Blog - Title

Lambda Expressions

Lambda Expressions

  • Comments 67

[Blog Map]  [Table of Contents]  [Next Topic]

Note: This article is a topic in a much larger tutorial on writing pure functional code in C#.  Lambda expressions are only one of the pieces in a much larger puzzle.  Functional programming in C# allows us to write shorter programs that perform better, and are much more robust.  Query Composition using Functional Programming Techniques explains lambda expressions in their proper context.

This blog is inactive.
New blog: EricWhite.com/blog

Blog TOC
In order to learn functional programming and a more declarative style of writing code, we need first to cover some basic material.  One of the first concepts is that of lambda expressions.  Lambda expressions can be summarized in one sentence:

Lambda expressions are simply functions/methods.

They have a different syntax, primarily so that they can be written in expression context (more on this shortly) instead of as a member of a class.  However, that is all they are.  For instance, the following lambda expression:

c => c + 1

is a function that takes one argument, c, and returns the value c + 1.

Actually, they are slightly more complicated than this, but not much more.  For the purposes of this tutorial, you only use lambda expressions when calling a method that takes a delegate as a parameter.  Instead of writing a method, creating a delegate from the method, and passing the delegate to the method as a parameter, you can simply write a lambda expression in-line as a parameter to the method.

To show lambda expressions in context, consider the problem where you have an array with 10 digits in it, and you want to filter for all digits greater than 5.  In this case, you can use the Where extension method, passing a lambda expression as an argument to the Where method:

int[] source = new[] { 3, 8, 4, 6, 1, 7, 9, 2, 4, 8 };
 
foreach (int i in source.Where(x => x > 5))
    Console.WriteLine(i);

To understand the semantics of this code, you needn't find some method elsewhere in the source code that does the selection; the code that you need to read is much tighter and smaller.  It reflects your intent in a much cleaner fashion.

Later on in this tutorial, you'll see a number of uses of the standard query operators.  Many of the standard query operators, including Where, take delegates as an argument, so this means that we can call them passing a lambda as an argument.

First, a quick review of delegates:

Defining, Creating, and Using a Delegate

In C#, a delegate is a data structure that refers to either a static method, or an object and an instance method of its class.  When you initialize a delegate, you initialize it with either a static method, or a class instance and an instance method.

The following code shows the definition of a delegate and a method that can be used to initialize the delegate:

// Defines a delegate that takes an int and returns an int
public delegate int ChangeInt(int x);
 
// Define a method to which the delegate can point
static public int DoubleIt(int x)
{
    return x * 2;
}

Now, you can create and initialize an instance of the delegate, and then call it:

ChangeInt myDelegate = new ChangeInt(DelegateSample.DoubleIt);
Console.WriteLine("{0}", myDelegate(5));

This, as you would expect, writes 10 to the console.

Using an Anonymous Method

With C# 2.0, anonymous methods allow you to write a method and initialize a delegate in place:

ChangeInt myDelegate = new ChangeInt(
    delegate(int x)
    {
        return x * 2;
    }
);
Console.WriteLine("{0}", myDelegate(5));

Using a Lambda Expression

With Lambda expressions, the syntax gets even terser:

ChangeInt myDelegate = x => x * 2;
Console.WriteLine("{0}", myDelegate(5));

This lambda expression is an anonymous method that takes one argument x, and returns x * 2.  In this case, the type of x and the type that the lambda returns are inferred from the type of the delegate to which the lambda is assigned.

If you wanted to, you could have specified the type of the argument, as follows:

ChangeInt myDelegate = (int x) => x * 2;
Console.WriteLine("{0}", myDelegate(5));

Using a Lambda with Two Arguments

When using the Standard Query Operators, on occasion, you need to write a lambda expression that takes two arguments.

If you have a delegate that takes two arguments:

// Defines a delegate that takes two ints and returns an int
public delegate int MultiplyInts(int arg, int arg2);

You can declare and initialize a delegate:

MultiplyInts myDelegate = (a, b) => a * b;
Console.WriteLine("{0}", myDelegate(5, 2));

Statement Lambda Expressions

You can write a more complicated lambda expression using statements, enclosing the statements in braces.  If you use this syntax, you must use the return statement, unless the lambda returns void:

int[] source = new[] { 3, 8, 4, 6, 1, 7, 9, 2, 4, 8 };
 
foreach (int i in source.Where(
        x =>
        {
            if (x <= 3)
                return true;
            else if (x >= 7)
                return true;
            return false;
        }
    ))
    Console.WriteLine(i);

Sometimes developers wonder how to pronounce the => token.

If the lambda expression is a predicate, expressing some condition: c => c.State == "WA" then the => can be spoken as "such that".  In this example, you could say "c such that c dot state equals Washington".  If the lambda expression is a projection, returning a new type: c => new XElement("CustomerID", c.CustomerID); then the => can be spoken as "becomes".  In the above example, you could say "c becomes new XElement with a name of CustomerID and its value is c dot CustomerID".  Or "maps to", or "evaluate to", as suggested in the comments below.  But most often, I just say "arrow".  J

A quick note: predicates are simply boolean expressions that are passed to some method that will use the boolean expression to filter something.  A lambda expression used for projection takes one type, and returns a different type.  More on both of these concepts later.

Lambda Expressions that Return Void

A lambda expression that returns void is not very useful in the context of functional programming because the only possible reason for such a function is that it has side-effects, and is not pure (more on this later in the tutorial), but it is part of C# 3.0 syntax, so I'll cover it here.  Sometimes developers will use a void statement lambda expression for writing an event handler.  This has the benefit that the syntax is terser, and the program is smaller.  In addition, the lambda expression can refer to local variables in the enclosing scope.  This is part of C#'s implementation of closures.  The only way to write a lambda expression that returns void is to write a statement lambda expression.  The following example shows defining a void delegate, declaring an instance of it, and calling it.

// Defines a delegate that takes a string and returns void
public delegate void OutputToConsole(string arg);
 
static void Main(string[] args)
{
    OutputToConsole o = a => {
        Console.WriteLine(a);
    };
    o("Hello, World");
}

If you write a lambda expression for a delegate that returns void and takes no arguments, it results in interesting syntax:

// Defines a delegate that takes no arguments and returns void
public delegate void OutputHelloToConsole();
 
static void Main(string[] args)
{
    OutputHelloToConsole o = () =>
    {
        Console.WriteLine("Hello, World");
    };
    o();
}

The Func Delegate Types

The framework defines a number of parameterized delegate types:

public delegate TR Func<TR>();
public delegate TR Func<T0, TR>(T0 a0);
public delegate TR Func<T0, T1, TR>(T0 a0, T1 a1);
public delegate TR Func<T0, T1, T2, TR>(T0 a0, T1 a1, T2 a2);
public delegate TR Func<T0, T1, T2, T3, TR>(T0 a0, T1 a1, T2 a2, T3 a3);

In the above delegate types, notice that if there is only one type parameter, it is the return type of the delegate. If there are two type parameters, the first type parameter is the type of the one and only argument, and the second type is the return type of the delegate, and so on. Many of the standard query operators (which are just methods that you call) take as an argument a delegate of one of these types. These delegate definitions are useful to you when writing your own methods that take a delegate as an argument.

using System;
using System.Collections.Generic;
 
class Program
{
    static List<T> MyWhereMethod<T>(IEnumerable<T> source,
        Func<T, bool> predicate)
    {
        List<T> l = new List<T>();
        foreach (T item in source)
            if (predicate(item))
                l.Add(item);
        return l;
    }
 
    static void Main(string[] args)
    {
        int[] source = new[] { 3, 8, 4, 6, 1, 7, 9, 2, 4, 8 };
 
        List<int> filteredList = MyWhereMethod(source,
            i => i >= 5);
        foreach (int z in filteredList)
            Console.WriteLine(z);
    }
}

The Action Delegate Types

The framework defines a number of parameterized delegate types for delegates that return void:

public delegate void Action();
public delegate void Action<T0>(T0 a0);
public delegate void Action<T0, T1>(T0 a0, T1 a1);
public delegate void Action<T0, T1, T2>(T0 a0, T1 a1, T2 a2);
public delegate void Action<T0, T1, T2, T3>(T0 a0, T1 a1, T2 a2, T3 a3);

Sometimes API designers will include an event that takes one of these delegate types as an argument, and you can write a lambda expression for the argument.  As with the Func delegate types, these delegate definitions are useful to you when writing your own methods that take a delegate as an argument.  This uses the interesting syntax of () => { /* body of void function here */ };

using System;
using System.Collections.Generic;
using System.Threading;
 
class Program
{
    static void SomeAsynchronousMethod(Action complete)
    {
        // just pretending to be asynchronous in this example
        Thread.Sleep(1000);
        complete();
    }
 
    static void Main(string[] args)
    {
        SomeAsynchronousMethod(() => { Console.WriteLine("Done"); });
    }
}

Expression Trees

Lambda expressions can also be used as expression trees.  This is an interesting topic, but is not part of this discussion on writing pure functional transformations.

[Blog Map]  [Table of Contents]  [Next Topic]

Leave a Comment
  • Please add 6 and 8 and type the answer here:
  • Post
  • Excellent work!  You really made this much easier to grasp and more importantly use in my code...

    Thanks

  • I've recently switched over from Java to C#, subsequently bought two books from Manning but was struggling to understand delegates and lambda expressions. I understood it immediately when I read your article. It's excellent, thanks.

  • Very good article and thanks!

    I just wonder what is: () => Could you please explain this one also? I encountered this a lot in "Composite Application Guidance for WPF and Silverlight" which I keep confused.

    Example:

               var myAction = new ActionHelper()

                                  {

                                      ActionToExecute =

                                          () => calledThreadID = Thread.CurrentThread.ManagedThreadId

                                  };

    and:

                   ServiceLocator.SetLocatorProvider(

                       () => new MockServiceLocator(

                           () => mockRegionContentRegistry));

    Thanks

    Alan

  • Hi Alan,

    What you're seeing is a statement lambda expression that takes no arguments, and returns void.  Here is similar code that you can paste into a console application and run:

    Action a = () =>

    {

        Console.WriteLine("in a statement lambda expression that takes no arguments and returns void.");

    };

     

    a();

    There are various flavors of the Action delegate type, just like there are various flabors of the Func delegate type.  Actions are delegate types that return void.

    What you're seeing in the "Composite Application Guidance for WPF and Silverlight" is that there is a void delegate that takes no arguments, and they are using Lambda expression syntax as a convenient way to initialize the delegate.  They could have written a function, declared and initialized a delegate, and assigned the delegate, but this would be more verbose.  (I've updated the post to include examples of this variety of lambda expression.)

    Does that help?

    -Eric

  • Eic:

    Thank you very much for your timely response. The code makes sense to me now. I've been Googling the () => and couldn't find any thing until your posting showed up.

    Thanks

    Alan

  • Nice article with clear breakup of lambda expresssion.

  • Thank you, it helped to grab the basics of Lambda Expressions.

  • Excellent job breaking down how lamda works. This was very helpful. I will definitely be reading more of your postings.

    Thank you

  • Very nice explanation. I especially like combining lambda expressions with Func class

  • why so overcomplicated:

    x =>

    {

      if (x <= 3)

         return true;

      else if (x >= 7)

         return true;

      return false;

    }

    if one can do this:

    x =>

    {

      return (x <= 3 || x >= 7);

    }

  • Hi, ebody, my main point in that example was to demonstrate a statement lambda expression.  Of course, that particular example can be simplified even further to x => x <= 3 || x >= 7.  Typical use of a statement lambda expression is where you need to implement fairly complex semantics, computing separate return values in different code blocks, so that is why I wrote the example as I did.  But I didn't want to pick an example where the code of the example interferred with the point I was making.  Does that make sense?

    -Eric

  • Good argumentation. I'm on your side now :]

  • Can you tell me please @EricWhite or anyone reading the comments what is the performance of this example:

    int[] source = new[] { 3, 8, 4, 6, 1, 7, 9, 2, 4, 8 };

    foreach (int i in source.Where(x => x > 5))

       Console.WriteLine(i);

    is this looping twice on that array, first filtering the values and then looping on the result or is there internally only one loop where each element is tested and either returned or skipped?

    I wonder whether this is not faster:

    foreach (int i in source)

      if(i > 5)

         Console.WriteLine(i);

  • Hi, @ebody, that's a great question.  The two examples you present basically perform almost identically.  The Where extension method is 'lazy'.  Internally, there is only one loop where each element is tested, and then either returned or skipped.  For a detailed explanation of laziness, see Lazy Evaluation, and in contrast, Eager Evaluation.  There is a little bit of overhead when using laziness, in that there is a closure object created by the call to Where.  This is an object that keeps the state of where you are in the iteration, as well as references to any variables that you refer to in the lambda expression (they are placed on the heap, even if they are local variables, see Closures for more info).  There is a function call to a function that is internal to the closure for every item in the collection, so that is a bit of overhead.  Also, using the Where extension method creates a new, short-lived object on the heap for every item that is returned.  However, the garbage collector is optimized for this scenario.  I have written some recursive LINQ to XML transformations that work on LARGE Open XML documents.  These transformations allocate a huge number of short-lived objects, but their performance is amazingly fast, even on a slow computer.  (I do almost all of my development and testing on an old, slow laptop - basically a netbook.)  There have been a very few cases where I needed to optimize code to perform faster, and I never focused on elmination of closures.  Instead, I focused on selective materialization of intermediate results into arrays or collections, and I've always been able to achieve my performance goals.  For instance, if I am querying a huge Open XML document, instead of iterating over the entire, huge document every time, I may first 'materialize' the paragraphs I'm interested in, and contents of the paragraphs into a List<T>.  Then subsequent iterations are over the much shorter List<T> instead of over the whole document.

    -Eric

  • In the Using an Anonymous Method paragraph there is this piece of code:

    ChangeInt myDelegate = new ChangeInt(

       delegate(int x)

       {

           return x * 2;

       }

    );

    The only thing I don't understand about this is why you give the delegate yet another delegate. Why isn't it given a method?

Page 3 of 5 (67 items) 12345