Lambda動的条件クエリー

using System;

using System.Linq;

using System.Linq.Expressions;

using System.Collections.Generic;

    public static class PredicateBuilder

    {

        public static Expression<Func<T, bool>> True<T>() { return f => true; }

        public static Expression<Func<T, bool>> False<T>() { return f => false; }



        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,

                                                            Expression<Func<T, bool>> expr2)

        {

            var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());

            return Expression.Lambda<Func<T, bool>>

                  (Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);

        }



        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,

                                                             Expression<Func<T, bool>> expr2)

        {

            var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());

            return Expression.Lambda<Func<T, bool>>

                  (Expression.AndAlso(expr1.Body, invokedExpr), expr1.Parameters);

        }

        public IList<Memo> GetMemoList(string department, bool includePublic)

        {

            var filterExpression = PredicateBuilder.True<Memo>();

            filterExpression = filterExpression.And(x => x.ID > 0);



            if (department != "")

            {

                filterExpression = filterExpression.And(x => x.Department == department);

            }



            if (includePublic)

            {

                var orExpression = PredicateBuilder.False<Memo>();

                orExpression = orExpression.Or(x => x.IsPublic == true);

                filterExpression = filterExpression.Or(orExpression);

            }



            return db.Memo.Where(filterExpression.Compile())

                .OrderByDescending(x => x.IsTop)

                .ThenByDescending(x => x.ID).ToList();

        }

 
Ref: Dynamically Composing Expression Predicates