Expression.Invoke for subexpressions; works on LINQ-to-SQL and LINQ-to-Objects, but not EF in 3.5SP1 (for IEnumerable<T>, call .AsQueryable() first):
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);
}
example LINQ-to-SQL output (EF explodes in sparks):
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]
identity-manager short-circuit without roundtrip - i.e.
var obj = ctx.Single(x=>x.Id == id);
var obj = ctx.Where(x=>x.Id == id).Single();
etc should not need to go to the database if an object with that identity has already been materialized and stored in the identity-manager; applies also to First, SingleOrDefault, FirstOrDefault. See LINQ-to-SQL (also here and here; you can verify by attaching to .Log); example:
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();
}
log output shows only only two trips; one to get the object the first time, and one for the count; it also shows the same object reference is returned by the materializer:
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 support; for a simple example that also works for LINQ-to-Objects:
partial class MyDataContext {
[Function(Name="NEWID", IsComposable=true)]
public Guid Random() { return Guid.NewGuid();}
}
and then order by x => ctx.Random(); example:
using (var ctx = new DataClasses1DataContext())
{
ctx.Log = Console.Out;
var anyAtRandom = (from cust in ctx.Customers
orderby ctx.Random()
select cust).First();
}
with output:
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()
If I was feeling truly evil, recursive lambda; probably not worth supporting this in any way... likewise, 4.0 expression (DLR) node-types.
