Question

例如：

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

编译后如何翻译？幕后发生了什么？

Answer 1

它按以下方式编译：

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<Car, Car>(c => c);
    foreach (Car aCar in query)
    {
         Console.WriteLine(aCar.Name);
    }
}

如果db.Cars的类型为IEnumerable<Car>（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<Car, Car> CachedAnonymousMethodDelegate1;
public static void Main()
{
    if (CachedAnonymousMethodDelegate1 == null)
        CachedAnonymousMethodDelegate1 = new Func<Car, Car>(lambda0);
    var query = db.Cars.Select<Car, Car>(CachedAnonymousMethodDelegate1);
    foreach // ...
}

实际上，该方法不是lambda0，而是类似<Main>b__0（其中Main是包含方法的名称）。同样，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<Car>，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<Func<Car, Car>>(parameter, new ParameterExpression[] { parameter }));
    var query = db.Cars.Select<Car, Car>(lambda);
    foreach // ...
}

foreach循环转换为try/finally块（两者都相同）：

IEnumerator<Car> 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();
}

最后，这是以预期的方式编译成IL。以下内容适用于IEnumerable<Car>：

// 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<Car, Car> Program::CachedAnonymousMethodDelegate1
L_0021: brtrue.s L_0034
L_0023: ldnull 
L_0024: ldftn Car Program::lambda0(Car)
L_002a: newobj instance void Func<Car, Car>::.ctor(object, native int)
L_002f: stsfld Func<Car, Car> Program::CachedAnonymousMethodDelegate1


// Put the delegate for “c => c” on the stack
L_0034: ldsfld Func<Car, Car> Program::CachedAnonymousMethodDelegate1


// Call to Enumerable.Select()
L_0039: call IEnumerable<!!1> Enumerable::Select<Car, Car>(IEnumerable<!!0>, Func<!!0, !!1>)
L_003e: stloc.1


// “try” block starts here
L_003f: ldloc.1 
L_0040: callvirt instance IEnumerator<!0> IEnumerable<Car>::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<Car>::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<Car>版本的已编译代码也符合预期。以下是与上述不同的重要部分（局部变量现在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<!!0> Expression::Lambda<Func<Car, Car>>(Expression, ParameterExpression[])


// var query = Queryable.Select(...);
L_004b: call IQueryable<!!1> Queryable::Select<Car, Car>(IQueryable<!!0>, Expression<Func<!!0, !!1>>)
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。

LINQ如何编译成CIL？

2 个答案: