c# – LINQ实际编译的是什么？

var productNames = 
    from p in products 
    where p.Id > 100 and p.Id < 5000
    select p.ProductName;

变为：

IEnumerable<string> productNames = products
                                       .Where(p => p.Id > 100 and p.Id < 5000)
                                       .Select(p => p.ProductName);

这使用了Enumerable类型中定义的扩展方法,因此实际编译为：

IEnumerable<string> productNames = 
     Enumerable.Select(
        Enumerable.Where(products,p => p.Id > 100 and p.Id < 5000),p => p.ProductName
     );

处理此问题的lambda表达式由编译器转换为方法.将where中的lambda转换为一个方法,该方法可以设置为Func< Product,Boolean>,并选择为Func< Product,String>.

有关详细说明,请参阅Jon Skeet’s blog series: Reimplementing LINQ to Objects.他将详细介绍其工作原理,包括编译器转换(从查询语法到方法调用),方法实现方式等.

注意,LINQ to Sql和IQueryable< T>实现是不同的.表达式< T>由lambda生成的是传递给查询提供程序,而查询提供程序又以某种方式(由提供程序如何执行此操作)“转换”为调用,通常在ORM的情况下在服务器上运行.

对于此方法,例如：

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;
    }

获取编译为以下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>,!!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,这些是方法调用的正常调用指令. lambdas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));
}