LINQ如何编译成CIL？

例如：

var query = from c in db.Cars select c; foreach(Car aCar in query) { Console.WriteLine(aCar.Name); } 

一旦编译，这将如何翻译？ 幕后发生了什么？

Answer 1

它按以下方式编译：

1. 首先，将LINQ查询表达式转换为方法调用：

    public static void Main() { var query = db.Cars.Select(c => c); foreach (Car aCar in query) { Console.WriteLine(aCar.Name); } } 

2. 如果db.Cars的类型为IEnumerable （它用于LINQ-to-Objects），则lambda表达式将转换为单独的方法：

    private Car lambda0(Car c) { return c; } private Func CachedAnonymousMethodDelegate1; public static void Main() { if (CachedAnonymousMethodDelegate1 == null) CachedAnonymousMethodDelegate1 = new Func(lambda0); var query = db.Cars.Select(CachedAnonymousMethodDelegate1); foreach // ... } 

   实际上，该方法不称为lambda0而是类似

   b__0
   （其中Main是包含方法的名称）。 类似地，缓存的委托实际上称为CS$<>9__CachedAnonymousMethodDelegate1 。

   如果您使用的是LINQ-to-SQL，那么db.Cars的类型为IQueryable ，此步骤非常不同。 它会将lambda表达式转换为表达式树：

    public static void Main() { var parameter = Expression.Parameter(typeof(Car), "c"); var lambda = Expression.Lambda>(parameter, new ParameterExpression[] { parameter })); var query = db.Cars.Select(lambda); foreach // ... } 

3. foreach循环转换为try/finally块（两者都是一样的）：

    IEnumerator enumerator = null; try { enumerator = query.GetEnumerator(); Car aCar; while (enumerator.MoveNext()) { aCar = enumerator.Current; Console.WriteLine(aCar.Name); } } finally { if (enumerator != null) ((IDisposable)enumerator).Dispose(); } 

4. 最后，这是以预期的方式编译成IL。 以下是IEnumerable ：

    // Put db.Cars on the stack L_0016: ldloc.0 L_0017: callvirt instance !0 DatabaseContext::get_Cars() // “if” starts here L_001c: ldsfld Func Program::CachedAnonymousMethodDelegate1 L_0021: brtrue.s L_0034 L_0023: ldnull L_0024: ldftn Car Program::lambda0(Car) L_002a: newobj instance void Func::.ctor(object, native int) L_002f: stsfld Func Program::CachedAnonymousMethodDelegate1 // Put the delegate for “c => c” on the stack L_0034: ldsfld Func Program::CachedAnonymousMethodDelegate1 // Call to Enumerable.Select() L_0039: call IEnumerable Enumerable::Select(IEnumerable, Func) L_003e: stloc.1 // “try” block starts here L_003f: ldloc.1 L_0040: callvirt instance IEnumerator IEnumerable::GetEnumerator() L_0045: stloc.3 // “while” inside try block starts here L_0046: br.s L_005a L_0048: ldloc.3 // body of while starts here L_0049: callvirt instance !0 IEnumerator::get_Current() L_004e: stloc.2 L_004f: ldloc.2 L_0050: ldfld string Car::Name L_0055: call void Console::WriteLine(string) L_005a: ldloc.3 // while condition starts here L_005b: callvirt instance bool IEnumerator::MoveNext() L_0060: brtrue.s L_0048 // end of while L_0062: leave.s L_006e // end of try // “finally” block starts here L_0064: ldloc.3 L_0065: brfalse.s L_006d L_0067: ldloc.3 L_0068: callvirt instance void IDisposable::Dispose() L_006d: endfinally 

   IQueryable版本的编译代码也是预期的。 以下是与上述不同的重要部分（局部变量现在将具有不同的偏移和名称，但让我们忽略它）：

    // typeof(Car) L_0021: ldtoken Car L_0026: call Type Type::GetTypeFromHandle(RuntimeTypeHandle) // Expression.Parameter(typeof(Car), "c") L_002b: ldstr "c" L_0030: call ParameterExpression Expression::Parameter(Type, string) L_0035: stloc.3 // Expression.Lambda(...) L_0036: ldloc.3 L_0037: ldc.i4.1 // var paramArray = new ParameterExpression[1] L_0038: newarr ParameterExpression L_003d: stloc.s paramArray L_003f: ldloc.s paramArray L_0041: ldc.i4.0 // paramArray[0] = parameter; L_0042: ldloc.3 L_0043: stelem.ref L_0044: ldloc.s paramArray L_0046: call Expression Expression::Lambda>(Expression, ParameterExpression[]) // var query = Queryable.Select(...); L_004b: call IQueryable Queryable::Select(IQueryable, Expression>) L_0050: stloc.1 

Answer 2

您应该编译它并对生成的可执行文件运行ildasm以查找。