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WTF Is a Lambda?

Jon Hilton on March 07, 2019

If you're just learning C# you've probably come across something like this and wondered what the weird x=>x is all about... things.SingleOrDef...

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Sung M. Kim • Mar 7 '19 • Edited

If anyone's coming from "JavaScript" background, you will find that it works very similar to the JavaScript's lambda.

To show the similarity (using the examples in this article)

1 - You can leave out () if you have only one argument passed.

things.SingleOrDefault((x) = >x.Name == "Bob");
// or
things.SingleOrDefault(x => x.Name == "Bob");

2 - You can pass the method name

var amy = People.SingleOrDefault(() => IsAmy());
// or
var amy = People.SingleOrDefault(IsAmy);

Both lambda syntax are the same in C# & JavaScript.

Hussein Duvigneau • Mar 9 '19 • Edited

You can go one step further:

things.SingleOrDefault(({name}) => name === "Bob");

Sung M. Kim • Mar 9 '19 • Edited

I wasn't able to replicate the object desconstruction in C# as you could do in JavaScript (even after implementing Deconstruct)

dotnetfiddle.net/Wbx3ac

using System;
using System.Linq;
using System.Collections.Generic;

// https://dev.to/jonhilt/wtf-is-a-lambda-2dg2

public class Person
{
    public string Name { get; set; }
    public int Age { get; set; }

    // Allow Object Deconstructuring/destructuring
    // https://docs.microsoft.com/en-us/dotnet/csharp/deconstruct#deconstructing-a-user-defined-type-with-discards
    public void Deconstruct(out string name, out int age) {
        name = Name;
        age = Age;
    }

    public override string ToString() {
        return $"Name={this.Name}, Age={this.Age}"; 
    }
}

public class Program
{
    static List<Person> people = new List<Person>
    {
        new Person { Name = "Bob", Age = 37 },
        new Person { Name = "Brian", Age = 21 },
        new Person { Name = "Amy", Age = 58 }
    };

    public static void Main()
    {
        // This works
//      var bob = people.SingleOrDefault(person => person.Name == "Bob");
        // But this doesn't
//      var bob = people.SingleOrDefault(({name}) => name == "Bob");
        var bob = people.SingleOrDefault(((Name, _)) => Name == "Bob");
        Console.WriteLine(bob);
    }
}

And Roslyn 2.0 compiler complains...


}
Compilation error (line 39, col 48): Invalid expression term '=>'
Compilation error (line 39, col 48): Syntax error, ',' expected
Compilation error (line 39, col 51): Syntax error, ',' expected
Compilation error (line 39, col 38): The type or namespace name 'Name' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 39, col 44): The type or namespace name '_' could not be found (are you missing a using directive or an assembly reference?)
Compilation error (line 39, col 37): Predefined type 'System.ValueTuple`2' is not defined or imported
Compilation error (line 39, col 51): The name 'Name' does not exist in the current context

Hussein Duvigneau • Mar 9 '19

Sorry, I was talking explicitly on JS terms

Sung M. Kim • Mar 9 '19

No worries, mate~ 😉

Tamás Szelei • Mar 9 '19

I think it's worth pointing out that a lambda is not simply a shorthand for function declaration. IsAmy is a function. The other two variants are both lambdas and they differ from functions in that they capture all the locals and globals that are visible in the scope where the lambda is defined. IsAmy has no access to those.

Jon Hilton • Mar 14 '19

Thanks for this, an important clarification and I appreciate you taking the time to outline it here :-)