C Lambda expressions Why should I use them

Question

I have quickly read over the Microsoft Lambda Expression documentation   This kind of example has helped me to understand better  though    delegate int del int i   del myDelegate   x   gt  x   x  int j   myDelegate 5     j   25   Still  I don t understand why it s such an innovation  It s just a method that dies when the  method variable  ends  right  Why should I use this instead of a real method

User · Answer

This is perhaps the best explanations on why to use lambda expressions -> https://youtu.be/j9nj5dTo54Q

In summary, it's to improve code readability, reduce chances of errors by reusing rather than replicating code, and leverage optimization happening behind the scenes.

User · Answer

It saves having to have methods that are only used once in a specific place from being defined far away from the place they are used  Good uses are as comparators for generic algorithms such as sorting  where you can then define a custom sort function where you are invoking the sort rather than further away forcing you to look elsewhere to see what you are sorting on   And it s not really an innovation  LISP has had lambda functions for about 30 years or more

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The innovation is in the type safety and transparency  Although you don t declare types of lambda expressions  they are inferred  and can be used by code search  static analysis  refactoring tools  and runtime reflection   For example  before you might have used SQL and could get an SQL injection attack  because a hacker passed a string where a number was normally expected  Now you would use a LINQ lambda expression  which is protected from that   Building a LINQ API on pure delegates is not possible  because it requires combining expression trees together before evaluating them   In 2016 most of the popular languages have lambda expression support  and C  was one of the pioneers in this evolution among the mainstream imperative languages

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It s a way of taking small operation and putting it very close to where it is used  not unlike declaring a variable close to its use point    This is supposed to make your code more readable   By anonymizing the expression  you re also making it a lot harder for someone to break your client code if it the function is used somewhere else and modified to  enhance  it   Similarly  why do you need to use foreach   You can do everything in foreach with a plain for loop or just using IEnumerable directly   Answer  you don t need it but it makes your code more readable

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Lambda s cleaned up C  2 0 s anonymous delegate syntax   for example  Strings Find s   gt  s     hello      Was done in C  2 0 like this   Strings Find delegate String s    return s     hello         Functionally  they do the exact same thing  its just a much more concise syntax

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Microsoft has given us a cleaner  more convenient way of creating anonymous delegates called Lambda expressions   However  there is not a lot of attention being paid to the expressions portion of this statement   Microsoft released a entire namespace  System Linq Expressions  which contains classes to create expression trees based on lambda expressions  Expression trees are made up of objects that represent logic   For example  x   y   z is an expression that might be part of an expression tree in  Net   Consider the following  simple  example   using System  using System Linq  using System Linq Expressions    namespace ExpressionTreeThingy       class Program               static void Main string   args                        Expression lt Func lt int  int gt  gt  expr    x    gt  x   1    this is not a delegate  but an object             var del   expr Compile      compiles the object to a CLR delegate  at runtime             Console WriteLine del 5      we are just invoking a delegate at this point             Console ReadKey                        This example is trivial   And I am sure you are thinking   This is useless as I could have directly created the delegate instead of creating an expression and compiling it at runtime    And you would be right   But this provides the foundation for expression trees   There are a number of expressions available in the Expressions namespaces  and you can build your own   I think you can see that this might be useful when you don t know exactly what the algorithm should be at design or compile time   I saw an example somewhere for using this to write a scientific calculator   You could also use it for Bayesian systems  or for genetic programming  AI    A few times in my career I have had to write Excel-like functionality that allowed users to enter simple expressions  addition  subtrations  etc  to operate on available data   In pre- Net 3 5 I have had to resort to some scripting language external to C   or had to use the code-emitting functionality in reflection to create  Net code on the fly   Now I would use expression trees

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Anonymous functions and expressions are useful for one-off methods that don t benefit from the extra work required to create a full method  Consider this example   List lt string gt  people   new List lt string gt     quot name1 quot    quot name2 quot    quot joe quot    quot another name quot    quot etc quot      string person   people Find person   gt  person Contains  quot Joe quot      versus  public string FindPerson string nameContains  List lt string gt  persons          foreach  string person in persons           if  person Contains nameContains                return person       return null      These are functionally equivalent

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I found them useful in a situation when I wanted to declare a handler for some control s event  using another control  To do it normally you would have to store controls  references in fields of the class so that you could use them in a different method than they were created   private ComboBox combo  private Label label   public CreateControls         combo   new ComboBox        label   new Label          some initializing code     combo SelectedIndexChanged    new EventHandler combo SelectedIndexChanged      void combo SelectedIndexChanged object sender  EventArgs e        label Text   combo SelectedValue      thanks to lambda expressions you can use it like this   public CreateControls         ComboBox combo   new ComboBox        Label label   new Label          some initializing code     combo SelectedIndexChanged     s  e    gt   label Text   combo SelectedValue        Much easier

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This is just one way of using a lambda expression  You can use a lambda expression anywhere you can use a delegate  This allows you to do things like this   List lt string gt  strings   new List lt string gt     strings Add  Good    strings Add  Morning   strings Add  Starshine    strings Add  The    strings Add  Earth    strings Add  says    strings Add  hello     strings Find s   gt  s     hello      This code will search the list for an entry that matches the word  hello   The other way to do this is to actually pass a delegate to the Find method  like this   List lt string gt  strings   new List lt string gt     strings Add  Good    strings Add  Morning   strings Add  Starshine    strings Add  The    strings Add  Earth    strings Add  says    strings Add  hello     private static bool FindHello String s        return s     hello      strings Find FindHello     EDIT   In C  2 0  this could be done using the anonymous delegate syntax     strings Find delegate String s    return s     hello         Lambda s significantly cleaned up that syntax

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You can also find the use of lambda expressions in writing generic codes to act on your methods   For example  Generic function to calculate the time taken by a method call   i e  Action in here   public  static  long  Measure Action  action        Stopwatch  sw     new  Stopwatch        sw Start        action        sw Stop        return  sw ElapsedMilliseconds      And you can call the above method using the lambda expression as follows   var  timeTaken     Measure       gt   yourMethod param      Expression allows you to get return value from your method and out param as well  var  timeTaken     Measure       gt   returnValue     yourMethod param   out  outParam

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Microsoft has given us a cleaner  more convenient way of creating anonymous delegates called Lambda expressions   However  there is not a lot of attention being paid to the expressions portion of this statement   Microsoft released a entire namespace  System Linq Expressions  which contains classes to create expression trees based on lambda expressions  Expression trees are made up of objects that represent logic   For example  x   y   z is an expression that might be part of an expression tree in  Net   Consider the following  simple  example   using System  using System Linq  using System Linq Expressions    namespace ExpressionTreeThingy       class Program               static void Main string   args                        Expression lt Func lt int  int gt  gt  expr    x    gt  x   1    this is not a delegate  but an object             var del   expr Compile      compiles the object to a CLR delegate  at runtime             Console WriteLine del 5      we are just invoking a delegate at this point             Console ReadKey                        This example is trivial   And I am sure you are thinking   This is useless as I could have directly created the delegate instead of creating an expression and compiling it at runtime    And you would be right   But this provides the foundation for expression trees   There are a number of expressions available in the Expressions namespaces  and you can build your own   I think you can see that this might be useful when you don t know exactly what the algorithm should be at design or compile time   I saw an example somewhere for using this to write a scientific calculator   You could also use it for Bayesian systems  or for genetic programming  AI    A few times in my career I have had to write Excel-like functionality that allowed users to enter simple expressions  addition  subtrations  etc  to operate on available data   In pre- Net 3 5 I have had to resort to some scripting language external to C   or had to use the code-emitting functionality in reflection to create  Net code on the fly   Now I would use expression trees

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Lambda expressions are a simpler syntax for anonymous delegates and can be used everywhere an anonymous delegate can be used   However  the opposite is not true  lambda expressions can be converted to expression trees which allows for a lot of the magic like LINQ to SQL     The following is an example of a LINQ to Objects expression using anonymous delegates then lambda expressions to show how much easier on the eye they are      anonymous delegate var evens   Enumerable                  Range 1  100                   Where delegate int x    return  x   2     0                      ToList        lambda expression var evens   Enumerable                  Range 1  100                   Where x   gt   x   2     0                   ToList      Lambda expressions and anonymous delegates have an advantage over writing a separate function  they implement closures which can allow you to pass local state to the function without adding parameters to the function or creating one-time-use objects   Expression trees are a very powerful new feature of C  3 0 that allow an API to look at the structure of an expression instead of just getting a reference to a method that can be executed   An API just has to make a delegate parameter into an Expression lt T gt  parameter and the compiler will generate an expression tree from a lambda instead of an anonymous delegate   void Example Predicate lt int gt  aDelegate     called like   Example x   gt  x  gt  5     becomes   void Example Expression lt Predicate lt int gt  gt  expressionTree     The latter will get passed a representation of the abstract syntax tree that describes the expression x  gt  5   LINQ to SQL relies on this behavior to be able to turn C  expressions in to the SQL expressions desired for filtering   ordering   etc  on the server side

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The biggest benefit of lambda expressions and anonymous functions is the fact that they allow the client  programmer  of a library framework to inject functionality by means of code in the given library framework   as it is the LINQ  ASP NET Core and many others   in a way that the regular methods cannot  However  their strength is not obvious for a single application programmer but to the one that creates libraries that will be later used by others who will want to configure the behaviour of the library code or the one that uses libraries  So the context of effectively using a lambda expression is the usage creation of a library framework   Also since they describe one-time usage code they don t have to be members of a class where that will led to more code complexity  Imagine to have to declare a class with unclear focus every time we wanted to configure the operation of a class object

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A lambda expression is like an anonymous method written in place of a delegate instance  delegate int MyDelagate  int i   MyDelagate delSquareFunction   x   gt  x   x   Consider the lambda expression x   gt  x   x   The input parameter value is x  on the left side of   gt   The function logic is x   x  on the right side of   gt    A lambda expression s code can be a statement block instead of an expression  x   gt   return x   x     Example Note  Func is a predefined generic delegate      Console WriteLine MyMethod x   gt   quot Hi  quot    x         public static string MyMethod Func lt string  string gt  strategy                return strategy  quot Lijo quot   ToString           References  How can a delegate  amp  interface be used interchangeably

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I found them useful in a situation when I wanted to declare a handler for some control s event  using another control  To do it normally you would have to store controls  references in fields of the class so that you could use them in a different method than they were created   private ComboBox combo  private Label label   public CreateControls         combo   new ComboBox        label   new Label          some initializing code     combo SelectedIndexChanged    new EventHandler combo SelectedIndexChanged      void combo SelectedIndexChanged object sender  EventArgs e        label Text   combo SelectedValue      thanks to lambda expressions you can use it like this   public CreateControls         ComboBox combo   new ComboBox        Label label   new Label          some initializing code     combo SelectedIndexChanged     s  e    gt   label Text   combo SelectedValue        Much easier

User · Answer

Lambda expression is a concise way to represent an anonymous method  Both anonymous methods and Lambda expressions allow you define the method implementation inline  however  an anonymous method explicitly requires you to define the parameter types and the return type for a method  Lambda expression uses the type inference feature of C  3 0 which allows the compiler to infer the type of the variable based on the context  It   s is very convenient because that saves us a lot of typing

User · Answer

A lot of the times  you are only using the functionality in one place  so making a method just clutters up the class

[c#] C# Lambda expressions: Why should I use them?

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