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首先想到：结果不正确！

            var data0 = db.T_Plants2; //这里加.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、纳姆达方式

            var data0 = db.T_Plants2.AsQueryable(); //这里加.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();// 这里Iqueryable 无法转化为list必须用新对象data2接收；

 

三元表达式：

            //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();

 

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

多条件查询，自己构造 filter条件

例如：

 

//对某一字段的查询，查询关键字是两个或两个以上的字符串，但查出来的相邻两个关键字的间距必须在20个字符以内
void Main()
{var keys = new List<string>();keys.Add("女子");keys.Add("须发");
//    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();
}

 多条件查询通用方法，重构 where、orderby 方法

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

 

参考文章

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

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

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