public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
        {
            var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());
            return Expression.Lambda<Func<T, bool>>(Expression.Or(expression1.Body, invokedExpression), expression1.Parameters);
        }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expression1, Expression<Func<T, bool>> expression2)
        {
            var invokedExpression = Expression.Invoke(expression2, expression1.Parameters.Cast<Expression>());
            return Expression.Lambda<Func<T, bool>>(Expression.And(expression1.Body, invokedExpression), expression1.Parameters);
        }

用法：

  List<EasyuiFilter> filter = Newtonsoft.Json.JsonConvert.DeserializeObject<List<EasyuiFilter>>(filterRules);
                Expression<Func<WebError, bool>> lmd = c => c.ID != 0;

                if (filter.Any(c => c.Field == "Code"))
                {
                    EasyuiFilter item = filter.Where(f => f.Field == "Code").FirstOrDefault();
                    if (item.Op == "equal")
                    {
                        lmd = lmd.And(c => c.Code == Convert.ToInt32(item.Value));
                    }
                    else
                    {
                        lmd = lmd.And(c => c.Code != Convert.ToInt32(item.Value));
                    }
                }

                if (filter.Any(c => c.Field == "IP"))
                {
                    EasyuiFilter item = filter.Where(f => f.Field == "IP").FirstOrDefault();
                    if (item.Op == "equal")
                    {
                        lmd = lmd.And(c => c.IP == item.Value);
                    }
                    else
                    {
                        lmd = lmd.And(c => c.IP != item.Value);
                    }
                }

//使用lamada变量
_context.WebErrors.Where(lmd).Count()