对于LINQ to Objects,这将被编译成一组静态方法调用:
var productNames =
from p in products
where p.Id > 100 and p.Id < 5000
select p.ProductName;
请提供需要翻译的内容。
Becomes:
Please provide the content to be translated.
IEnumerable<string> productNames = products
.Where(p => p.Id > 100 and p.Id < 5000)
.Select(p => p.ProductName);
这里使用了在Enumerable
类型中定义的扩展方法,因此实际上编译为:
IEnumerable<string> productNames =
Enumerable.Select(
Enumerable.Where(products, p => p.Id > 100 and p.Id < 5000),
p => p.ProductName
);
lambda表达式会被编译器转化为方法。在where中的lambda会被转化为一个可以设置为
Func<Product, Boolean>
的方法,而select则是
Func<Product, String>
。
要详细了解,请参见
Jon Skeet的博客系列:重新实现LINQ to Objects。他详细介绍了整个过程,包括编译器的转换(从查询语法到方法调用),如何实现方法等。
请注意,LINQ to Sql和
IQueryable<T>
实现不同。由lambda生成的
Expression<T>
传递给查询提供程序,后者以某种方式“转换”(由提供程序决定如何执行),通常在ORM的情况下在服务器上运行。
例如,对于此方法:
private static IEnumerable<string> ProductNames(IEnumerable<Product> products)
{
var productNames =
from p in products
where p.Id > 100 && p.Id < 5000
select p.ProductName;
return productNames;
}
编译为以下IL代码:
.method private hidebysig static class [mscorlib]System.Collections.Generic.IEnumerable`1<string> ProductNames(class [mscorlib]System.Collections.Generic.IEnumerable`1<class ConsoleApplication3.Product> products) cil managed
{
.maxstack 3
.locals init (
[0] class [mscorlib]System.Collections.Generic.IEnumerable`1<string> enumerable,
[1] class [mscorlib]System.Collections.Generic.IEnumerable`1<string> enumerable2)
L_0000: nop
L_0001: ldarg.0
L_0002: ldsfld class [mscorlib]System.Func`2<class ConsoleApplication3.Product, bool> ConsoleApplication3.Program::CS$<>9__CachedAnonymousMethodDelegate3
L_0007: dup
L_0008: brtrue.s L_001d
L_000a: pop
L_000b: ldnull
L_000c: ldftn bool ConsoleApplication3.Program::<ProductNames>b__2(class ConsoleApplication3.Product)
L_0012: newobj instance void [mscorlib]System.Func`2<class ConsoleApplication3.Product, bool>::.ctor(object, native int)
L_0017: dup
L_0018: stsfld class [mscorlib]System.Func`2<class ConsoleApplication3.Product, bool> ConsoleApplication3.Program::CS$<>9__CachedAnonymousMethodDelegate3
L_001d: call class [mscorlib]System.Collections.Generic.IEnumerable`1<!!0> [System.Core]System.Linq.Enumerable::Where<class ConsoleApplication3.Product>(class [mscorlib]System.Collections.Generic.IEnumerable`1<!!0>, class [mscorlib]System.Func`2<!!0, bool>)
L_0022: ldsfld class [mscorlib]System.Func`2<class ConsoleApplication3.Product, string> ConsoleApplication3.Program::CS$<>9__CachedAnonymousMethodDelegate5
L_0027: dup
L_0028: brtrue.s L_003d
L_002a: pop
L_002b: ldnull
L_002c: ldftn string ConsoleApplication3.Program::<ProductNames>b__4(class ConsoleApplication3.Product)
L_0032: newobj instance void [mscorlib]System.Func`2<class ConsoleApplication3.Product, string>::.ctor(object, native int)
L_0037: dup
L_0038: stsfld class [mscorlib]System.Func`2<class ConsoleApplication3.Product, string> ConsoleApplication3.Program::CS$<>9__CachedAnonymousMethodDelegate5
L_003d: call class [mscorlib]System.Collections.Generic.IEnumerable`1<!!1> [System.Core]System.Linq.Enumerable::Select<class ConsoleApplication3.Product, string>(class [mscorlib]System.Collections.Generic.IEnumerable`1<!!0>, class [mscorlib]System.Func`2<!!0, !!1>)
L_0042: stloc.0
L_0043: ldloc.0
L_0044: stloc.1
L_0045: br.s L_0047
L_0047: ldloc.1
L_0048: ret
}
请注意,这些是方法调用的普通call
指令。Lambda表达式会转换为其他方法,例如:
[CompilerGenerated]
private static bool <ProductNames>b__2(Product p)
{
return ((p.Id > 100) && (p.Id < 0x1388));
}
Join
、GroupBy
、Distinct
、ToLookup
等这样的东西,那当然可以。但是Where
根本不会使用基于哈希的算法/结构。 - Servy