linq 多条件查询

Linq 进行多条件查询的时候使用PredicateBuilder帮助类可以很好的解决。

类的源码：

 public static class PredicateBuilder
    {

        /// <summary>
        /// 应用True时：单个AND有效，多个AND有效；单个OR无效，多个OR无效；混合使用时写在AND后的OR有效
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <returns></returns>
        public static Expression<Func<T, bool>> True<T>() { return f => true; }

        /// <summary>
        /// 应用False时：单个AND无效，多个AND无效；单个OR有效，多个OR有效；混合使用时写在OR后面的AND有效
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <returns></returns>
        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.Or(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.And(expr1.Body, invokedExpr), expr1.Parameters);
        }
    }

测试方法：   private void testList_True()
        {
            List<Person> listPerson = new List<Person>()
            {
                ,Name=, Birthday=Convert.ToDateTime("1991-01-01") },
                ,Name=, Birthday=Convert.ToDateTime("1992-01-01")},
                ,Name=, Birthday=Convert.ToDateTime("1993-01-01")}
            };
            var where = PredicateBuilder.True<Person>();
            //var  where= PredicateBuilder_Object.True<Person>();

            //string strUserName = "chm1";
            //where = where.And(p => p.Name.Contains(strUserName));

            ));

            string strUserName = "chm1";
            where = where.And(p => p.Name.Contains(strUserName));
            ));
            var reulst = listPerson.AsQueryable().Where(where).ToList();
        }
        private void testList_False()
        {
            List<Person> listPerson = new List<Person>()
            {
                ,Name=, Birthday=Convert.ToDateTime("1991-01-01") },
                ,Name=, Birthday=Convert.ToDateTime("1992-01-01")},
                ,Name=, Birthday=Convert.ToDateTime("1993-01-01")}
            };
            var where = PredicateBuilder.False<Person>();

            string strUserName = "chm1";
            where = where.And(p => p.Name.Contains(strUserName));

            ));
            string strUserName2 = "chm3";
            where = where.And(p => p.Name.Contains(strUserName2));

            var reulst = listPerson.AsQueryable().Where(where).ToList();
        }

测试结果：

true： and (v) 　　and (v)　　or(v)

true： or(x) 　　and (v)　　or(v)

============================

false： or(v) 　　　 or(v)　

false： and (x) 　　and (x)

false： and (x) 　　or(v)　　and(v)