通过lambda表达式创建的委托是否可以保证被缓存？

Question

It seems to be working that way, but is it stated somewhere in the spec or just an implementation detail I can't really depend on? I'm trying to speed up property/field name extraction faster by creating the expression tree only once and caching it. I do this by wrapping the tree in a lambda and using it as a key for the cache. And it will break down miserably if the runtime decides to create a new delegate every time it hits the same lambda expression.

// KeyValuePair<string, T> GetPair<T>(Func<Expression<Func<T>>> val)...
var item = new Item { Num = 42 };
var pair = GetPair(() => () => item.Num); // guaranteed to be the same instance?
// pair.Key = "Num"
// pair.Value = 42

Edit: Ok, here is the full thing. It seems it works and doesn't seem to generate any garbage in the process.

Another Edit: Ok, changed it, this doesn't seem to capture anything, and it works even faster!

using System;
using System.Diagnostics;
using System.Linq.Expressions;
using System.Runtime.CompilerServices;

class Program
{
    static void Main(string[] args) {
        var pair = new Pair<int>();
        var pair2 = new Pair<string>();
        var item = new Item { Num = 42, Word = "Answer" };
        double ratio = 1;
        var sw = Stopwatch.StartNew();
        for (int i = 0;; i++) {
            if ((i & 0xFFF) == 0 && sw.ElapsedMilliseconds > 2000) {
                Console.WriteLine("literal: {0:N0}", i);
                ratio *= i;
                break;
            }
            Assign(pair, "Num", item.Num); Assign(pair2, "Word", item.Word);
            Assign(pair, "Num", item.Num); Assign(pair2, "Word", item.Word);
            Assign(pair, "Num", item.Num); Assign(pair2, "Word", item.Word);
            Assign(pair, "Num", item.Num); Assign(pair2, "Word", item.Word);
        }
        sw = Stopwatch.StartNew();
        for (int i = 0; ; i++) {
            if ((i & 0xFFF) == 0 && sw.ElapsedMilliseconds > 2000) {
                item = new Item { Num = 42, Word = "Answer" };
                Console.WriteLine("expression: {0:N0}", i);
                ratio /= i;
                break;
            }
            Assign4(pair, item, () => it => it.Num); Assign4(pair2, item, () => it => it.Word);
            Assign4(pair, item, () => it => it.Num); Assign4(pair2, item, () => it => it.Word);
            Assign4(pair, item, () => it => it.Num); Assign4(pair2, item, () => it => it.Word);
            Assign4(pair, item, () => it => it.Num); Assign4(pair2, item, () => it => it.Word);

        }
        Console.WriteLine(ratio.ToString("F3"));
        Console.ReadLine();
    }

    static void Assign<T>(Pair<T> pair, string name, T value) {
        pair.Name = name;
        pair.Value = value;
    }

    static void Assign4<T, U>(Pair<T> pair, U item, Func<Expression<Func<U, T>>> value,
        [CallerFilePath]string path = "", [CallerLineNumber]int line = 0) {

        int key = ((path.Length << 20) + line) % Cache<U, T>.Length;
//        int key = value.GetHashCode() % Cache<T>.Length;
        while (true) {
            var bucket = Cache<U, T>.Records[key];
            if (bucket.Literal == null) break;
            if (object.ReferenceEquals(bucket.Literal, value)) {
                pair.Name = bucket.FieldName;
                pair.Value = bucket.Getter(item);
                return;
            }
            key += 1;
            if (key == Cache<U, T>.Length) key = 0;
        }
        var tree = value();
        var getter = tree.Compile();
        string name = (tree.Body as MemberExpression).Member.Name;
        Cache<U, T>.Records[key] = new Cache<U, T>.Record {
            Literal = value,
            FieldName = name,
            Getter = getter,
        };
        pair.Name = name;
        pair.Value = getter(item);
    }
}

class Cache<U, T>
{
    public struct Record
    {
        public Func<Expression<Func<U, T>>> Literal;
        public string FieldName;
        public Func<U, T> Getter;
    }
    public const int Length = 997;
    public static Record[] Records = new Record[Length];
}

class Pair<T>
{
    public string Name;
    public T Value;
}

class Item
{
    public int Num;
    public string Word;
}

Answer 1

It can't be the same instance in this case - the captured variable ( item ) will be different each time you execute this pair of lines.

Even where it can be the same instance, it isn't guaranteed. From what I remember of the MS C# compiler, lambda expressions which don't capture any variables (not even this ) will be cached in static variables - but I'm not sure that anything else is.