Inline Functions in C#

C++ has an inline function, which is modified by the inline keyword. The compiler optimizes it by inserting this function's code at the call site.

Excerpt from C Language Chinese Network

When a function is called, memory space must first be allocated on the stack for the formal parameters and local variables. Then, the values of the actual parameters need to be copied to the formal parameters. Next, the return address (which indicates where the program should continue execution after the function finishes) is placed in the stack, and only then does the program jump to the function body for execution. This process is time-consuming.

Additionally, when a function executes the return statement, it needs to reclaim the memory space occupied by the formal parameters and local variables from the stack, retrieve the return address from the stack, and then jump to that address to continue execution. This process also takes time.

In C#, you can use attributes on a function to instruct the compiler to optimize it, achieving the same purpose.

    [MethodImpl(MethodImplOptions.AggressiveInlining)]

Here’s an example:

using System;
using System.Diagnostics;
using System.Runtime.CompilerServices;

class Program
{
    const int _max = 10000000;
    static void Main()
    {
        int sum = 0;
        Stopwatch s1 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            sum += Method1();
        }
        s1.Stop();

        Stopwatch s2 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            sum += Method2();
        }

        s2.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) /
            _max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) /
            _max).ToString("0.00 ns"));
        Console.Read();
    }

    static int Method1()
    {
        return "one".Length + "two".Length + "three".Length +
            "four".Length + "five".Length + "six".Length +
            "seven".Length + "eight".Length + "nine".Length +
            "ten".Length;
    }

    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    static int Method2()
    {
        return "one".Length + "two".Length + "three".Length +
            "four".Length + "five".Length + "six".Length +
            "seven".Length + "eight".Length + "nine".Length +
            "ten".Length;
    }
}

Testing results:

21.92 ns
3.22 ns