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C#•2w ago

Merineth 🇸🇪

✅ Can someone explain Delegates to me like i'm 5?

:(

167 Replies

Pobiega•2w ago

I believe we already did :p

Merineth 🇸🇪•2w ago

I didn't get it :I

Pobiega•2w ago

Imagine you have a bunch of toys, and you like to play with them in different ways. Sometimes you want to race your toy cars, other times you want to build a tower with blocks. Now, imagine you have a special box called "Delegate Box". This box doesn't play with the toys by itself, but it holds instructions on how you want to play. So, you can tell the box: "Today, I want to race the cars," and it remembers that. Tomorrow, you can tell it: "Now I want to build a tower!" The box doesn't care what the instructions are—it just remembers them and gives them to the toys. In C#, a delegate is like that box. It holds instructions (a reference to a method) for doing something, and when you need it, you tell the delegate to do whatever it has been asked to do, without caring how it works inside. Does that make sense? The delegate is like a magic helper that holds instructions on how to do things!

Actually a pretty good ELI5

Merineth 🇸🇪•2w ago

Ok so delegates holds instructions for how to do something? (a method)

Pobiega•2w ago

a delegate is a type (like class, or struct, or enum) instead of holding objects or values, a delegate hold a method a delegates parameters declare what parameters/return value a method must have to "fit" the delegate

public delegate decimal PerformCalculation(int x, int y);

public delegate decimal PerformCalculation(int x, int y);

this one for example requires a method to return a decimal and take two ints in to fit

Merineth 🇸🇪•2w ago

Ok, so we pass a method to PerformCalculation and not int x, int y? But the method we pass must match the parameters of the delegate?

Pobiega•2w ago

yes

Merineth 🇸🇪•2w ago

What's the benefit of passing a method rather than just passing the values/types to a normal method instead?

Pobiega•2w ago

public static int Add(int a, int b) { ... }
public static decimal Remove(int k, int x) { ... }

public static int Add(int a, int b) { ... }
public static decimal Remove(int k, int x) { ... }

Which of these would match the above delegate? Because we can pass the method reference around (as a delegate) to let some other code decide when to use it

Merineth 🇸🇪•2w ago

the second one remove (except the typo inte) since it returns a decimal and takes 2 integers k and x

Pobiega•2w ago

correct. the names dont matter only the types matter

Merineth 🇸🇪•7d ago

Couldn't we just call the removedirectly instead of having to do a delegate? especially since it's public and static

Pobiega•7d ago

we could, but where delegates become useful is when we need to separate the decision of what method to call and when to actually call it

Merineth 🇸🇪•7d ago

Hmm, i'm 100% clear on what it is now, i'm just a little unclear what that last part means If i were writing some code, for eg a calculator, i would just call the methods i need directly such as Add, Sub, Mult etc instead of passing the method of Add, Sub, Mult to the delegate which then in turn performs the calculations But i'm probably missing the point of delegates

Pobiega•7d ago

Yeah let me find a good example

Merineth 🇸🇪•7d ago

:catthinking:

Pobiega•7d ago

okay You are writing a console app one thing you do a lot in console apps is ask the user for input so you, as a clever developer, decide to write a method to help you make this easier like string username = AskUser("What is your username?"); with me so far?

Merineth 🇸🇪•7d ago

Yeah

Pobiega•7d ago

okay. We use this method for several things. Like, asking for a username, their date of birth, their first and last names... etc all is good

Merineth 🇸🇪•7d ago

Right

Pobiega•7d ago

then someone types "CHEESECAKE" as their birthdate and your program crashes okay, thats a problem. We need to validate the input One way to do this would be to write something like...

string birthdate = AskUser("Enter your birthdate: ");
if(StringIsValidBirthdate(birthdate))
{
    Console.WriteLine("You entered a valid birthdate.");
}
else
{
    Console.WriteLine("You entered an invalid birthdate.");
    birthDate = AskUser("Enter your birthdate again: ");
}

string birthdate = AskUser("Enter your birthdate: ");
if(StringIsValidBirthdate(birthdate))
{
    Console.WriteLine("You entered a valid birthdate.");
}
else
{
    Console.WriteLine("You entered an invalid birthdate.");
    birthDate = AskUser("Enter your birthdate again: ");
}

but alas, if the user keeps entering invalid values, we have problems okay, so we use a loop to re-ask until the user enters a valid value! but we need to do this EVERYWHERE we ask for user input.... and what is a "valid value" changes depending on what we asked like, everyone knows lastnames can only end in sson birthdates are purely numeric, except for the dashes. ages are numeric with no other signs allowed etc so now our code is super messy

Merineth 🇸🇪•7d ago

Yeah for sure, having to take into account every single possible problem would create a lot of code

Pobiega•7d ago

so now our code looks like this

string birthdate = AskUser("Enter your birthdate: ");
while (!ValidBirthdate(birthdate))
{
    birthdate = AskUser("Invalid birthdate. Please enter a valid birthdate: ");
}

string lastname = AskUser("Enter your lastname: ");
while (!ValidLastname(lastname))
{
    lastname = AskUser("Invalid lastname. Please enter a valid lastname: ");
}

string birthdate = AskUser("Enter your birthdate: ");
while (!ValidBirthdate(birthdate))
{
    birthdate = AskUser("Invalid birthdate. Please enter a valid birthdate: ");
}

string lastname = AskUser("Enter your lastname: ");
while (!ValidLastname(lastname))
{
    lastname = AskUser("Invalid lastname. Please enter a valid lastname: ");
}

we can see that there is a pattern here, right? we store the given value, then we validate it and ask again with a loop and we do this for every input, with the difference beeing what we ask for, the validation method and what the error message is if its invalid so what if we could change our AskUser method to take these in? something like...

string username = AskUser("Enter username:", ValidateUsername, "Invalid username, try again");

string username = AskUser("Enter username:", ValidateUsername, "Invalid username, try again");

wouldnt that be nice? like, if we could do that, the code above would turn into...

string username = AskUser("Enter username:", ValidateUsername, "Invalid username, try again");
string birthdate = AskUser("Enter your birthdate: ", ValidateBirthdate, "Invalid birthdate, try again");
string lastname = AskUser("Enter your lastname: ", ValidateLastname, "Invalid lastname, try again");

string username = AskUser("Enter username:", ValidateUsername, "Invalid username, try again");
string birthdate = AskUser("Enter your birthdate: ", ValidateBirthdate, "Invalid birthdate, try again");
string lastname = AskUser("Enter your lastname: ", ValidateLastname, "Invalid lastname, try again");

damn, thats so much nicer! Are you still with me?

Merineth 🇸🇪•7d ago

I think so? I'm not familiar with tying string as my parameters tho

Pobiega•7d ago

wdym?

Merineth 🇸🇪•7d ago

In the parameters for AskUser you are typing strings

Pobiega•7d ago

its just a string as a parameter. same thing as Add(5,10); except there its ints

Merineth 🇸🇪•7d ago

In that case 5 and 10 are arguments You are using the strings as parameters

Pobiega•7d ago

so?

Merineth 🇸🇪•7d ago

I've never done that before, didn't even know that was allowed

Pobiega•7d ago

there is no difference, except int instead of string of course its allowed

Merineth 🇸🇪•7d ago

Parameters are a type followed by a name I don't see that in ur example I see a text of string, a name and then a text of string again

Pobiega•7d ago

thats only in declarations of methods not in usages when you USE a method, you just give a value. Either as a literal (typed as is then and there) or a variable

Merineth 🇸🇪•7d ago

Ok, so you are passing 2 strings and a aValidateUsername method?

Pobiega•7d ago

yeah, something like that

Merineth 🇸🇪•7d ago

:/

Pobiega•7d ago

our old method looked like this:

string AskUser(string prompt)
{
    Console.WriteLine(prompt);
    return Console.ReadLine();
}

string AskUser(string prompt)
{
    Console.WriteLine(prompt);
    return Console.ReadLine();
}

when it just had one parameter the new one looks like this:

string AskUser(string prompt, what_do_we_type_here validator, string errorMessage)
...

string AskUser(string prompt, what_do_we_type_here validator, string errorMessage)
...

Merineth 🇸🇪•7d ago

Ók so how does the AskUser method look like declared?

Pobiega•7d ago

but now I ask you... what can we type there? what type exists in C# that can accept a method of a known signature?

Merineth 🇸🇪•7d ago

It's a delegate, but i got lost in the last part of the example.

Pobiega•7d ago

ok lets rewind and fix that confusion first then can you tell me exactly where I lost you?

Clint•7d ago

Delegates are pointers to a function, it's just a way of passing the location of a function (and its signature) around. Let's say you have a list of integers and a function that squares integers and you want to end up with a list of squared integers.

int Square(int num) {
  return num ^ 2;
}

List<int> SquareIntsTheOldWay(List<int> ints) {
  var result = new List<int>();

  foreach(var num in ints) {
    result.Add(Square(num));
  }

  return result;
}

List<int> DoSomethingToInts(List<int> nums, Func<int, int> something) {
  var result = new List<int>();
  foreach(var num in nums) {
    result.Add(something(num));
  }
}

int Square(int num) {
  return num ^ 2;
}

List<int> SquareIntsTheOldWay(List<int> ints) {
  var result = new List<int>();

  foreach(var num in ints) {
    result.Add(Square(num));
  }

  return result;
}

List<int> DoSomethingToInts(List<int> nums, Func<int, int> something) {
  var result = new List<int>();
  foreach(var num in nums) {
    result.Add(something(num));
  }
}

Then going further you could expand out to a generic use case:

IEnumerable<T> DoSomethingToInts(IEnumerable<int> nums, Func<int, T> something) {
  foreach(var num in nums) {
    yield return something(num);
  }
}

IEnumerable<T> DoSomethingToInts(IEnumerable<int> nums, Func<int, T> something) {
  foreach(var num in nums) {
    yield return something(num);
  }
}

Which you could then call do to all sorts of things!

var Squared = DoSomethingToInts(myInts, (i) => i ^ 2);

var toString = DoSomethingToInts(myInts, (i) => i.ToString());

var Squared = DoSomethingToInts(myInts, (i) => i ^ 2);

var toString = DoSomethingToInts(myInts, (i) => i.ToString());

And so on so forth. This is (basically) what LINQ's Select method is doing:

var squared = myInts.Select((i) => i ^ 2);

var squared = myInts.Select((i) => i ^ 2);

Pobiega•7d ago

Not sure how its helpful to drop a full page of code involving lambdas and generics to someone not understanding delegates my guy

Clint•7d ago

Gives some code to try and then watch what it gets up to, hard to explain the concept without involving the type signatures of what's going on.

Merineth 🇸🇪•7d ago

Is !ValidateLastname a CBL method?

Pobiega•7d ago

no, its a method you would write yourself we can already tell it returns a bool (since its used as a condition for a while), and that it takes in a string

bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

for example

Merineth 🇸🇪•7d ago

I'm with you to this part. Everytime we ask the user for input we have to validate what they wrote, age, name, etc.

Pobiega•7d ago

yep the thing is, we dont want to write a new method for each step like, I dont want to write a AskUserForName method, and a AskUserForLastname method, and a AskUserForBirthdate method

Merineth 🇸🇪•7d ago

We have to? The error message and the condition changes for every input the user puts in?

Pobiega•7d ago

I want to write ONE method for asking for input, and using parameters to change what it does in the calculator example, you dont write a AddFivePlusSeven() method you write a single Add(int x, int y) method

Merineth 🇸🇪•7d ago

Ok, can a delegate handle all of these possible inputs and error messages?

Pobiega•7d ago

wait what? what does the error message have to do with the delegates?

Merineth 🇸🇪•7d ago

Idk? You tell me You are trying to tell me that you can handle every error message, validate the string the user inputs and output the correct message with just one method

Pobiega•7d ago

yeah, but only the validation has anything to do with delegates the prompt and the error message are just completely normal method parameters for example, here is AskUser without custom validation:

string AskUser(string prompt, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        var input = Console.ReadLine();


        if (input == null)
        {
            Console.WriteLine(errorMessage);
        }
        else
        {
            return input;
        }
    }
}

string AskUser(string prompt, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        var input = Console.ReadLine();


        if (input == null)
        {
            Console.WriteLine(errorMessage);
        }
        else
        {
            return input;
        }
    }
}

Merineth 🇸🇪•7d ago

Ok. In that case the parameter for the delegate would just be a string and the return would be void. 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 🤣 this makes no sense now

Pobiega•7d ago

what makes no sense? we haven't gotten to delegates yet :p

Merineth 🇸🇪•7d ago

\o/ Idk the example ig You lost me when you tried introducing two strings as messages and a validate method

Pobiega•7d ago

ignore the validation method for now I dont understand how strings are confusing you thou

Merineth 🇸🇪•7d ago

Not sure how to respond to that If i don't get it i don't get it :/

Pobiega•7d ago

a string is just another datatype

Merineth 🇸🇪•7d ago

If i wanted to ask the user for user input

Pobiega•7d ago

there is nothing different about them as opposed to say ints, or decimals, or doubles continue this line of thought, please

Merineth 🇸🇪•7d ago

Nah idk anymore i'm 100% fully lost now I know what a delegate is now but the usage for it make absolutely zero sence

Pobiega•7d ago

I cant even get to the delegate example because you got stuck on string parameters :/

Merineth 🇸🇪•7d ago

Why would i ever want to pass something to a delegate when i can just call the method i want directly? It makes nooooooooo sense. It's just an unnecessary step

Pobiega•7d ago

it really isnt but look you said yourself that this was perfectly fine and not confusing

string birthdate = AskUser("Enter your birthdate: ");

string birthdate = AskUser("Enter your birthdate: ");

Merineth 🇸🇪•7d ago

mm'

Pobiega•7d ago

so why did it become confusing when I added a second string?

string birthdate = AskUser("Enter your birthdate: ", "Invalid birthdate, try again.");

string birthdate = AskUser("Enter your birthdate: ", "Invalid birthdate, try again.");

Merineth 🇸🇪•7d ago

@Clint why are you dming me? Like i have this channel open for a reason :/

Clint•7d ago

Just letting you know that I'm available if you want options and alternative ways of going through this, I think it's great you're looking to learn this, but appreciate that a busy chat window isn't always the best forum for it 🙂 No worries if not though, no problem.

Pobiega•7d ago

Its absolutely fine to drop options and alternatives in the thread. Unsolicited DMs are discouraged.

Clint•7d ago

Ah I wasn't aware, apologies.

Pobiega•7d ago

No worries

Clint•7d ago

But my offer still stands, we will have you understanding how delegates work cos they're an extremely useful tool! Pobiega is right, multitude of uses And they show up all over the place in the core library

Pobiega•7d ago

I think we are pretty close to introducing them, if we can just get over the string confusion 🙂

Merineth 🇸🇪•7d ago

Ok so you are creating a method AskUser which has the parameters and return type string Askuser(string username, method ValidateUsername, string error message)?

Clint•7d ago

What if take a step back? Merineth what problem is you're trying to solve? Perhaps we can tackle from first principles and slot the delegates in gradually?

Pobiega•7d ago

yeah pretty much!

Merineth 🇸🇪•7d ago

Ok

Pobiega•7d ago

the thing is that what the validate method does changes depending on what we ask for right? we validate usernames differently from lastnames, or birthdates

Merineth 🇸🇪•7d ago

Yes

Pobiega•7d ago

but everything else about the "asking user for input" remains exactly the same and we know that delegates are how we pass methods around... so we reach this point:

delegate ? ValidatorDelegate(? input);

string AskUser(string prompt, ValidatorDelegate validator, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        string input = Console.ReadLine();

        if (validator(input))
        {
            return input;
        }

        Console.WriteLine(errorMessage);
    }
}

delegate ? ValidatorDelegate(? input);

string AskUser(string prompt, ValidatorDelegate validator, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        string input = Console.ReadLine();

        if (validator(input))
        {
            return input;
        }

        Console.WriteLine(errorMessage);
    }
}

What do you think we should replace the two ? with? remember, delegates are types, but also strictly typed - they declare what method signatures are valid for the delegate hint: if(validator(input)) tells you a lot about the delegate

Clint•7d ago

And if it helps, think of:

delegate ? ValidatorDelegate(? input);

delegate ? ValidatorDelegate(? input);

Like the way you'd write a method signature, just with an extra keyword at the start, and NO implementation body.

Pobiega•7d ago

yep

Merineth 🇸🇪•7d ago

string and string?

Pobiega•7d ago

is if("hello") valid? what does an if() expect to be passed in?

Merineth 🇸🇪•7d ago

    internal class Program
    {
        static void Main(string[] args)
        {
            string username = AskUsername("What's your username");
            string age = AskAge("What's your age?");
        }

        private static string AskAge(string v)
        {
            if(/*it isn't an integer between 18 and 100*/)
            {
                Console.WriteLine("Wrong input");
                AskAge(v);
            }
        }

        private static string AskUsername(string v)
        {
            if (/*doesn't end it sson*/)
            {
                Console.WriteLine("Wrong input");
                AskUsername(v);
            }
            else
            {
                return v;
            }
        }
    }

    internal class Program
    {
        static void Main(string[] args)
        {
            string username = AskUsername("What's your username");
            string age = AskAge("What's your age?");
        }

        private static string AskAge(string v)
        {
            if(/*it isn't an integer between 18 and 100*/)
            {
                Console.WriteLine("Wrong input");
                AskAge(v);
            }
        }

        private static string AskUsername(string v)
        {
            if (/*doesn't end it sson*/)
            {
                Console.WriteLine("Wrong input");
                AskUsername(v);
            }
            else
            {
                return v;
            }
        }
    }

This is what i'd do for your example.

Pobiega•7d ago

yeah, makes sense. thats how you solve this problem without delegates but look how similar those methods are they are in fact almost identical

Merineth 🇸🇪•7d ago

yes, in terms of parameters and return value

Pobiega•7d ago

exactly so, can we maybe return to the delegate example? we really are SO CLOSE

Merineth 🇸🇪•7d ago

Haha yeah we are probably close for you to understand it :/ but i don't get it

Pobiega•7d ago

I mean, it feels like you are not really giving me/it a chance here

Merineth 🇸🇪•7d ago

I fail to see how you can take a parameter, a return value and have them do two different things without making two different methods for them What more can i say, i'm trying

Clint•7d ago

public bool ValidateLastName(string value) {
  // return some validation pass / fail result
}

public bool ValidateLastName(string value) {
  // return some validation pass / fail result
}

Take the types of the return value and the arguments, what do you think the delegate signature looks like for that?

Pobiega•7d ago

what must the delegate declaration look like for that method to be valid for the delegate?

Merineth 🇸🇪•7d ago

public delegate string AskUser(string input);

public delegate string AskUser(string input);

Like that ig?

Clint•7d ago

so close

Pobiega•7d ago

Why string as return value? what return value does the method have?

Merineth 🇸🇪•7d ago

my example or yours?

Pobiega•7d ago

yours

Merineth 🇸🇪•7d ago

it returns v, a string

Clint•7d ago

Oh okay, I see the confusion here, your ask methods are returning string but we're getting tripped up on the validation part

Pobiega•7d ago

Oh yeah, skip that. We're back to using separate validation methods, not your "all in one" methods

bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

for example this is my suggested lastname validator that is a little biased regarding what constitutes a valid lastname

Merineth 🇸🇪•7d ago

EndsWith is a CBL method which returens a bool?

Clint•7d ago

public string AskUser(string message, ? validator, string failureMessage) {
  var value = Console.ReadLine(message);
  if (!validator(value)) {
    Console.WriteLine(failureMessage);
  }

  // code to go around while we don't have a valid value not included
}

public string AskUser(string message, ? validator, string failureMessage) {
  var value = Console.ReadLine(message);
  if (!validator(value)) {
    Console.WriteLine(failureMessage);
  }

  // code to go around while we don't have a valid value not included
}

That's more like what we're aiming for, yes? Where ? is the delegate to perform the validation check.

Pobiega•7d ago

yes (but its called BCL :D)

Merineth 🇸🇪•7d ago

Ok

    internal class Program
    {
        static void Main(string[] args)
        {
            string username = AskUsername("What's your username");
            string age = AskAge("What's your age?");
        }

        private static string AskAge(string v)
        {
            if(/*it isn't an integer between 18 and 100*/)
            {
                Console.WriteLine("Wrong input");
                AskAge(v);
            }
        }
        bool ValidateLastname(string name)
        {
            return name.EndsWith("sson");
        }

        private static string AskUsername(string v)
        {
            if (/*doesn't end it sson*/)
            {
                Console.WriteLine("Wrong input");
                AskUsername(v);
            }
            else
            {
                return v;
            }
        }

    internal class Program
    {
        static void Main(string[] args)
        {
            string username = AskUsername("What's your username");
            string age = AskAge("What's your age?");
        }

        private static string AskAge(string v)
        {
            if(/*it isn't an integer between 18 and 100*/)
            {
                Console.WriteLine("Wrong input");
                AskAge(v);
            }
        }
        bool ValidateLastname(string name)
        {
            return name.EndsWith("sson");
        }

        private static string AskUsername(string v)
        {
            if (/*doesn't end it sson*/)
            {
                Console.WriteLine("Wrong input");
                AskUsername(v);
            }
            else
            {
                return v;
            }
        }

In that case it looks like this inside the if for askusername i'd include it there

Pobiega•7d ago

why are we back to your example?

Merineth 🇸🇪•7d ago

I thought we never left?

Pobiega•7d ago

As said, thats how we do things without delegates but you are trying to learn delegates

Merineth 🇸🇪•7d ago

Ok, so where are we then? What code are we basing it of

Pobiega•7d ago

string username = AskUser("Enter username:", ValidateUsername, "Invalid username, try again");
string birthdate = AskUser("Enter your birthdate: ", ValidateBirthdate, "Invalid birthdate, try again");
string lastname = AskUser("Enter your lastname: ", ValidateLastname, "Invalid lastname, try again");

bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

string AskUser(string prompt, ValidatorDelegate validator, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        string input = Console.ReadLine();

        if (validator(input))
        {
            return input;
        }

        Console.WriteLine(errorMessage);
    }
}

delegate bool ValidatorDelegate(string input);

string username = AskUser("Enter username:", ValidateUsername, "Invalid username, try again");
string birthdate = AskUser("Enter your birthdate: ", ValidateBirthdate, "Invalid birthdate, try again");
string lastname = AskUser("Enter your lastname: ", ValidateLastname, "Invalid lastname, try again");

bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

string AskUser(string prompt, ValidatorDelegate validator, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        string input = Console.ReadLine();

        if (validator(input))
        {
            return input;
        }

        Console.WriteLine(errorMessage);
    }
}

delegate bool ValidatorDelegate(string input);

Merineth 🇸🇪•7d ago

ok I don't get the last delegate part, everything else is fine

Pobiega•7d ago

well, a validation method here takes in a string (the suggested value) and returns a bool (if it was valid or not) the delegate declares what signature a method must have to be a validator any method that takes in a string and returns a bool can be used as a validator

Merineth 🇸🇪•7d ago

Ok, but you are still missing ValidateBirthdate and ValidateLastname?

Pobiega•7d ago

username and birthdate, yes they would have the exact same signature, but different bodies

bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

bool ValidateBirthdate(string birthdate)
{
    return DateTime.TryParse(birthdate, out _);
}

bool ValidateUsername(string username)
{
    return username.Length > 3;
}

bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

bool ValidateBirthdate(string birthdate)
{
    return DateTime.TryParse(birthdate, out _);
}

bool ValidateUsername(string username)
{
    return username.Length > 3;
}

here are some suggestions of potential bodies notice how they all have the same return value, and the same input parameters, but different names, different bodies

Merineth 🇸🇪•7d ago

Yes.

Pobiega•7d ago

so with a delegate, we can separate out the part that was actually different, and re-use the parts that were the same asking the user for input, taking in the value, checking if it was valid, all that was the same. The only part that actually changed between our three calls was "what constitutes a valid value for this variable?"

Merineth 🇸🇪•7d ago

string username = AskUser("Enter username:", ValidateUsername, "Invalid username, try again");
string birthdate = AskUser("Enter your birthdate: ", ValidateBirthdate, "Invalid birthdate, try again");
string lastname = AskUser("Enter your lastname: ", ValidateLastname, "Invalid lastname, try again");

bool ValidateBirthdate(string birthdate)
{
    return DateTime.TryParse(birthdate, out _);
}

bool ValidateUsername(string username)
{
    return username.Length > 3;
}
bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

string AskUser(string prompt, ValidatorDelegate validator, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        string input = Console.ReadLine();

        if (validator(input))
        {
            return input;
        }

        Console.WriteLine(errorMessage);
    }
}

delegate bool ValidatorDelegate(string input);

string username = AskUser("Enter username:", ValidateUsername, "Invalid username, try again");
string birthdate = AskUser("Enter your birthdate: ", ValidateBirthdate, "Invalid birthdate, try again");
string lastname = AskUser("Enter your lastname: ", ValidateLastname, "Invalid lastname, try again");

bool ValidateBirthdate(string birthdate)
{
    return DateTime.TryParse(birthdate, out _);
}

bool ValidateUsername(string username)
{
    return username.Length > 3;
}
bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

string AskUser(string prompt, ValidatorDelegate validator, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        string input = Console.ReadLine();

        if (validator(input))
        {
            return input;
        }

        Console.WriteLine(errorMessage);
    }
}

delegate bool ValidatorDelegate(string input);

So we are here?

Pobiega•7d ago

yes We can clean this up even further, but that would involve using generics and lambda methods, which you might not be comfortable with yet 🙂

Merineth 🇸🇪•7d ago

So instead of having 3 different Askuser functions for username, birthdate and lastname we create one generic askuser function and pass the criteria it needs as a method by utilizing a delegate?

Pobiega•7d ago

yes! exactly so and this is good, because if you later need to change something about how you ask users for input, you can change that in ONE place - instead of in each method

Merineth 🇸🇪•7d ago

Ok i do see the benefit of delegates, it's just so hard for me to utilize without practice that i'd end up doing the 3 askuser functions because it'd go faster for me

Pobiega•7d ago

I mean, thats to be expected, you have known methods for a while now but understood delegates maybe a few minutes ago Learning of something != naturalizing it

Merineth 🇸🇪•7d ago

You mentionder generics and lambda methods? Got until monday to learn everything lol, so i gotta try at least

Pobiega•7d ago

yeah. Do you perhaps remember us speaking of Action<T> and Func<TResult, T>?

Merineth 🇸🇪•7d ago

Yes

Pobiega•7d ago

we called them "generic delegates"

Merineth 🇸🇪•7d ago

and =>

Pobiega•7d ago

because thats what they are

Merineth 🇸🇪•7d ago

Yea, generic because of the T

Pobiega•7d ago

well, actually because of the <>

Merineth 🇸🇪•7d ago

iirc it's generic becuase we don't know the input type?

Pobiega•7d ago

yeah, its a placeholder so here is the same ask method with a generic delegate instead of your own

string AskUser(string prompt, Func<string, bool> validator, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        string input = Console.ReadLine();

        if (validator(input))
        {
            return input;
        }

        Console.WriteLine(errorMessage);
    }
}

string AskUser(string prompt, Func<string, bool> validator, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        string input = Console.ReadLine();

        if (validator(input))
        {
            return input;
        }

        Console.WriteLine(errorMessage);
    }
}

Merineth 🇸🇪•7d ago

Right, sorry <> . The T is just the name of the parameter

Pobiega•7d ago

exactly boom, same method, only thing changed was I replaced ValidatorDelegate with Func<string, bool>

Merineth 🇸🇪•7d ago

string username = AskUser("Enter username:", ValidateUsername, "Invalid username, try again");
string birthdate = AskUser("Enter your birthdate: ", ValidateBirthdate, "Invalid birthdate, try again");
string lastname = AskUser("Enter your lastname: ", ValidateLastname, "Invalid lastname, try again");

bool ValidateBirthdate(string birthdate)
{
    return DateTime.TryParse(birthdate, out _);
}

bool ValidateUsername(string username)
{
    return username.Length > 3;
}
bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

string AskUser(string prompt, Func<string, bool> validator, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        string input = Console.ReadLine();

        if (validator(input))
        {
            return input;
        }

        Console.WriteLine(errorMessage);
    }
}

string username = AskUser("Enter username:", ValidateUsername, "Invalid username, try again");
string birthdate = AskUser("Enter your birthdate: ", ValidateBirthdate, "Invalid birthdate, try again");
string lastname = AskUser("Enter your lastname: ", ValidateLastname, "Invalid lastname, try again");

bool ValidateBirthdate(string birthdate)
{
    return DateTime.TryParse(birthdate, out _);
}

bool ValidateUsername(string username)
{
    return username.Length > 3;
}
bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

string AskUser(string prompt, Func<string, bool> validator, string errorMessage)
{
    while (true)
    {
        Console.Write(prompt);
        string input = Console.ReadLine();

        if (validator(input))
        {
            return input;
        }

        Console.WriteLine(errorMessage);
    }
}

So we end up like this?

Pobiega•7d ago

almost. you can safely throw the delegate declaration at the bottom we dont need it anymore there you go

Merineth 🇸🇪•7d ago

The Func<> does that for us?

Pobiega•7d ago

yep look at the types it takes in string and bool the last type is the return value so Func<T, TResult> returns a TResult and takes a T in

Merineth 🇸🇪•7d ago

~~Ok so the left side of , is return value and right side of , is input parameter?~~ No opposite?

Pobiega•7d ago

yeah, opposite

Merineth 🇸🇪•7d ago

Ok so the right side of , is return value and left side of , is input parameter?

Pobiega•7d ago

No description

Pobiega•7d ago

look here at what Rider is showing me

Merineth 🇸🇪•7d ago

What's the difference between Func<T> and Action<T> I think it was called action?

Pobiega•7d ago

it is! Action has a void return type

Merineth 🇸🇪•7d ago

Ah

public delegate void AskUser(string input)

public delegate void AskUser(string input)

So this would be Action?

Pobiega•7d ago

yep! Action<string> would be matched by the same methods

Merineth 🇸🇪•7d ago

Makes sense what abt lambda?

Pobiega•7d ago

sooooooo

bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

bool ValidateLastname(string name)
{
    return name.EndsWith("sson");
}

this method is very short what if we could just.... skip declaring it and giving it a name? we only use it in one place anyways what if I told you we can do just that

string lastname = AskUser("Enter your lastname: ", name => name.EndsWith("sson"), "Invalid lastname, try again");

string lastname = AskUser("Enter your lastname: ", name => name.EndsWith("sson"), "Invalid lastname, try again");

BOOM this is called "an anonymous method"

Merineth 🇸🇪•7d ago

Right i remember that a method that isn't declared just instantly used

Pobiega•7d ago

sort of it is actually declared, this isnt a usage

Merineth 🇸🇪•7d ago

So essentially, delegates, func, action and lambda are things in C# to make code faster to write and easier to read?

Pobiega•7d ago

yeah remember, all this compiles down to CPU instructions at the end anyways and thats true for all programming languages so everything that isnt a raw CPU instruction is just a convenience for the developer 😄

Merineth 🇸🇪•7d ago

Does it become easier for the CPU to handle if we use delegates, or is it purely for the dev?

Pobiega•7d ago

purely for the devs

Merineth 🇸🇪•7d ago

Makes sense purely for the dev convience (except me)

Pobiega•7d ago

you have a point. Look at the Go programming language as an example

Merineth 🇸🇪•7d ago

But yea i got a decent basic understanding of delegates now

Pobiega•7d ago

its intentionally kept simple and low amount of features

Merineth 🇸🇪•7d ago

Not sure how i'm gonna practice it tho

Pobiega•7d ago

because introducing many alternative ways to do the same thing => different styles and confusion there is a certain value to that too thats fair. Try experimenting with this validation code we have here for example, what would happen if I suddenly wanted to ask the user for an int directly, instead of a string?

Merineth 🇸🇪•7d ago

We were only given one exercise which includes lambda ... and it's kind of complex

Pobiega•7d ago

here is a screenshot of my ConsoleHelper methods I use when I write console apps, as inspiration. Plenty of lambdas, generics and delegates here

No description

Merineth 🇸🇪•7d ago

public class Command<T> : ICommand
{
    private T target;
    private Action<T> command;

    public Command(T target, Action<T> command)
    {
        this.target = target;
        this.command = command;
    }
    public void Execute()
    {
        command(target);
    }

public class Command<T> : ICommand
{
    private T target;
    private Action<T> command;

    public Command(T target, Action<T> command)
    {
        this.target = target;
        this.command = command;
    }
    public void Execute()
    {
        command(target);
    }

No description

Merineth 🇸🇪•7d ago

Am i high? Or are there no lambda expression in this code

Can the Action<T> class be replaced with a user-
defined delegate type with the same functionality?

Shouldn't the Action<T>have two parameters? 💀 ok this is wayyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy to complicated thanks course responsible Anyhow i'll continue with my studies (not delegates) thanks for tthe help!

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