq__0) AND ([Extent1].[FolderID] = @p__linq__1)',N'@p__linq__0 uniqueidentifier,@p__linq__1 

vs当我使用功能进行选择时：

myTable.Where(p => p.OwnerID.Equals(owner) && p.FolderID == folderId)
       .Select(Select);

选择是

private SignedNativeAnalysis Select(NativeAnalysis source)
{
      return new SignedNativeAnalysis
      {
        ID = source.ID,
        Name = source.Name,
        Created = source.Created,
        FolderID = source.FolderID,
        Locked = source.Locked,
        Modified = source.Modified,
        OwnerID = source.OwnerID,
        IsSigned = source.SignedXml != null
      };
}

这会产生如下结果：

exec sp_executesql N'SELECT 
[Extent1].[ID] AS [ID], 
[Extent1].[Name] AS [Name], 
[Extent1].[Created] AS [Created], 
[Extent1].[FolderID] AS [FolderID], 
[Extent1].[Locked] AS [Locked], 
[Extent1].[Modified] AS [Modified], 
[Extent1].[OwnerID] AS [OwnerID], 
CASE WHEN ([Extent1].[SignedXml] IS NOT NULL) THEN cast(1 as bit) WHEN ([Extent1].[SignedXml] IS NULL) THEN cast(0 as bit) END AS [C1]
FROM [dbo].[NativeAnalyses] AS [Extent1]
WHERE ([Extent1].[OwnerID] = @p__linq__0) AND ([Extent1].[FolderID] = @p__linq__1)',N'@p__linq__0 uniqueidentifier,@p__linq__1 

exec sp_executesql N'SELECT 
[Extent1].[ID] AS [ID], 
[Extent1].[Name] AS [Name], 
[Extent1].[ModelXml] AS [ModelXml], 
[Extent1].[Created] AS [Created], 
[Extent1].[Modified] AS [Modified], 
[Extent1].[OwnerID] AS [OwnerID], 
[Extent1].[SignedXml] AS [SignedXml], 
[Extent1].[FolderID] AS [FolderID], 
[Extent1].[Locked] AS [Locked]
FROM [dbo].[NativeAnalyses] AS [Extent1]
WHERE ([Extent1].[OwnerID] = @p__linq__0) AND ([Extent1].[FolderID] = @p__linq__1)',N'@p__linq__0 uniqueidentifier,@p__linq__1 

而SignedNativeAnalysis只继承了NativeAnalysis的一个额外属性

internal class SignedNativeAnalysis : NativeAnalysis
{
   public bool IsSigned { get; set; }
}

更新： 根据建议，我尝试使用表达式来实现我想要的结果，但我得到了一个异常：

Expression<Func<NativeAnalysis, SignedNativeAnalysis>> signed = p => Select(p);
myTable.Where(p => p.OwnerID.Equals(owner) && p.FolderID == folderId)
       .Select(signed)

Expression signed=p=>Select（p）；
myTable.Where（p=>p.OwnerID.Equals（owner）和&p.FolderID==FolderID）
.选择（签名）

LINQ to实体无法识别方法的SignedNativeAnalysis 选择（www.EntityFramework.NativeAnalysis）方法，然后 方法无法转换为存储表达式

---

这是个诡计。实体框架选择不接受Func。它需要一个表达式

因此，当您传递lambda时，编译器将lambda转换为表达式，并将其传递给EF，EF将表达式转换为SQL代码

传递Func时，无法将其转换为表达式。而Enumerable.Select扩展方法——它接受一个普通Func，而不是一个表达式——起作用。因此EF在SQL中执行第一部分，直到Where。然后将结果传递给Linq to对象，该对象在内存中执行投影

---

您仍然可以执行动态操作，但必须创建表达式的实例，而不是委托。

好的，我将其更改为使用表达式，但这不起作用，因为我遇到了“此方法无法转换为表达式”错误？我看到了您编辑的问题。您所做的只是创建“callselect（p）method”表达式。但EF不知道如何将“调用选择（p）”转换为SQL代码。这不是你想要的。您需要的是“p=>newsignednativeanalysis{ID=p.ID，…}”表达式。但这基本上是我的第一个选择，包装在一个表达式中。我想要第二个选项（使用函数），这样我就可以在多个位置使用选择，而不必复制它…？您不能使用编译后的函数。但是您仍然可以将表达式存储在字段/变量/静态中，并在代码中共享它。比如：表达式投影=p=>new。。。；然后使用它myTable.Where（…）.Select（projection）；此链接提供有关表达式的更多详细信息，可能会有所帮助：