How can I dynamically evaluate LINQ expressions from strings in C#?

Dynamically Evaluating LINQ Expressions from Strings

Problem:

Given an input string representing a LINQ expression, such as "(Person.Age > 3 AND Person.Weight > 50) OR Person.Age < 3", the goal is to dynamically create a Func that can be evaluated against an instance of the Person class.

Overbaked Approach:

The proposed solution involving ANTLR grammar and the Predicate Builder framework may be unnecessarily complex.

Alternative Solution: Dynamic LINQ

The Dynamic LINQ library provides an elegant and straightforward solution. Here's how to use it:

using System;
using System.Linq.Expressions;
using System.Linq.Dynamic;

namespace ExpressionParser
{
    class Program
    {
        public class Person
        {
            public string Name { get; set; }
            public int Age { get; set; }
            public int Weight { get; set; }
            public DateTime FavouriteDay { get; set; }
        }

        static void Main()
        {
            // Input expression string
            const string exp = @"(Person.Age > 3 AND Person.Weight > 50) OR Person.Age < 3";

            // Compile expression into a lambda expression
            var p = Expression.Parameter(typeof(Person), "Person");
            var e = System.Linq.Dynamic.DynamicExpression.ParseLambda(new[] { p }, null, exp);

            // Create a Person instance
            var bob = new Person
            {
                Name = "Bob",
                Age = 30,
                Weight = 213,
                FavouriteDay = new DateTime(2000, 1, 1)
            };

            // Evaluate expression against the Person instance
            var result = e.Compile().DynamicInvoke(bob);

            // Print result
            Console.WriteLine(result);
            Console.ReadKey();
        }
    }
}

In this example, we dynamically parse the expression string into a lambda expression and then compile it into a Func. We can then evaluate the function against a Person instance to determine whether it matches the specified criteria.

Benefits of Dynamic LINQ:

• Ease of Use: Simple API that allows for dynamic construction of LINQ expressions from strings.
• Versatility: Supports a wide range of expression operators and types.
• Lightweight: Includes only the necessary functionality for parsing and evaluating expressions.

Note: Remember to include the System.Linq.Dynamic nuget package for this code to work.

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