Apply a dynamically created lambda to an object instance

c# expression expression-trees lambda

Question

I have some code which dynamically create a lambda starting from strings. For example, I have a filter class like the following:

public class Criteria {
    public string Property { get; set; }
    public string Operator { get; set; }
    public string Value { get; set; }
}

And I am able to create a lambda like x => x.Name == "Foo" starting from a Criteria instance like this

Criteria c = new Criteria() {
    Property = "Name",
    Operator = "equal",
    Value = "Foo"
}

Supposing to have a class like

public class Receipt {
    public string Name { get; set; }
    public int Amount { get; set; }
    [other props omitted]
    public ICollection<ReceiptDetail> Details { get; set; }
}

I would like to:

  1. Apply the lambda to any object (I know that the lambda should be created with a ParameterExpression of the Receipt class)
  2. Getting back the boolean result of the lambda (e.g. Is the name equals to Foo?)
  3. Apply the same logic to collections Count() method (e.g. Building a lambda which checks against receipt.Details.Count()

Is this possible?

EDIT: As per the comments, I am elaborating my needs a bit more. This code will give me the chance to answer to a requirement that I have and that says: If there is a rule specified for my object then the application should behave a bit differently. While this is a common requirement, I would like to create a code which will permit me to extend it as more rules will be added. Actually I only have 5 rule types:

  • Verify if the input comes in a specific day of the week
  • Verify if the input comes in a specific time range
  • Verify if the field "X" of the input is less/equal/greather than a value
  • Verify if the field "Y" of the input contains a value
  • Verify if the field "Z" of the input, which is a collection, has a count that is less/equal/greather than a value

For the first 4 points I have been able to dynamically create a lambda expression, with code like in P.Brian.Mackey answer, which I could apply, using the Specification pattern, to the object itself.

The last point needs to be implemented almost in the same way but the only difference is that the left part of the expression was a method call and not a Property (specifically the ICollection<T>.Count() method)

1
2
5/23/2017 12:12:56 PM

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Accepted Answer

Here's something to get you started. There is plenty of room for improvement. Especially the ugly factory. This demo is intended to show how to use Expressions to solve the problems, not as a best practices or factory pattern demo. If anything is unclear please feel free to ask for clarification.

Usage

    [Test]
    public void ApplySameLogicToCollectionsCount()
    {
        var receipt = new Receipt();
        var details = new ReceiptDetail();
        var details2 = new ReceiptDetail();
        receipt.Details.Add(details);
        receipt.Details.Add(details2);
        var result = LambdaGeneratorFactory<ICollection<ReceiptDetail>>.Run(detailsCount);
        Assert.IsTrue(result(receipt.Details));
    }

Factory

 public static class LambdaGeneratorFactory<T>
    {
        //This is an ugly implementation of a Factory pattern.
        //You should improve this possibly with interfaces, maybe abstract factory.  I'd start with an ICriteria.
        public static Predicate<T> Run(Criteria criteria)
        {
            if (typeof(T) == typeof (Receipt))
            {
                return CreateLambda(criteria);
            }
            else if (typeof (T) == typeof (ICollection<ReceiptDetail>))
            {
                return CreateLambdaWithCount(criteria);
            }

            return null;
        }
        private static Predicate<T> CreateLambda(Criteria criteria)
        {
            ParameterExpression pe = Expression.Parameter(typeof(T), "i");

            Expression left = Expression.Property(pe, typeof(T).GetProperty(criteria.Property));
            Expression right = Expression.Constant(criteria.Value);

            Expression predicateBody = Expression.Equal(left, right);

            var predicate = Expression.Lambda<Predicate<T>>(predicateBody, new ParameterExpression[] { pe }).Compile();

            return predicate;
        }

        private static Predicate<T> CreateLambdaWithCount(Criteria criteria)
        {
            ParameterExpression pe = Expression.Parameter(typeof(T), "i");

            Expression count = Expression.Property(pe, typeof(T).GetProperty("Count"));
            Expression left = Expression.Call(count, typeof(Object).GetMethod("ToString"));
            Expression right = Expression.Constant(criteria.Value);

            Expression predicateBody = Expression.Equal(left, right);

            var predicate = Expression.Lambda<Predicate<T>>(predicateBody, new ParameterExpression[] { pe }).Compile();

            return predicate;
        }
    }

Criteria

    private Criteria detailsCount = new Criteria()
    {
        Property = "Details",
        Operator = "equal",
        Value = "2"
    };

Switch to ICriteria and things will b cleaner. A better factory and no need for ToString. Program to an interface.

All that being said, this code feels a bit funky. What is the point of generating functions from strings? I get the feeling this is heading towards generating C# from a grammar. I'm not convinced that will scale well. For non-trivial implementations consider lex/yacc first. You can find more details for doing this in the Pragmatic Programmer "Implementing a mini language".

3
3/14/2015 4:19:10 AM

Popular Answer

Yours is a fascinating question, and I'd like to understand the requirements. I created a demo, and I'm wondering, how does the demo differ from what you're trying to accomplish? There is a working version here https://dotnetfiddle.net/AEBZ1w too.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;

public class Program
{
    public static void Main()
    {
        Criteria c = new Criteria() { 
            Property = "Name", 
            Operator = "==", 
            Value = "Foo" };

        var queryable = (new List<Receipt>() { 
            new Receipt { Name = "Foo", Amount = 1 },
            new Receipt { Name = "Foo", Amount = 2 }, 
            new Receipt { Name = "Bar" }  
        }).AsQueryable();

        var parameter = Expression.Parameter(typeof(Receipt), "x");
        var property = Expression.Property(parameter, typeof(Receipt).GetProperty(c.Property));
        var constant = Expression.Constant(c.Value);
        var operation = Expression.Equal(property, constant);
        var expression = Expression.Call(
            typeof(Queryable),
            "Where",
            new Type[] { queryable.ElementType },
            queryable.Expression, 
            Expression.Lambda<Func<Receipt, bool>>(operation, new ParameterExpression[] { parameter })
        );

        Console.WriteLine("Linq Expression: {0} \n", expression.ToString());
        Console.WriteLine("Results: \n");

        var results = queryable.Provider.CreateQuery<Receipt>(expression);
        foreach(var r in results)
        {
            Console.WriteLine("{0}:{1}", r.Name, r.Amount);
        }
    }
}

public class Criteria
{
    public string Property, Operator, Value;
}

public class ReceiptDetail
{
    public string ItemName;
}

public class Receipt
{
    public string Name { get; set; }
    public int Amount;
    public ICollection<ReceiptDetail> Details;
}

References

Shaun Luttin

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