C#•3y ago

(Concept Question) Delegates and Lambda Expressions [Answered]

I'm so confused about the concept of Delegates and Lambda Expression

namespace LINQ;

internal static class Program
{
    static void Main()
    {
        var titanic = new Movie("Titanic", 1998, 4.5f);
        PrettyPrint(titanic, (movie) => new string('*', (int)movie.Rating)); // "Lambda Expression"
    }

    private static void PrettyPrint(Movie movie, Func<Movie, string> printRating)
    {    
        // var rating = printRating.Invoke(movie);
        // Or simply
        var rating = printRating(movie);
        
        Console.WriteLine($"Name: {movie.Name}, Rating: [{rating}]");
    }
    
    // Note: This is "expression body"
    private static int RandomMethod() => 20;
}

class Movie
{
    public Movie(string name, int releaseYear, float rating)
    {
        Name = name;
        ReleaseYear = releaseYear;
        Rating = rating;
    }

    public string Name { get; set; }

    public int ReleaseYear { get; set; }

    public float Rating { get; set; }
}

namespace LINQ;

internal static class Program
{
    static void Main()
    {
        var titanic = new Movie("Titanic", 1998, 4.5f);
        PrettyPrint(titanic, (movie) => new string('*', (int)movie.Rating)); // "Lambda Expression"
    }

    private static void PrettyPrint(Movie movie, Func<Movie, string> printRating)
    {    
        // var rating = printRating.Invoke(movie);
        // Or simply
        var rating = printRating(movie);
        
        Console.WriteLine($"Name: {movie.Name}, Rating: [{rating}]");
    }
    
    // Note: This is "expression body"
    private static int RandomMethod() => 20;
}

class Movie
{
    public Movie(string name, int releaseYear, float rating)
    {
        Name = name;
        ReleaseYear = releaseYear;
        Rating = rating;
    }

    public string Name { get; set; }

    public int ReleaseYear { get; set; }

    public float Rating { get; set; }
}

Specifically this code (movie) => new string('*', (int)movie.Rating) Is this suppose to create a new method in place of (movie)? I looked through the lowered C# code on sharplab but it's even more confusing o-0

71 Replies

Aaron•3y ago

SomeMethod((movie) => new string('*', (int)movie.Rating);

SomeMethod((movie) => new string('*', (int)movie.Rating);

is the same as doing

SomeMethod(OtherMethod);

string OtherMethod(Movie movie)
{
    return new string('*', (int)movie.Rating);
}

SomeMethod(OtherMethod);

string OtherMethod(Movie movie)
{
    return new string('*', (int)movie.Rating);
}

Bobby BobOP•3y ago

What does the (movie) denote? Like what does that syntax do?

Kouhai•3y ago

Adding to Aaron's answer, Lambda's can work as closures, unlike normal methods.

Bobby BobOP•3y ago

I think closures are a bit too far-fetched concept for me atm Looking at the the lambda code above

Aaron•3y ago

its saying it takes a parameter called movie you can be more explicit if you'd like

Bobby BobOP•3y ago

So it generates a random method OtherMethod and then takes a unnamed parameter called movie? Wait so hold on, it doesn't give any names to the method and the parameter itself?

Aaron•3y ago

SomeMethod(void (Movie movie) => new string('*', (int)movie.Rating)) is also valid

Bobby BobOP•3y ago

ooooooooh Hold on, how is this any different to expression bodies?

Aaron•3y ago

they're also called "anonymous functions"

Bobby BobOP•3y ago

From the code above, the author put down

    // Note: This is "expression body"
    private static int RandomMethod() => 20;

    // Note: This is "expression body"
    private static int RandomMethod() => 20;

Aaron•3y ago

they don't have a name

Bobby BobOP•3y ago

Isn't this the same thing? oooooh

Aaron•3y ago

they can only be accessed as a delegate

Bobby BobOP•3y ago

Yeah that's what I'm confused about What's the use with delegates? Like what is it's application?

Aaron•3y ago

they let you send a method as a parameter or store a method in a field the most visible example is LINQ as an application of them

Bobby BobOP•3y ago

Can't we just write

SomeMethod(new string('*', (int)movie.Rating);

SomeMethod(new string('*', (int)movie.Rating);

rather then

SomeMethod((movie) => new string('*', (int)movie.Rating);

SomeMethod((movie) => new string('*', (int)movie.Rating);

Wouldn't that be the same thing?

Aaron•3y ago

where does it get movie from in the first example?

Bobby BobOP•3y ago

Why is an anonymous function needed to parse the argument?

Aaron•3y ago

its not passing an argument

Bobby BobOP•3y ago

oh....

Aaron•3y ago

so like, with LINQ

Bobby BobOP•3y ago

I think it might be getting the object movie from the same namespace

Aaron•3y ago

lets take FirstOrDefault have you ever used LINQ before?

Bobby BobOP•3y ago

No, I have never used LINQ but I'm learning it right now This is the course I'm currently following

Bobby BobOP•3y ago

https://www.educative.io/courses/getting-started-linq-c-sharp

Educative: Interactive Courses for Software Developers

Getting Started with LINQ in C# - Learn Interactively

LINQ is the best feature in C#. If you’re interested in writing more expressive code to work with collections, this course is for you. In this course, you will learn about what LINQ is and what lambda expressions are. You will learn the most common LINQ methods and some common pitfalls to avoid when using LINQ. You will also write your first met...

Bobby BobOP•3y ago

And it's introducing me to lambda expressions and delegates

Aaron•3y ago

this is what FirstOrDefault Looks like

public static class Extension
{
    public static T? FirstOrDefault<T>(IEnumerable<T> enumerable, Func<T, bool> func)
    {
        foreach (var thing in enumerable)
        {
            if (func(thing))
            {
                return thing;
            }
        }
        return default(T);
    }
}

public static class Extension
{
    public static T? FirstOrDefault<T>(IEnumerable<T> enumerable, Func<T, bool> func)
    {
        foreach (var thing in enumerable)
        {
            if (func(thing))
            {
                return thing;
            }
        }
        return default(T);
    }
}

it goes through the enumerable (an enumerable is anything you can use foreach on) and if anything passes the logic you give it

Bobby BobOP•3y ago

Is FirstOrDefault a LINQ method?

Aaron•3y ago

it will give you that thing yes

Aaron•3y ago

its right here https://learn.microsoft.com/en-us/dotnet/api/system.linq.enumerable.firstordefault?view=net-6.0#system-linq-enumerable-firstordefault-1(system-collections-generic-ienumerable((-0))-system-func((-0-system-boolean)))

Enumerable.FirstOrDefault Method (System.Linq)

Returns the first element of a sequence, or a default value if no element is found.

Bobby BobOP•3y ago

hmmmmm... such weird syntaxes

Kouhai•3y ago

Which part is weird?

Bobby BobOP•3y ago

Yeah I'm trying to understand the syntax. I've never seen generics defined that way before public static T? FirstOrDefault<T> Usually, I see generics defined as public static FirstOrDefault<T> But there's a T? nullable thingy in the middle?

Kouhai•3y ago

T is the type T? means it could return the type or null

Aaron•3y ago

but you use it like this

int[] ints = new int[] { 1, 2, 3, 4, 5 };
var four = ints.FirstOrDefault(i => i == 4);

int[] ints = new int[] { 1, 2, 3, 4, 5 };
var four = ints.FirstOrDefault(i => i == 4);

that second thing you showed isnt valid you didnt put a return type T? is the return type of the method

Bobby BobOP•3y ago

OOOOOOOOH T? is a return type????

Aaron•3y ago

yes! it returns whatever the generic is, but can possibly be null

Kouhai•3y ago

Or null if applicable

Bobby BobOP•3y ago

Holy shit you're right! I'm confusing it with generic classes, I just realized this is my first time seeing a generic method

Kouhai•3y ago

Note in a value type it won't be null

Bobby BobOP•3y ago

Bobby BobOP•3y ago

Yep, I'm starting to get the overall picture So coming back to that LINQ method

Aaron•3y ago

yeah, you can use it like this

int[] ints = new int[] { 1, 2, 3, 4, 5 };
var four = ints.FirstOrDefault(bool (int i) => i == 4);

int[] ints = new int[] { 1, 2, 3, 4, 5 };
var four = ints.FirstOrDefault(bool (int i) => i == 4);

it will give you the first item in the array that returns true from i == 4

Bobby BobOP•3y ago

It takes an IEnumerable<T> object and a Func<T, bool> object which is a delegate that accepts the parameter T (generic) and returns a boolean am I understanding that correctly?

Aaron•3y ago

yes

Bobby BobOP•3y ago

And this line here if (func(thing)). I'm guessing thing is suppose to be T the generic type right?

Aaron•3y ago

yeah, sorry for the var

Bobby BobOP•3y ago

Man this is confusing, C# just gets exponentially harder the more I learn Okay, so to review again In the context of the example code above, I must use

SomeMethod((movie) => new string('*', (int)movie.Rating);

SomeMethod((movie) => new string('*', (int)movie.Rating);

and not

SomeMethod(new string('*', (int)movie.Rating));

SomeMethod(new string('*', (int)movie.Rating));

Because movie is not defined right? There's no movie object within that statement or the source code. But the lambda expression is able to interpret movie as an identifier for a Movie class? Am I getting this right?

Aaron•3y ago

the reason you must use it is because the method wants you to pass a delegate, because it wants to run whatever code you give it (that method could actually be changed to just take a parameter and it would work fine)

Kouhai•3y ago

Func<Movie, string> printRating means pass a method that takes Movie as a parameter, and returns a string

Bobby BobOP•3y ago

OOOOOOOOOH Ok I'm starting to understand now PrettyPrint takes Func<Movie, string> as it's second argument, that's why we are creating a lambda expression, a anonymous function to pass to PrettyPrint I think I got it it now, damn that was pretty difficult So everytime I think of delegates, I should think of lambda expression since they go hand in hand right?

Aaron•3y ago

eh, they're related but not the exact same lambda expressions are delegates not all delegates come from a lambda

Kouhai•3y ago

No, they are different, delegates aren't necessarily lambdas

Aaron•3y ago

for example

MODiX•3y ago

Windows10CE#8553

REPL Result: Success

string GetString()
{
    return "my string!";
}

Func<string> getDelegate = GetString;
getDelegate()

string GetString()
{
    return "my string!";
}

Func<string> getDelegate = GetString;
getDelegate()

Result: string

my string!

my string!

Compile: 609.198ms | Execution: 30.801ms | React with ❌ to remove this embed.

Aaron•3y ago

there is not lambda there but it still uses delegates delegates are methods stuffed into a variable lambdas are methods with no name

Kouhai•3y ago

Adding again to Aaron's amazing answers, lambdas can be closures, so they sometimes behave differently from normal methods

Aaron•3y ago

closures are weird, still good to know closures are things like this:

MODiX•3y ago

Windows10CE#8553

REPL Result: Success

string myString = "i'm in a variable!";
Func<string> iReturnWhateverIsInTheVarable = string () => myString;
iReturnWhateverIsInTheVarable()

string myString = "i'm in a variable!";
Func<string> iReturnWhateverIsInTheVarable = string () => myString;
iReturnWhateverIsInTheVarable()

Result: string

i'm in a variable!

i'm in a variable!

Compile: 486.734ms | Execution: 47.877ms | React with ❌ to remove this embed.

Aaron•3y ago

they have the magic power of using things that are in variables they can even change variables! oops

MODiX•3y ago

Windows10CE#8553

REPL Result: Success

static void JustInvokeTheAction(Action a)
{
a();
}

int i = 2;
Action a = void () => i = 5;
Console.WriteLine(i);
JustInvokeTheAction(a);
Console.WriteLine(i);

static void JustInvokeTheAction(Action a)
{
a();
}

int i = 2;
Action a = void () => i = 5;
Console.WriteLine(i);
JustInvokeTheAction(a);
Console.WriteLine(i);

Console Output

2
5

2
5

Compile: 608.731ms | Execution: 79.437ms | React with ❌ to remove this embed.

Aaron•3y ago

how exactly they do that is a bit of compiler nonsense that isnt super important to understand right now but its very very useful

Bobby BobOP•3y ago

Ty all for helping out 😄 I think I understand lambda expressions and delegates now, although am a bit slow on recognizing the syntax

Accord•3y ago

✅ This post has been marked as answered!

Bobby BobOP•3y ago

Sorry to opening this up again, I just came across another Lambda expression and got confused again. For Lambda expressions, I can understand not having a name for anything, but we can omit the return type in a lambda expression? For example

SomeMethod((movie) => "Guardians of the Galaxy")

SomeMethod((movie) => "Guardians of the Galaxy")

Doesn't specify that the return type of is a string Also just came across this weird syntax which I don't quite understand .OrderBy(b => b.BlogId) is that the same thing as saying .OrderBy((b) => b.BlogId)? So I'm guessing something like

void SomeMethod(B b)
{
  return b.BlogId
}

void SomeMethod(B b)
{
  return b.BlogId
}

? But that makes zero sense because there's no class called "B" in the context of the code that I am readin

Aaron•3y ago

all types related to lambas can be inferred by the compiler in fact, being able to specify the types wasn't even a feature until very recently

Bobby BobOP•3y ago

oooo that's intersting Also for the last question I think I figured it out

Bobby BobOP•3y ago

Bobby BobOP•3y ago

I was confused on that part, until I realized b had the same colour as other objects

Tvde1•3y ago

the (movie) => is just to make the first parameter named movie just like

void PlayMovie(Movie movie)
{
   // ...

void PlayMovie(Movie movie)
{
   // ...

Accord•3y ago

✅ This post has been marked as answered!

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(Concept Question) Delegates and Lambda Expressions [Answered]

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