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How to understand. NET 3.5 extension methods and Lambda expressions, I believe that many inexperienced people do not know what to do, so this article summarizes the causes of the problem and solutions, through this article I hope you can solve this problem.

For the simplification requirements above, you can write something like this using Lambda expressions and the built-in .NET 3.5 extension method:

Static List

< int>

EvenSquareLambda (IEnumerable

< int>

Source) {return source.Where (I = > I% 2 = = 0) .Select (I = > I * I) .ToList ();}

Delay effect of the .NET 3.5 extension method

Many friends should already know that extension methods have a "delayed" effect when dealing with collections in NET 3.5, that is, delegates (two Lambda expressions) in Where and Select are executed only when the ToList method is called. This is both an advantage and a trap, and we still need to know how effective these methods are when using these methods. However, these methods actually do not have any "tricks". In other words, their behavior is consistent with the results of our normal thinking. If you can figure it out, you can write a similar method yourself, or you can "justify yourself", nine times out of ten there will be no deviation. But if you don't understand how they are constructed, make sure through experiments. The way of the experiment is actually very simple, as long as we used to verify the "double counting" trap, that is, to observe the timing and order of delegate execution to judge.

Okay, let's get back to our current problem. We know the "delay" effect, and we know that Where and Select will not deal with it until ToList. However, how they are handled is to create temporary containers, such as List, as our "normal method" does.

< T>

And fill in the return "? we don't analyze this much, but we find the answer by" observing the timing and order in which the delegate executes. "the key to using this approach is to print out some information when the delegate is executed. To do this, we need such a Wrap method (you can also use this method when you do your own experiments):

Static Func

< T, TResult>

Wrap

< T, TResult>

(Func

< T, TResult>

Func, string messgaeFormat) {return I = > {var result = func (I); Console.WriteLine (messgaeFormat, I, result); return result;};}

The purpose of the Wrap method is to set a Func

< T, TResult>

The delegate object is encapsulated and a delegate object of the same type is returned. Each time the encapsulated delegate is executed, the delegate object we provide is executed and the output is formatted according to the messageFormat we passed. For example:

Var wrapper = Wrap

< int, int>

(I = > I + 1, "{0} + 1 = {1}"); for (var I = 0; I

< 3; i++) wrapper(i); 则会输出： 0 + 1 = 1 1 + 1 = 2 2 + 1 = 3 那么，我们下面这段代码会打印出什么内容呢？ List< int>

Source = new List

< int>

(); for (var I = 0; I

< 10; i++) source.Add(i); var finalSource = source .Where(Wrap< int, bool>

(I = > I% 3 = = 0, "{0} can be divided by 3? {1}") .Select (Wrap

< int, int>

(I = > I * I, "The square of {0} equals {1}.") .Where (Wrap

< int, bool>

(I = > I% 2 = = 0, "The result {0} can be devided by 2? {1}")); Console.WriteLine ("= Start ="); foreach (var item in finalSource) {Console.WriteLine ("= Print {0} =", item);}

We prepare a list of ten elements from 0 to 9 and Where them. Select... Where processing, can you guess the content on the screen after foreach?

= Start = 0 can be divided by 3? True The square of 0 equals 0. The result 0 can be devided by 2? True = Print 0 = 1 can be divided by 3? False 2 can be divided by 3? False 3 can be divided by 3? True The square of 3 equals 9. The result 9 can be devided by 2? False 4 can be divided by 3? False 5 can be divided by 3? False 6 can be divided by 3? True The square of 6 equals 36. The result 36 can be devided by 2? True = Print 36 = 7 can be divided by 3? False 8 can be divided by 3? False 9 can be divided by 3? True The square of 9 equals 81. The result 81 can be devided by 2? False

The order in which the elements in the list are executed is as follows:

* element "0" passes through Where... Select... Where,*** is Print out.

The second element "1" is aborted by Where.

The third element "2" is aborted by Where.

The fourth element "4" passes through Where... Select... Where, abort.

……

The magic of the .NET 3.5 extension method

This shows that we use the Where or Select methods that come with the .NET Framework, and the final effect is similar to the "merge loop" in the previous section. Because, if a temporary container is created to hold elements, all elements will be executed by * * delegates in * Where (I = > I% 3 = = 0), and then the filtered elements will be handed over to delegates in Select (I = > I * I) for execution. Note that here the effect of the "merge loop" is hidden from the outside, and our code still seems to process the collection step by step. In other words, we use the clear way of "decomposing loops", but get an efficient implementation of "merge loops". This is the magic of these extension methods of the .NET Framework 1.

Before we do any specific performance testing, let's think again, which GoF 23 pattern is implemented by so many IEnumerable objects here? Enumerator? When you see IEnumerable, you say the enumerator is too cliche. In fact, the "decorator" mode is also used here. After each Where or Select, a new IEnumerable object is actually used to encapsulate the original object, so we get a "decorated" effect when we traverse the new enumerator. So, in the future, if someone asks you, "what is the embodiment of the decorator pattern in the .NET Framework?" in addition to the well-known Stream, you can also answer, "some extension methods in the System.Linq.Enumeral class in .NET 3.5." how cool.

After reading the above, do you know how to understand the NET 3.5 extension method and Lambda expression method? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!

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