背景:在接下来的一个月里，我将做三次关于LINQ的演讲，或者至少将LINQ包含在c#的上下文中。我想知道哪些话题值得花相当多的精力，这取决于人们可能很难理解哪些话题，或者他们可能有错误的印象。我不会具体讨论LINQ to SQL或实体框架，只是作为如何使用表达式树(通常是IQueryable)远程执行查询的示例。
那么，你发现LINQ有什么难的地方吗?在误解方面你看到了什么?例子可能是以下任何一个，但请不要限制自己!
c#编译器如何处理查询表达式 Lambda表达式 表达式树 扩展方法 匿名类型 这个IQueryable 延迟执行与立即执行 流与缓冲执行(例如，OrderBy被延迟但被缓冲) 隐式类型局部变量 读取复杂的泛型签名(例如Enumerable.Join)
当前回答
The fact that you can't chain IQueryable because they are method calls (while still nothing else but SQL translateable!) and that it is almost impossible to work around it is mindboggling and creates a huge violation of DRY. I need my IQueryable's for ad-hoc in which I don't have compiled queries (I only have compiled queries for the heavy scenarios), but in compiled queries I can't use them and instead need to write regular query syntax again. Now I'm doing the same subqueries in 2 places, need to remember to update both if something changes, and so forth. A nightmare.
其他回答
一些错误消息，特别是从LINQ到SQL的错误消息可能非常令人困惑。露齿而笑
和其他人一样，我也被推迟执行的问题困扰过几次。我认为对我来说最困惑的事情是SQL Server查询提供程序，以及你能用它做什么和不能做什么。
我仍然惊讶于你不能在有时为空的小数/货币列上执行Sum()。使用DefaultIfEmpty()是行不通的。：(
正如大多数人所说，我认为最被误解的部分是假设LINQ只是T-SQL的替代品。我的经理认为自己是一个TSQL专家，他不让我们在项目中使用LINQ，甚至讨厌微软发布这样的东西!!
The fact that you can't chain IQueryable because they are method calls (while still nothing else but SQL translateable!) and that it is almost impossible to work around it is mindboggling and creates a huge violation of DRY. I need my IQueryable's for ad-hoc in which I don't have compiled queries (I only have compiled queries for the heavy scenarios), but in compiled queries I can't use them and instead need to write regular query syntax again. Now I'm doing the same subqueries in 2 places, need to remember to update both if something changes, and so forth. A nightmare.
表达式<Func<T1, T2, T3，…>> and Func<T1, T2, T3，…>，没有给出第二个情况下性能下降的提示。
下面是代码示例，演示了我的意思:
[TestMethod]
public void QueryComplexityTest()
{
var users = _dataContext.Users;
Func<User, bool> funcSelector = q => q.UserName.StartsWith("Test");
Expression<Func<User, bool>> expressionSelector = q => q.UserName.StartsWith("Test");
// Returns IEnumerable, and do filtering of data on client-side
IQueryable<User> func = users.Where(funcSelector).AsQueryable();
// Returns IQuerible and do filtering of data on server side
// SELECT ... FROM [dbo].[User] AS [t0] WHERE [t0].[user_name] LIKE @p0
IQueryable<User> exp = users.Where(expressionSelector);
}
大O符号。LINQ使得编写O(n^4)算法变得非常容易，如果你不知道自己在做什么的话。
