using System;

using System.Linq;

using System.Linq.Expressions;

​

public class Program

{

    public static void Main()

    {

        InnerNotCompiled();

        InnerCompiled();

    }

    private static void InnerNotCompiled() {

        Type propType = typeof(string);

        var whereParam = Expression.Parameter(propType, "whereParam");

        var whereExp = Expression.Call(typeof(System.Linq.Enumerable).GetMethods().First(a => a.Name == "Where" && /*Func<TSource,TResult>*/ a.GetParameters().Last().ParameterType.GenericTypeArguments.Length == 2).MakeGenericMethod(propType),

                                       Expression.Constant(new[] { "test", "test2" }), 

                                       Expression.Lambda(Expression.NotEqual(Expression.Property(whereParam, "Length"), Expression.Constant(4)), whereParam));

        //build select

        var selectParam = Expression.Parameter(propType, "selectParam");

        var selectExp = Expression.Call(typeof(System.Linq.Enumerable).GetMethods().First(a => a.Name == "Select" && /*Func<TSource,TResult>*/ a.GetParameters().Last().ParameterType.GenericTypeArguments.Length == 2).MakeGenericMethod(propType, typeof(int)),

                                        whereExp,

                                        Expression.Lambda(Expression.Property(selectParam, "Length"), selectParam));

        //first or default

        var exp = Expression.Call(typeof(System.Linq.Enumerable).GetMethods().Where(a => a.Name == "FirstOrDefault" && a.ContainsGenericParameters && a.GetGenericArguments().Length == 1 && a.GetParameters().Length == 1).Single().MakeGenericMethod(typeof(int)), selectExp);

        var f = (Func<int>)Expression.Lambda(exp).Compile();

        Console.WriteLine(f());

    }

    private static void InnerCompiled() {

        Type propType = typeof(string);

        var whereParam = Expression.Parameter(propType, "whereParam");

        var whereExp = Expression.Call(typeof(System.Linq.Enumerable).GetMethods().First(a => a.Name == "Where" && /*Func<TSource,TResult>*/ a.GetParameters().Last().ParameterType.GenericTypeArguments.Length == 2).MakeGenericMethod(propType),

                                       Expression.Constant(new[] { "test", "test2" }), 

                                       Expression.Constant(Expression.Lambda(Expression.NotEqual(Expression.Property(whereParam, "Length"), Expression.Constant(4)), whereParam).Compile())); //compile

        //build select

        var selectParam = Expression.Parameter(propType, "selectParam");

        var selectExp = Expression.Call(typeof(System.Linq.Enumerable).GetMethods().First(a => a.Name == "Select" && /*Func<TSource,TResult>*/ a.GetParameters().Last().ParameterType.GenericTypeArguments.Length == 2).MakeGenericMethod(propType, typeof(int)),

                                        whereExp,

                                        Expression.Constant(Expression.Lambda(Expression.Property(selectParam, "Length"), selectParam).Compile())); //compile

        //first or default

        var exp = Expression.Call(typeof(System.Linq.Enumerable).GetMethods().Where(a => a.Name == "FirstOrDefault" && a.ContainsGenericParameters && a.GetGenericArguments().Length == 1 && a.GetParameters().Length == 1).Single().MakeGenericMethod(typeof(int)), selectExp);

        var f = (Func<int>)Expression.Lambda(exp).Compile();

        Console.WriteLine(f());

    }

}