What's the hardest or most misunderstood aspect of LINQ?

Background: Over the next month, I'll be giving three talks about or at least including LINQ in the context of C# . I'd like to know which topics are worth giving a fair amount of attention to, based on what people may find hard to understand, or what they may have a mistaken impression of. I won't be specifically talking about LINQ to SQL or the Entity Framework except as examples of how queries can be executed remotely using expression trees (and usually IQueryable ).

So, what have you found hard about LINQ ? What have you seen in terms of misunderstandings? Examples might be any of the following, but please don't limit yourself!

How the C# compiler treats query expressions

Lambda expressions

Expression trees

Extension methods

Anonymous types

IQueryable

Deferred vs immediate execution

Streaming vs buffered execution (eg OrderBy is deferred but buffered)

Implicitly typed local variables

Reading complex generic signatures (eg Enumerable.Join)

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延迟执行

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I know the deferred execution concept should be beaten into me by now, but this example really helped me get a practical grasp of it:

static void Linq_Deferred_Execution_Demo()
{
    List<String> items = new List<string> { "Bob", "Alice", "Trent" };

    var results = from s in items select s;

    Console.WriteLine("Before add:");
    foreach (var result in results)
    {
        Console.WriteLine(result);
    }

    items.Add("Mallory");

    //
    //  Enumerating the results again will return the new item, even
    //  though we did not re-assign the Linq expression to it!
    //

    Console.WriteLine("nAfter add:");
    foreach (var result in results)
    {
        Console.WriteLine(result);
    }
}

The above code returns the following:

Before add:
Bob
Alice
Trent

After add:
Bob
Alice
Trent
Mallory

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除了LINQ to SQL ，这些功能不仅仅是嵌入在语言中的SQL解析器。

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