public static implicit operator Func<T, bool>(Specification<T> spec)
{
    return spec.Function;
}

public static implicit operator Expression<Func<T, bool>>(Specification<T> spec)
{
    return spec.Predicate;
}

public static bool operator true(Specification<T> spec)
{
    return false;
}

public static bool operator false(Specification<T> spec)
{
    return false;
}

public static Specification<T> operator !(Specification<T> spec)
{
    return new Specification<T>(
        Expression.Lambda<Func<T, bool>>(
            Expression.Not(spec.Predicate.Body),
            spec.Predicate.Parameters));
}

public static Specification<T> operator &(Specification<T> left, Specification<T> right)
{
    var leftExpr = left.Predicate;
    var rightExpr = right.Predicate;
    var leftParam = leftExpr.Parameters[0];
    var rightParam = rightExpr.Parameters[0];

    return new Specification<T>(
        Expression.Lambda<Func<T, bool>>(
            Expression.AndAlso(
                leftExpr.Body,
                new ParameterReplacer(rightParam, leftParam).Visit(rightExpr.Body)),
            leftParam));
}

public static Specification<T> operator |(Specification<T> left, Specification<T> right)
{
    var leftExpr = left.Predicate;
    var rightExpr = right.Predicate;
    var leftParam = leftExpr.Parameters[0];
    var rightParam = rightExpr.Parameters[0];

    return new Specification<T>(
        Expression.Lambda<Func<T, bool>>(
            Expression.OrElse(
                leftExpr.Body,
                new ParameterReplacer(rightParam, leftParam).Visit(rightExpr.Body)),
            leftParam));
}

internal class ParameterReplacer : ExpressionVisitor
{
    readonly ParameterExpression _parameter;
    readonly ParameterExpression _replacement;

    public ParameterReplacer(ParameterExpression parameter, ParameterExpression replacement)
    {
        _parameter = parameter;
        _replacement = replacement;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        return base.VisitParameter(_parameter == node ? _replacement : node);
    }
}

public class SpecificationExpander : ExpressionVisitor
{
    protected override Expression VisitUnary(UnaryExpression node)
    {
        if (node.NodeType == ExpressionType.Convert)
        {
            MethodInfo method = node.Method;

            if (method != null && method.Name == "op_Implicit")
            {
                Type declaringType = method.DeclaringType;

                if (declaringType.GetTypeInfo().IsGenericType
                    && declaringType.GetGenericTypeDefinition() == typeof(Specification<>))
                {
                    const string name = nameof(Specification<object>.ToExpression);

                    MethodInfo toExpression = declaringType.GetMethod(name);

                    return ExpandSpecification(node.Operand, toExpression);
                }
            }
        }

        return base.VisitUnary(node);
    }

    protected override Expression VisitMethodCall(MethodCallExpression node)
    {
        MethodInfo method = node.Method;

        if (method.Name == nameof(ISpecification<object>.ToExpression))
        {
            Type declaringType = method.DeclaringType;
            Type[] interfaces = declaringType.GetTypeInfo().GetInterfaces();

            if (interfaces.Any(i => i.GetTypeInfo().IsGenericType
                && i.GetGenericTypeDefinition() == typeof(ISpecification<>)))
            {
                return ExpandSpecification(node.Object, method);
            }
        }

        return base.VisitMethodCall(node);
    }

    private Expression ExpandSpecification(Expression instance, MethodInfo toExpression)
    {
        return Visit((Expression)GetValue(Expression.Call(instance, toExpression)));
    }

    // http://stackoverflow.com/a/2616980/1402923
    private static object GetValue(Expression expression)
    {
        var objectMember = Expression.Convert(expression, typeof(object));
        var getterLambda = Expression.Lambda<Func<object>>(objectMember);
        return getterLambda.Compile().Invoke();
    }
}

public static IQueryable<T> AsExpandable<T>(this IQueryable<T> queryable)
{
    return queryable.AsVisitable(new ExtensionExpander());
}

public static IQueryable<T> AsVisitable<T>(
    this IQueryable<T> queryable, params ExpressionVisitor[] visitors)
{
    return queryable as VisitableQuery<T>
        ?? VisitableQueryFactory<T>.Create(queryable, visitors);
}

internal class VisitableQuery<T> : IQueryable<T>, IOrderedQueryable<T>, IOrderedQueryable
{
    readonly ExpressionVisitor[] _visitors;
    readonly IQueryable<T> _queryable;
    readonly VisitableQueryProvider<T> _provider;

    internal ExpressionVisitor[] Visitors => _visitors;
    internal IQueryable<T> InnerQuery => _queryable;

    public VisitableQuery(IQueryable<T> queryable, params ExpressionVisitor[] visitors)
    {
        _queryable = queryable;
        _visitors = visitors;
        _provider = new VisitableQueryProvider<T>(this);
    }

    Expression IQueryable.Expression => _queryable.Expression;

    Type IQueryable.ElementType => typeof(T);

    IQueryProvider IQueryable.Provider => _provider;

    public IEnumerator<T> GetEnumerator()
    {
        return _queryable.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return _queryable.GetEnumerator();
    }
}

internal class VisitableQueryProvider<T> : IQueryProvider
{
    readonly VisitableQuery<T> _query;

    public VisitableQueryProvider(VisitableQuery<T> query)
    {
        _query = query;
    }

    IQueryable<TElement> IQueryProvider.CreateQuery<TElement>(Expression expression)
    {
        expression = _query.Visitors.Visit(expression);
        return _query.InnerQuery.Provider
            .CreateQuery<TElement>(expression)
            .AsVisitable(_query.Visitors);
    }

    IQueryable IQueryProvider.CreateQuery(Expression expression)
    {
        expression = _query.Visitors.Visit(expression);
        return _query.InnerQuery.Provider.CreateQuery(expression);
    }

    TResult IQueryProvider.Execute<TResult>(Expression expression)
    {
        expression = _query.Visitors.Visit(expression);
        return _query.InnerQuery.Provider.Execute<TResult>(expression);
    }

    object IQueryProvider.Execute(Expression expression)
    {
        expression = _query.Visitors.Visit(expression);
        return _query.InnerQuery.Provider.Execute(expression);
    }
}

internal static class VisitorExtensions
{
    public static Expression Visit(this ExpressionVisitor[] visitors, Expression node)
    {
        if (visitors != null)
        {
            foreach (var visitor in visitors)
            {
                node = visitor.Visit(node);
            }
        }
        return node;
    }
}

public class ExtensionExpander : ExpressionVisitor
{
    protected override Expression VisitMethodCall(MethodCallExpression node)
    {
        MethodInfo method = node.Method;

        if (method.IsDefined(typeof(ExtensionAttribute), true)
            && method.IsDefined(typeof(ExpandableAttribute), true))
        {
            ParameterInfo[] methodParams = method.GetParameters();
            Type queryableType = methodParams.First().ParameterType;
            Type entityType = queryableType.GetGenericArguments().Single();

            object inputQueryable = MakeEnumerableQuery(entityType);

            object[] arguments = new object[methodParams.Length];

            arguments[0] = inputQueryable;

            var argumentReplacements = new List<KeyValuePair<string, Expression>>();

            for (int i = 1; i < methodParams.Length; i++)
            {
                try
                {
                    arguments[i] = GetValue(node.Arguments[i]);
                }
                catch (InvalidOperationException)
                {
                    ParameterInfo paramInfo = methodParams[i];
                    Type paramType = paramInfo.GetType();

                    arguments[i] = paramType.GetTypeInfo().IsValueType
                        ? Activator.CreateInstance(paramType) : null;

                    argumentReplacements.Add(
                        new KeyValuePair<string, Expression>(paramInfo.Name, node.Arguments[i]));
                }
            }

            object outputQueryable = method.Invoke(null, arguments);

            Expression expression = ((IQueryable)outputQueryable).Expression;

            Expression realQueryable = node.Arguments[0];

            if (!typeof(IQueryable).IsAssignableFrom(realQueryable.Type))
            {
                MethodInfo asQueryable = _asQueryable.MakeGenericMethod(entityType);
                realQueryable = Expression.Call(asQueryable, realQueryable);
            }

            expression = new ExtensionRebinder(
                inputQueryable, realQueryable, argumentReplacements).Visit(expression);

            return Visit(expression);
        }
        return base.VisitMethodCall(node);
    }

    private static object MakeEnumerableQuery(Type entityType)
    {
        return _queryableEmpty.MakeGenericMethod(entityType).Invoke(null, null);
    }

    private static readonly MethodInfo _asQueryable = typeof(Queryable)
        .GetMethods(BindingFlags.Static | BindingFlags.Public)
        .First(m => m.Name == nameof(Queryable.AsQueryable) && m.IsGenericMethod);

    private static readonly MethodInfo _queryableEmpty = (typeof(ExtensionExpander))
        .GetMethod(nameof(QueryableEmpty), BindingFlags.Static | BindingFlags.NonPublic);

    private static IQueryable<T> QueryableEmpty<T>()
    {
        return Enumerable.Empty<T>().AsQueryable();
    }

    // http://stackoverflow.com/a/2616980/1402923
    private static object GetValue(Expression expression)
    {
        var objectMember = Expression.Convert(expression, typeof(object));
        var getterLambda = Expression.Lambda<Func<object>>(objectMember);
        return getterLambda.Compile().Invoke();
    }
}

internal class ExtensionRebinder : ExpressionVisitor
{
    readonly object _originalQueryable;
    readonly Expression _replacementQueryable;
    readonly List<KeyValuePair<string, Expression>> _argumentReplacements;

    public ExtensionRebinder(
        object originalQueryable, Expression replacementQueryable,
        List<KeyValuePair<string, Expression>> argumentReplacements)
    {
        _originalQueryable = originalQueryable;
        _replacementQueryable = replacementQueryable;
        _argumentReplacements = argumentReplacements;
    }

    protected override Expression VisitConstant(ConstantExpression node)
    {
        return node.Value == _originalQueryable ? _replacementQueryable : node;
    }

    protected override Expression VisitMember(MemberExpression node)
    {
        if (node.NodeType == ExpressionType.MemberAccess
            && node.Expression.NodeType == ExpressionType.Constant
            && node.Expression.Type.GetTypeInfo().IsDefined(typeof(CompilerGeneratedAttribute)))
        {
            string argumentName = node.Member.Name;

            Expression replacement = _argumentReplacements
                .Where(p => p.Key == argumentName)
                .Select(p => p.Value)
                .FirstOrDefault();

            if (replacement != null)
            {
                return replacement;
            }
        }
        return base.VisitMember(node);
    }
}

public class DatabaseQueryBenchmark
{
    readonly DbConnection _connection = Context.CreateConnection();

    [Benchmark(Baseline = true)]
    public object RawQuery()
    {
        using (var context = new Context(_connection))
        {
            DateTime today = DateTime.Now.Date;

            return context.Users
                .Where(u => u.Posts.Any(p => p.Date > today))
                .FirstOrDefault();
        }
    }

    [Benchmark]
    public object ExpandableQuery()
    {
        using (var context = new Context(_connection))
        {
            return context.Users
                .AsExpandable()
                .Where(u => u.Posts.FilterToday().Any())
                .ToList();
        }
    }

    readonly Random _random = new Random();

    [Benchmark]
    public object NotCachedQuery()
    {
        using (var context = new Context(_connection))
        {
            int[] postIds = new[] { _random.Next(), _random.Next() };

            return context.Users
                .Where(u => u.Posts.Any(p => postIds.Contains(p.Id)))
                .ToList();
        }
    }
}

          Method |        Median |     StdDev | Scaled | Scaled-SD |
---------------- |-------------- |----------- |------- |---------- |
        RawQuery |   555.6202 ?s | 15.1837 ?s |   1.00 |      0.00 |
 ExpandableQuery |   644.6258 ?s |  3.7793 ?s |   1.15 |      0.03 | <<<
  NotCachedQuery | 2,277.7138 ?s | 10.9754 ?s |   4.06 |      0.10 |

• https://martinfowler.com/eaaCatalog/repository.html
• https://softwareengineering.stackexchange.com/questions/180851/why-shouldnt-i-use-the-repository-pattern-with-entity-framework
• http://enterprisecraftsmanship.com/2016/02/08/specification-pattern-c-implementation
• https://aspnetboilerplate.com/Pages/Documents/Repositories
• https://aspnetboilerplate.com/Pages/Documents/Specifications
• http://www.albahari.com/nutshell/linqkit.aspx
• https://msdn.microsoft.com/en-us/library/aa691312(v=vs.71).aspx