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首先想到：結果不正確！

var data0 = db.T_Plants2; //這里加.AsQueryable()if (locationType == 1){.Where(d => d.NaturalEcosystem == true);}else{.Where(d => d.BuiltUpArea == true);}.Select(d => new{AnimalID = d.PlantID,Species = d.Species,}).ToList();

一、納姆達方式

var data0 = db.T_Plants2.AsQueryable(); //這里加.AsQueryable()if (locationType == 1){data0= data0.Where(d => d.NaturalEcosystem == true);}else{data0 = data0.Where(d => d.BuiltUpArea == true);}var data2= data0.Select(d => new{AnimalID = d.PlantID,Species = d.Species,}).ToList();// 這里Iqueryable 無法轉化為list必須用新對象data2接收；

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三元表達式：

//var data2 = db.T_Plants2
// .Where(d=> locationType == 1 ? d.NaturalEcosystem == true : d.BuiltUpArea == true)
// .Select(d => new//{// AnimalID = d.PlantID,// Species = d.Species,// Genus = d.Genus,// Family = d.Family//}).ToList();

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二、Linq方式

//案例一

from p in db.productsselect new{Owner = (p.price > 0 ?from q in db.Users select q.Name :from r in db.ExternalUsers select r.Name)}

?//案例二

from p in db.products if p.price>0 select new {Owner=from q in db.Usersselect q.Name } else select new {Owner = from r in db.ExternalUsersselect r.Name }

?//案例三

private string getValue(float price) {if(price >0)return "debit";return "credit"; }select new {p.PriceID, Type = getValue(p.Price)};

?三、拓展

多條件查詢，自己構造 filter條件

View Code

例如：

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//對某一字段的查詢，查詢關鍵字是兩個或兩個以上的字符串，但查出來的相鄰兩個關鍵字的間距必須在20個字符以內 void Main() {var keys = new List<string>();keys.Add("女子");keys.Add("須發"); // keys.Add("如男子");var filters = new List<System.Linq.Expressions.Expression<Func<LINQPad.User.CM_BookPageDetail, bool>>>();foreach(var key in keys)filters.Add(item=>item.Content.Contains(key));for(var i=0;i<keys.Count ;i++){if(i>0){var lastKey = keys[i-1];var currentKey = keys[i];filters.Add(item=> item.Content.IndexOf(currentKey) - item.Content.IndexOf(lastKey) < 20);}}var query = from item in CM_BookPageDetailsselect item;foreach(var filter in filters)query = query.Where(filter);query.Take(10).Dump(); }

?多條件查詢通用方法，重構 where、orderby 方法

https://www.cnblogs.com/hao-1234-1234/p/11271840.html

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參考文章

http://www.it1352.com/401595.html

https://stackoverflow.com/questions/443044/if-else-in-linq

https://q.cnblogs.com/q/73479/

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轉載于:https://www.cnblogs.com/hao-1234-1234/p/11271672.html

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