For LINQ to Objects, this is compiled into a set of static method calls:
var productNames =
from p in products
where p.Id > 100 and p.Id < 5000
select p.ProductName;
Becomes:
IEnumerable<string> productNames = products
.Where(p => p.Id > 100 and p.Id < 5000)
.Select(p => p.ProductName);
This uses extension methods defined in the Enumerable
type, so is actually compiled to:
IEnumerable<string> productNames =
Enumerable.Select(
Enumerable.Where(products, p => p.Id > 100 and p.Id < 5000),
p => p.ProductName
);
The lambda expressions to handle this are turned into methods by the compiler. The lambda in the where is turned into a method which can be set to a Func<Product, Boolean>
, and the select into a Func<Product, String>
.
For a thorough explanation, see Jon Skeet's blog series: Reimplementing LINQ to Objects. He walks through the entire process of how this works, including the compiler transformations (from query syntax to method calls), how the methods are implemented, etc.
Note that LINQ to Sql and IQueryable<T>
implementations are different. The Expression<T>
that is generated by the lambda is passed into the query provider, which in turn is "transformed" in some manner (it's up to the provider how to do this) into calls, typically run on the server in the case of an ORM.
For this method, for example:
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;
}
Gets compiled to the following IL:
.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
}
Note that these are normal call
instructions for the method calls. The lambdas get converted into other methods, such as:
[CompilerGenerated]
private static bool <ProductNames>b__2(Product p)
{
return ((p.Id > 100) && (p.Id < 0x1388));
}