非常复杂的LINQ(to SQL)查询示例

Expression.Invoke 对于子表达式;用于linq to sql和linq to对象,但不用于3.5sp1中的ef(用于 IEnumerable<T> ,呼叫 .AsQueryable() 第一):

    Expression<Func<Customer, bool>> pred1 = cust=>cust.Country=="UK";
    Expression<Func<Customer, bool>> pred2 = cust=>cust.Country=="France";
    var param = Expression.Parameter(typeof(Customer), "x");
    var final = Expression.Lambda<Func<Customer, bool>>(
        Expression.OrElse(
            Expression.Invoke(pred1, param),
            Expression.Invoke(pred2, param)
        ), param);
    using (var ctx = new DataClasses1DataContext())
    {
        ctx.Log = Console.Out;
        int ukPlusFrance = ctx.Customers.Count(final);
    }

LINQ to SQL输出示例(EF在火花中爆炸):

SELECT COUNT(*) AS [value]
FROM [dbo].[Customers] AS [t0]
WHERE ([t0].[Country] = @p0) OR ([t0].[Country] = @p1)
-- @p0: Input NVarChar (Size = 2; Prec = 0; Scale = 0) [UK]
-- @p1: Input NVarChar (Size = 6; Prec = 0; Scale = 0) [France]

身份管理器无往返短路-即

var obj = ctx.Single(x=>x.Id == id);
var obj = ctx.Where(x=>x.Id == id).Single();

等应 不 如果具有该标识的对象已具体化并存储在标识管理器中,则需要转到数据库;也适用于 First ,请 SingleOrDefault , FirstOrDefault . 请参见Linq to SQL(另请参见 here 和 here ;您可以通过附加到 .Log );举例:

using (var ctx = new DataClasses1DataContext())
{
    ctx.Log = Console.Out;
    var first = ctx.Customers.First();
    string id = first.CustomerID;
    Console.WriteLine("Any more trips?");
    var firstDup = ctx.Customers.First(x=>x.CustomerID==id);
    Console.WriteLine(ReferenceEquals(first, firstDup)); // true
    Console.WriteLine("Prove still attached");
    int count = ctx.Customers.Count();
}

日志输出只显示两次行程;一次是第一次获取对象,一次是计数;它还显示物化器返回相同的对象引用:

SELECT TOP (1) [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[
ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t
0].[Country], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 3.5.30729.492
6

Any more trips?
True <==== this is object reference equality, not "are there any more trips"
Prove still attached
SELECT COUNT(*) AS [value]
FROM [dbo].[Customers] AS [t0]
-- Context: SqlProvider(Sql2008) Model: AttributedMetaModel Build: 3.5.30729.492
6

UDF支持;对于同样适用于Linq to对象的简单示例:

partial class MyDataContext {
     [Function(Name="NEWID", IsComposable=true)] 
     public Guid Random()  { return Guid.NewGuid();}
}

然后点菜 x => ctx.Random() 例如:

using (var ctx = new DataClasses1DataContext())
{
    ctx.Log = Console.Out;
    var anyAtRandom = (from cust in ctx.Customers
                       orderby ctx.Random()
                       select cust).First();
}

输出:

SELECT TOP (1) [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[        ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
ORDER BY NEWID()

如果我感觉 真正地 邪恶的, recursive lambda 可能不值得支持 任何 方式…同样,4.0表达式(DLR)节点类型。