Evaluation of lambda expression which is stored in string parameter

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Please check below code

C#

List<Emp> empD = new List<Emp> { new Emp { Name = "a", age = 1 }, new Emp { Name = "b", age = 2 } };
            var test = "empD.Where(x=>x.age ==1 )";
            var param1 = Expression.Parameter(typeof(Emp), "test");
            var lambda = System.Linq.Dynamic.DynamicExpression.ParseLambda(new[] { param1 }, typeof(Emp), test);
            var rest = lambda.Compile();

Here i am trying to evaluate lambda expression but input will be given in string param , which stores expression.How can i evaluate to get list of entity.

Thanks in Advance
Jayanth

Posted 14-Dec-14 22:24pm

Member 8118083

Updated 14-Dec-14 22:55pm

Tomas Takac

v3

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Tomas Takac 15-Dec-14 4:57am

Shouldn't the name of the parameter param1 be x?

Member 8118083 15-Dec-14 5:48am

trying to get typeof Emp from the string value "test"; as i tried with giving "x" or "empD" it gives me an exception.

Thinking to solve this in different approach (may be by changing tree to iqueryable).But not sure what extent its correct.

Thanks

Maciej Los 15-Dec-14 5:47am

Why?

BillWoodruff 15-Dec-14 9:37am

Thanks for the heads-up on my response here: I really missed the boat on this one :)

Maciej Los 15-Dec-14 10:12am

;) You're very welcome, Bill ;)

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Maciej Los · Answer 1 · 2014-12-15T04:19:00

Please, read my comment to the question. I asked there: why do you want to evaluate lambda expression. You didn't answered. Nevertheless...

In a few words: lambda expression is nothing else than inline delegate!

Have a look here:
Func<T1, T2, TResult> Delegate[^]
Exploring Lambda Expression in C#[^]

Conclusion: you can use it even as a parameter of function.
See:
Dynamic Queries and LINQ Expressions[^]
http://stackoverflow.com/questions/14297633/c-sharp-pass-lambda-expression-as-method-parameter[^]

Example (works like a charm):

C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LinqDelegate
{
    class Program
    {
        static void Main(string[] args)
        {
            List<Emp> empD = new List<Emp> { new Emp("a", 1),
                                    new Emp("b", 2),
                                    new Emp("c", 1),
                                    new Emp("d", 4) };

            Func<Emp, int, bool> predicate = (e, i) => e.Age==1;

            var qry = empD.Where(predicate).Select(x => x);
            foreach (var e in qry)
                Console.WriteLine("{0} = {1}", e.Name, e.Age);

            Console.ReadKey();

        }

        class Emp
        {
            private string sName = String.Empty;
            private int iAge = 0;

            public Emp(string _Name, int _Age)
            {
                sName = _Name;
                iAge = _Age;
            }

            public string Name
            {
                get { return sName; }
                set { sName = value; }
            }

            public int Age
            {
                get { return iAge; }
                set { iAge = value; }
            }
        }
    }
}

Result:

a = 1
c = 1