Dynamic expresso foreach with lambda?

Question

I am using Dynamic-Expresso library. I would like to evaluate expression containing foreach statement. Is that possible? The only way I can see is to write parametrized method and send the whole class as a parameter to the Interpreter

var target = new Interpreter();
target.SetVariable("a", a, typeof(Tools));
target.SetVariable("b", b, typeof(List<param>));
if (target.Eval("a.MethodWithForeach(b)").Equals(true))
{
 ...
}

Answer 1

You are right, for now there isn't an easy way to work with collections. I already have a plan to work on this feature but I don't known an exact release date.

For now, as a workaround, you can write some helpers like the one below:

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.Linq;
using System.Linq.Expressions;
using System.Collections.Generic;

namespace DynamicExpresso.UnitTest
{
    [TestClass]
    public class CollectionHelperTests
    {
        [TestMethod]
        public void Where()
        {
            var target = new Interpreter();

            target.SetVariable("IntCollectionHelper", new CollectionHelper<int>());

            var list = new List<int> { 1, 10, 19, 21 };

            var results = target.Eval("IntCollectionHelper.Where(list, \"x > 19\")", new Parameter("list", list))
                                    as IEnumerable<int>;

            Assert.AreEqual(1, results.Count());
            Assert.AreEqual(21, results.First());
        }
    }

    public class CollectionHelper<T>
    {
        readonly Interpreter _interpreter;

        public CollectionHelper()
        {
            _interpreter = new Interpreter();
        }

        public IEnumerable<T> Where(IEnumerable<T> values, string expression)
        {
            var predicate = _interpreter.Parse<Func<T, bool>>(expression, "x");

            return values.Where(predicate);
        }
    }
}

Basically I have created a little helper to simulate some LINQ functions (Where in the above example). You can then call this helper from your expressions.

Answer 2

Starting with version 2.9.0, Dynamic Expresso supports lambda expressions!

Note: lambda expressions parsing is disabled by default, because it has a slight performance impact. It must be enabled via the InterpreterOptions.LambdaExpressions flag:

var options = InterpreterOptions.Default | InterpreterOptions.LambdaExpressions;
var interpreter = new Interpreter(options);

It's now possible to write something like this:

var options = InterpreterOptions.Default | InterpreterOptions.LambdaExpressions;
var interpreter = new Interpreter(options);

var list = new List<string> { "this", "is", "awesome" };
interpreter.SetVariable("myList", list);

var expr = "myList.Where(str => str.Length > 5).Select(str => str.ToUpper())";
var results = interpreter.Eval<IEnumerable<string>>(expr);

Assert.AreEqual(new[] { "AWESOME" }, results);