Question

I assumed lambda functions, delegates and anonymous functions with the same body would have the same “speed”, however, running the following simple program:

static void Main(string[] args)
{
    List<int> items = new List<int>();

    Random random = new Random();

    for (int i = 0; i < 10000000; i++)
    {
        items.Add(random.Next());
    }

    Stopwatch watch;
    IEnumerable<int> result;

    Func<int, bool> @delegate = delegate(int i)
    {
        return i < 500;
    };
    watch = Stopwatch.StartNew();
    result = items.Where(@delegate);
    watch.Stop();
    Console.WriteLine("Delegate: {0}", watch.Elapsed.TotalMilliseconds);

    Func<int, bool> lambda = i => i < 500;
    watch = Stopwatch.StartNew();
    result = items.Where(lambda);
    watch.Stop();
    Console.WriteLine("Lambda: {0}", watch.Elapsed.TotalMilliseconds);

    watch = Stopwatch.StartNew();
    result = items.Where(i => i < 500);
    watch.Stop();
    Console.WriteLine("Inline: {0}", watch.Elapsed.TotalMilliseconds);

    Console.ReadLine();
}

I get:

Delegate: 4.2948 ms

Lambda: 0.0019 ms

Anonymous: 0.0034 ms

Although negligible, why are these three – apparently identical – methods running at different speeds? What’s happening under the hood?

---

Update:

As suggested by the comments, the following “forces” the Where by calling ToList() on it. In addition, a loop is added to offer more run data:

while (true) 
{
    List<int> items = new List<int>();

    Random random = new Random();

    for (int i = 0; i < 10000000; i++)
    {
        items.Add(random.Next());
    }

    Stopwatch watch;
    IEnumerable<int> result;

    Func<int, bool> @delegate = delegate(int i)
    {
        return i < 500;
    };
    watch = Stopwatch.StartNew();
    result = items.Where(@delegate).ToList();
    watch.Stop();
    Console.WriteLine("Delegate: {0}", watch.Elapsed.TotalMilliseconds);

    Func<int, bool> lambda = i => i < 500;
    watch = Stopwatch.StartNew();
    result = items.Where(lambda).ToList();
    watch.Stop();
    Console.WriteLine("Lambda: {0}", watch.Elapsed.TotalMilliseconds);

    watch = Stopwatch.StartNew();
    result = items.Where(i => i < 500).ToList();
    watch.Stop();
    Console.WriteLine("Inline: {0}", watch.Elapsed.TotalMilliseconds);
    Console.WriteLine(new string('-', 12));

}

The above code results in ~120 ms for each function.

Answer 1

A lambda expression is an anonymous function. “Anonymous function” refers to either a lambda expression or an anonymous method (which is what you’ve called a “delegate” in your code).

All three operations are using delegates. The second and third are both using lambda expressions. All three will execute in the same way, with the same performance characteristics.

Note that there can be a difference in performance between:

Func<int, int> func = x => ...;
for (int i = 0; i < 10000; i++) {
    CallFunc(func);
}

and

for (int i = 0; i < 10000; i++) {
    CallFunc(x => ...) // Same lambda as before
}

It depends on whether the compiler is able to cache the delegate created by the lambda expression. That will in turn depend on whether it captures variables etc.

For example, consider this code:

using System;
using System.Diagnostics;

class Test
{
    const int Iterations = 1000000000;

    static void Main()
    {
        AllocateOnce();
        AllocateInLoop();
    }

    static void AllocateOnce()
    {
        int x = 10;

        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        Func<int, int> allocateOnce = y => y + x;
        for (int i = 0; i < Iterations; i++)
        {
            sum += Apply(i, allocateOnce);
        }
        sw.Stop();
        Console.WriteLine("Allocated once: {0}ms", sw.ElapsedMilliseconds);
    }

    static void AllocateInLoop()
    {
        int x = 10;

        Stopwatch sw = Stopwatch.StartNew();
        int sum = 0;
        for (int i = 0; i < Iterations; i++)
        {
            sum += Apply(i, y => y + x);
        }
        sw.Stop();
        Console.WriteLine("Allocated in loop: {0}ms", sw.ElapsedMilliseconds);
    }

    static int Apply(int loopCounter, Func<int, int> func)
    {
        return func(loopCounter);
    }
}

The compiler is smart, but there’s still a difference. Using Reflector, we can see that AllocateInLoop is effectively compiled to:

private static void AllocateInLoop()
{
    Func<int, int> func = null;
    int x = 10;
    Stopwatch stopwatch = Stopwatch.StartNew();
    int sum = 0;
    for (int i = 0; i < Iterations; i++)
    {
        if (func == null)
        {
            func = y => y + x;
        }
        sum += Apply(i, func);
    }
    stopwatch.Stop();
    Console.WriteLine("Allocated in loop: {0}ms", stopwatch.ElapsedMilliseconds);
}

So still only a single delegate instance is created, but there’s extra logic within the loop – an extra nullity test on each iteration, basically.

On my machine that makes about a 15% difference in performance.