using System;

using System.Collections.Generic;

using System.Linq;

using System.Linq.Expressions;

​

public class Program

{

    private static readonly List<Merchant> Merchants = new List<Merchant> { new Merchant { ExternalId = "1", Name = "Foo" }, new Merchant { ExternalId = "2", Name = "Bar" } };

    public static void Main()

    {

        Console.WriteLine(Get(m => m.ExternalId == "2").Name);

    }

    private static Merchant Get(Expression<Func<MerchantViewModel, bool>> predicate)

    {

        return Merchants.AsQueryable().SingleOrDefault(predicate.CastParameter<MerchantViewModel, Merchant>());

    }

}

​

public class Merchant

{

    public string ExternalId { get; set; } 

    public string Name { get; set; } 

}

​

public class MerchantViewModel

{

    public string ExternalId { get; set; } 

}

​

public static class PredicateMapper

{

    public static Expression<Func<TTo, bool>> CastParameter<TFrom, TTo>(this Expression<Func<TFrom, bool>> predicate)

    {

        var parameter = Expression.Parameter(typeof(TTo));

        var body = new ParameterReplacer<TTo>(parameter).Visit(predicate.Body);

        return Expression.Lambda<Func<TTo, bool>>(body, parameter);

    }

    private class ParameterReplacer<TTo> : ExpressionVisitor

    {

        private readonly ParameterExpression parameter;

        public ParameterReplacer(ParameterExpression parameter)

        {

            this.parameter = parameter;

        }

        protected override Expression VisitParameter(ParameterExpression node)

        {

            return this.parameter;

        }

        protected override Expression VisitMember(MemberExpression node)

        {

            var matchingMember = typeof(TTo).GetProperty(node.Member.Name);

            return Expression.Property(this.Visit(node.Expression), matchingMember);

        }

    }

}