是否可以缓存在 lambda 表达式中评估的值？

Question

在以下代码的 ContainsIngredients 方法中，是否可以缓存 p.Ingredients 值而不是多次显式引用它？这是一个相当简单的例子，我只是为了说明目的而编写的，但我正在处理的代码引用了 p 内部的值，例如。 p.InnerObject.ExpensiveMethod().Value

编辑： 我正在使用来自http://www.albahari.com/nutshell/predicatebuilder.html的 PredicateBuilder@

public class IngredientBag
{
    private readonly Dictionary<string, string> _ingredients = new Dictionary<string, string>();

    public void Add(string type, string name)
    {
        _ingredients.Add(type, name);
    }

    public string Get(string type)
    {
        return _ingredients[type];
    }

    public bool Contains(string type)
    {
        return _ingredients.ContainsKey(type);
    }
}

public class Potion
{
    public IngredientBag Ingredients { get; private set;}
    public string Name {get; private set;}        

    public Potion(string name) : this(name, null)
    {

    }

    public Potion(string name, IngredientBag ingredients)
    {
        Name = name;
        Ingredients = ingredients;
    }

    public static Expression<Func<Potion, bool>> 
        ContainsIngredients(string ingredientType, params string[] ingredients)
    {
        var predicate = PredicateBuilder.False<Potion>();
        // Here, I'm accessing p.Ingredients several times in one 
        // expression.  Is there any way to cache this value and
        // reference the cached value in the expression?
        foreach (var ingredient in ingredients)
        {
            var temp = ingredient;
            predicate = predicate.Or (
                p => p.Ingredients != null &&
                p.Ingredients.Contains(ingredientType) &&
                p.Ingredients.Get(ingredientType).Contains(temp));
        }

        return predicate;
    }

}


[STAThread]
static void Main()
{
    var potions = new List<Potion>
    {
        new Potion("Invisibility", new IngredientBag()),
        new Potion("Bonus"),
        new Potion("Speed", new IngredientBag()),
        new Potion("Strength", new IngredientBag()),
        new Potion("Dummy Potion")
    };

    potions[0].Ingredients.Add("solid", "Eye of Newt");
    potions[0].Ingredients.Add("liquid", "Gall of Peacock");
    potions[0].Ingredients.Add("gas", "Breath of Spider");

    potions[2].Ingredients.Add("solid", "Hair of Toad");
    potions[2].Ingredients.Add("gas", "Peacock's anguish");

    potions[3].Ingredients.Add("liquid", "Peacock Sweat");
    potions[3].Ingredients.Add("gas", "Newt's aura");

    var predicate = Potion.ContainsIngredients("solid", "Newt", "Toad")
        .Or(Potion.ContainsIngredients("gas", "Spider", "Scorpion"));

    foreach (var result in 
                from p in potions
                where(predicate).Compile()(p)
                select p)
    {
        Console.WriteLine(result.Name);
    }
}

Answer 1

你考虑过Memoization吗？

基本思路是这样的；如果你有一个昂贵的函数调用，有一个函数将在第一次调用时计算昂贵的值，但之后返回一个缓存版本。函数如下所示；

static Func<T> Remember<T>(Func<T> GetExpensiveValue)
{
    bool isCached= false;
    T cachedResult = default(T);

    return () =>
    {
        if (!isCached)
        {
            cachedResult = GetExpensiveValue();
            isCached = true;
        }
        return cachedResult;

    };
}

这意味着你可以写这个；

    // here's something that takes ages to calculate
    Func<string> MyExpensiveMethod = () => 
    { 
        System.Threading.Thread.Sleep(5000); 
        return "that took ages!"; 
    };

    // and heres a function call that only calculates it the once.
    Func<string> CachedMethod = Remember(() => MyExpensiveMethod());

    // only the first line takes five seconds; 
    // the second and third calls are instant.
    Console.WriteLine(CachedMethod());
    Console.WriteLine(CachedMethod());
    Console.WriteLine(CachedMethod());

作为一般策略，它可能会有所帮助。

Answer 2

您不能简单地将布尔表达式写在您从 lambda 调用的单独静态函数中 - 将 p.Ingredients 作为参数传递...

private static bool IsIngredientPresent(IngredientBag i, string ingredientType, string ingredient)
{
    return i != null && i.Contains(ingredientType) && i.Get(ingredientType).Contains(ingredient);
}

public static Expression<Func<Potion, bool>>
                ContainsIngredients(string ingredientType, params string[] ingredients)
{
    var predicate = PredicateBuilder.False<Potion>();
    // Here, I'm accessing p.Ingredients several times in one 
    // expression.  Is there any way to cache this value and
    // reference the cached value in the expression?
    foreach (var ingredient in ingredients)
    {
        var temp = ingredient;
        predicate = predicate.Or(
            p => IsIngredientPresent(p.Ingredients, ingredientType, temp));
    }

    return predicate;
}

Answer 3

好吧，在这种情况下，如果你不能使用记忆，你会受到相当的限制，因为你实际上只能将堆栈用作缓存：你无法在你的范围内声明一个新变量'会需要。我能想到的（我并不是说它会很漂亮）会做你想做的，但保留你需要的可组合性就像......

private static bool TestWith<T>(T cached, Func<T, bool> predicate)
{
    return predicate(cached);
}

public static Expression<Func<Potion, bool>>
                ContainsIngredients(string ingredientType, params string[] ingredients)
{
    var predicate = PredicateBuilder.False<Potion>();
    // Here, I'm accessing p.Ingredients several times in one 
    // expression.  Is there any way to cache this value and
    // reference the cached value in the expression?
    foreach (var ingredient in ingredients)
    {
        var temp = ingredient;
        predicate = predicate.Or (
            p => TestWith(p.Ingredients,
                i => i != null &&
                     i.Contains(ingredientType) &&
                     i.Get(ingredientType).Contains(temp));
    }

    return predicate;
}

您可以在需要时将多个 TestWith 调用的结果组合成一个更复杂的布尔表达式 - 为每个调用缓存适当的昂贵值 - 或者您可以将它们嵌套在作为第二个参数传递的 lambda 中，以处理复杂的深度层次结构。

但阅读代码会非常困难，而且由于您可能会在所有 TestWith 调用中引入更多堆栈转换，因此它是否会提高性能将取决于您的 ExpensiveCall() 的成本。

请注意，正如另一个答案所建议的那样，原始示例中不会有任何内联，因为据我所知，表达式编译器没有进行该级别的优化。

Answer 4

在这种情况下我会说不。我假设编译器可以计算出它使用了 p.Ingredients 变量 3 次，并将变量保持在堆栈或寄存器或它使用的任何东西附近。

Answer 5

Turbulent Intellect 给出了完全正确的答案。

我只是想建议您可以从您正在使用的类型中去除一些空值和异常，以便更友好地使用它们。

    public class IngredientBag
    {
      private Dictionary<string, string> _ingredients = 
new Dictionary<string, string>();
      public void Add(string type, string name)
      {
        _ingredients[type] = name;
      }
      public string Get(string type)
      {
        return _ingredients.ContainsKey(type) ? _ingredients[type] : null;
      }
      public bool Has(string type, string name)
      {
        return name == null ? false : this.Get(type) == name;
      }
    }

    public Potion(string name) : this(name, new IngredientBag())    {    }

那么，如果你有这个结构中的查询参数...

Dictionary<string, List<string>> ingredients;

你可以这样写查询。

from p in Potions
where ingredients.Any(i => i.Value.Any(v => p.IngredientBag.Has(i.Key, v))
select p;

PS，为什么是只读的？