using System;

using System.Linq.Expressions;

using System.Collections;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var list = new List<int>() { 10, 20, 30 };

        Console.WriteLine("****************** C# Equivalent Output **************************\n\n");

        foreach (var item in list)

        {

            Console.WriteLine(item);

        }

        Console.WriteLine("\n\n***************** Expression Tree Output **************************\n\n");

        var collection = Expression.Parameter(typeof(List<int>), "collection");

        var loopVar = Expression.Parameter(typeof(int), "loopVar");

        var loopBody = Expression.Call(typeof(Console).GetMethod("WriteLine", new[] { typeof(int) }), loopVar);

        var loop = ForEach(collection, loopVar, loopBody);

        var results = Expression.Lambda<Action<List<int>>>(loop, collection).Compile();

        results(list);      

    }

    public static Expression ForEach(Expression collection, ParameterExpression loopVar, Expression loopContent)

    {

        var elementType = loopVar.Type;

        var enumerableType = typeof(IEnumerable<>).MakeGenericType(elementType);

        var enumeratorType = typeof(IEnumerator<>).MakeGenericType(elementType);

        var enumeratorVar = Expression.Variable(enumeratorType, "enumerator");

        var getEnumeratorCall = Expression.Call(collection, enumerableType.GetMethod("GetEnumerator"));

        var enumeratorAssign = Expression.Assign(enumeratorVar, getEnumeratorCall);

        var moveNextCall = Expression.Call(enumeratorVar, typeof(IEnumerator).GetMethod("MoveNext"));

        var breakLabel = Expression.Label("LoopBreak");

        var ifThenElseExpr = Expression.IfThenElse(

            Expression.Equal(moveNextCall, Expression.Constant(true)),

            Expression.Block(new[] { loopVar },

                Expression.Assign(loopVar, Expression.Property(enumeratorVar, "Current")),

                loopContent

            ),

            Expression.Break(breakLabel)

        );

        var loop = Expression.Loop(ifThenElseExpr, breakLabel);

        var block = Expression.Block(new[] { enumeratorVar },

            enumeratorAssign,

            loop

        );

        return block;

    }

}