IEnumerable 

• Is for LINQ to Objects
• Objects that are in memory on Heap

IQueryable 

• Is for working on LINQ to SQL
• Inside it, it using Expressions to frame and execute sql queries.

Func<T>

• Can return a value. The last value in the definition is the return.

Ex: 

Func<int,int> PrintNumber = x => (x);

            Console.WriteLine(PrintNumber(50));

Action<T>

• Cannot return any value. It is only to feed and use directly

Ex:

Action PrintEmpty = () => Console.WriteLine("Hi");

            PrintEmpty();

            Action<int> PrintOneNumber = x => Console.WriteLine(x);

            PrintOneNumber(5);

Expression:

• To be used to help the developer better understand the code.
• It is slightly different from Func<T> and Action<T>.
• Expression takes Func<T> as input and is to be used as a variable and not as a method that is the case with Func or Action

Ex:

Expression<Func<int, int, int>> PrintMulti = (x, y) => (x * y);

            Func<int, int, int> Mutli = PrintMulti.Compile();
            var zExp = Mutli(5, 10);
            Console.WriteLine(z);

A simple example for converting the normal linq query to a Parallel Linq query in dotnet.

public void NormalLinqQuery()

        {

            IEnumerable<int> localRange = Enumerable.Range(1, 100000000);

            Stopwatch watch = Stopwatch.StartNew();

            var output = localRange

                .Where(getNum => getNum % 1234567 == 0)

                .Select(rangeNum => rangeNum);

            foreach (var num in output)

                Console.WriteLine(num);

            Console.WriteLine("Normal Query in millisec: {0}",watch.ElapsedMilliseconds);

        }

        public void ParallelLinqQueryExample()

        {

            IEnumerable<int> localRange = Enumerable.Range(1, 100000000);

            Stopwatch watch = Stopwatch.StartNew();

            var output = localRange

                .AsParallel()

                .Where(getNum => getNum % 1234567 == 0)

                .Select(rangeNum => rangeNum);

            foreach (var num in output)

                Console.WriteLine(num);

            Console.WriteLine("Parallel Query in millisec: {0}", watch.ElapsedMilliseconds);

        }