ExpressionTreeBuildingSampleTwo.cs

using System.Linq.Expressions;

using ExpressionVisitor;

namespace ExpressionTreeSamples;

public class ExpressionTreeBuildingSampleTwo : Sample
{
    public override string Name { get; } = "Building Expression Trees, Sample 2: Building Loops and Conditionals";

    public override void Run()
    {
        // This code builds the equivalent of:
        Func<int, int> factorialFunc = (n) =>
        {
            var res = 1;
            while (n > 1)
            {
                res = res * n;
                n--;
            }
            return res;
        };

        var nArgument = Expression.Parameter(typeof(int), "n");
        var result = Expression.Variable(typeof(int), "result");

        // Creating a label that represents the return value
        LabelTarget label = Expression.Label(typeof(int));

        var initializeResult = Expression.Assign(result, Expression.Constant(1));

        // This is the inner block that performs the multiplication,
        // and decrements the value of 'n'
        var block = Expression.Block(
            Expression.Assign(result,
                Expression.Multiply(result, nArgument)),
            Expression.PostDecrementAssign(nArgument)
        );

        // Creating a method body.
        BlockExpression body = Expression.Block(
            new[] { result },
            initializeResult,
            Expression.Loop(
                Expression.IfThenElse(
                    Expression.GreaterThan(nArgument, Expression.Constant(1)),
                    block,
                    Expression.Break(label, result)
                ),
                label
            )
        );

        var factorial = Expression.Lambda(body, nArgument);

        // Compile and run an expression tree.
        var func = (Func<int, int>)factorial.Compile();

        Console.WriteLine(func(5));
        var visitor = Visitor.CreateFromExpression(factorial);
        visitor.Visit("");
    }
}