class Program
    {

        private static int Method1(string str, char symbol)
        {
            var result = 0;
            str.AsParallel().ForAll(x =>
            {
                if (x == symbol)
                {
                    Interlocked.Add(ref result, 1);
                    //result++;
                }
            });
            return result;
        }

        private static int Method2(string str, char symbol)
        {
            var result = 0;
            Action<char> lambda = x =>
            {
                if (x == symbol)
                {
                    //Interlocked.Add(ref result, 1);
                    result++;
                }
            };
            foreach (var @char in str)
            {
                lambda(@char);
            }
            return result;
        }

        static void Main(string[] args)
        {
            string str = File.ReadAllText("Толстой Лев. Война и мир. Том 1 - royallib.ru.txt");
            var res1 = Method1(str, 'a');
            var res2 = Method2(str, 'a');
            var sw = new Stopwatch();

            sw.Restart();
            res1 = Method1(str, 'a');
            sw.Stop();
            Console.WriteLine("Result {0}, time {1}", res1, sw.ElapsedMilliseconds);
            sw.Restart();
            res2 = Method2(str, 'a');
            sw.Stop();
            Console.WriteLine("Result {0}, time {1}", res1, sw.ElapsedMilliseconds);
            //1.ToString();
        }
    }

Result 1920, time 9
Result 1920, time 2

    public static void Main(string[] args)
    {
        const int RepeatCount = 100 * 1000;
        const int RepeatCountShort = 10 * 1000 * 1000;
        string longString = string.Join("", Enumerable.Range(0, 100).Select(i => Path.GetRandomFileName()));
        string shortString = "a";

        Measure("For", () =>
        {
            long result = 0;
            for (int i = 0; i < RepeatCount; i++)
            {
                result += Count(longString, 'a');
            }
            return result;
        });
        Measure("IndexOf", () =>
        {
            long result = 0;
            for (int i = 0; i < RepeatCount; i++)
            {
                result += CountFast(longString, 'a');
            }
            return result;
        });
        Console.WriteLine();
        Measure("ForShort", () =>
        {
            long result = 0;
            for (int i = 0; i < RepeatCountShort; i++)
            {
                result += Count(shortString, 'a');
            }
            return result;
        });
        Measure("IndexOfShort", () =>
        {
            long result = 0;
            for (int i = 0; i < RepeatCountShort; i++)
            {
                result += CountFast(shortString, 'a');
            }
            return result;
        });

        Console.WriteLine("Done.");
        Console.ReadKey();
    }

    static int Count(string inputString, char selector)
    {
        int result = 0;
        for (var i = 0; i < inputString.Length; i++)
        {
            if (inputString[i] == selector)
                result++;
        }
        return result;
    }

    static int CountFast(string inputString, char selector)
    {
        int result = 0;
        int index = 0;
        while (index < inputString.Length)
        {
            index = inputString.IndexOf(selector, index);
            if (index < 0)
            {
                break;
            }
            else
            {
                result++;
                index++;
            }
        }
        return result;
    }

    static void Measure(string name, Func<long> callback)
    {
        GC.Collect();
        GC.WaitForPendingFinalizers();
        GC.Collect();

        var mem = GC.GetTotalMemory(true);
        var gc = GC.CollectionCount(0);

        var sw = Stopwatch.StartNew();
        var result = callback();
        sw.Stop();

        var mem2 = GC.GetTotalMemory(false);
        var gc2 = GC.CollectionCount(0);

        var memDelta = (mem2 - mem) / 1024.0;
        var gcDelta = gc2 - gc;

        Console.WriteLine(
            "{0,14}: {1,5}ms, ips: {2,14:N} | Mem: {3,4:N2} kb, GC0: {4} => {5,6}",
            name, sw.ElapsedMilliseconds, result / sw.Elapsed.TotalSeconds, memDelta, gcDelta, result);
    }