using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Linq.Expressions;

​

namespace LISTING_2_59_Modifying_an_expression_tree

{

​

public class MultiplyToAdd : ExpressionVisitor

{

    public Expression Modify(Expression expression)

    {

        return Visit(expression);

    }

​

    protected override Expression VisitBinary(BinaryExpression b)

    {

        if (b.NodeType == ExpressionType.Multiply)

        {

            Expression left = this.Visit(b.Left);

            Expression right = this.Visit(b.Right);

​

            // Make this binary expression an Add rather than a multiply operation.  

            return Expression.Add(left, right);

        }

​

        return base.VisitBinary(b);

    }

}

​

    public class Program

    {

        public static void Main(string[] args)

        {

            // build the expression tree: Expression<Func<int,int>> square = num => num * num;

​

            // The parameter for the expression is an integer

            ParameterExpression numParam = Expression.Parameter(typeof(int), "num");

​

            // The opertion to be performed is to square the parameter

            BinaryExpression squareOperation = Expression.Multiply(numParam, numParam);

​

            // This creates an expression tree that describes the square operation

            Expression<Func<int, int>> square =

                Expression.Lambda<Func<int, int>>(

                    squareOperation,

                    new ParameterExpression[] { numParam });

​

            // Compile the tree to make an executable method and assign it to a delegate

            Func<int, int> doSquare = square.Compile();

​

            // Call the delegate

            Console.WriteLine("Square of 4: {0}", doSquare(4));

​

            // Modify the expression to replace the multiply with an add

​

            MultiplyToAdd m = new MultiplyToAdd();

​

            Expression<Func<int, int>> addExpression = (Expression<Func<int, int>>)m.Modify(square);

            Func<int, int> doAdd = addExpression.Compile();

​

            Console.WriteLine("Double of 4: {0}", doAdd(4));

        }

    }

}