How do I get a return value from a Button Click event?

Question

I just started learning C#. I saw an old question of someone trying to make a coca-cola machine and it seemed like a good exercise.

But I got stuck on the money buttons. I can't figure out how I can store the amount of money a button represents in a variable, accessible by the ColaMachine method.

I've got the following code:

using System;
using System.Windows.Forms;
using System.Drawing;

namespace QuickSharp
{
    public class ColaMachine : Form
    {
        public ColaMachine()
        {
            this.Text = "Cola Machine";
            this.Size = new Size(450 , 500);

            //Money & Money Buttons   

            Label Money;
            Money = new Label();
            Money.Text = "Insert Coins Here:";
            Money.Location = new Point(20, 100);
            this.Controls.Add(Money);

            Button MoneyButton1;
            MoneyButton1 = new Button();
            MoneyButton1.Text = "€0,05";
            MoneyButton1.Location = new Point(28,125);
            MoneyButton1.Click += new System.EventHandler(this.MoneyButton1_Click);
            this.Controls.Add(MoneyButton1);

            Button MoneyButton2;
            MoneyButton2 = new Button();
            MoneyButton2.Text = "€0,10";
            MoneyButton2.Location = new Point(28,165);
            MoneyButton2.Click += new System.EventHandler(this.MoneyButton2_Click);
            this.Controls.Add(MoneyButton2);

            Button MoneyButton3;
            MoneyButton3 = new Button();
            MoneyButton3.Text = "€0,20";
            MoneyButton3.Location = new Point(28,205);
            MoneyButton3.Click += new System.EventHandler(this.MoneyButton3_Click);
            this.Controls.Add(MoneyButton3);

            Button MoneyButton4;
            MoneyButton4 = new Button();
            MoneyButton4.Text = "€0,50";
            MoneyButton4.Location = new Point(28,245);
            MoneyButton4.Click += new System.EventHandler(this.MoneyButton4_Click);
            this.Controls.Add(MoneyButton4);

            Button MoneyButton5;
            MoneyButton5 = new Button();
            MoneyButton5.Text = "€1,00";
            MoneyButton5.Location = new Point(28,285);
            MoneyButton5.Click += new System.EventHandler(this.MoneyButton5_Click);
            this.Controls.Add(MoneyButton5);

            Button MoneyButton6;
            MoneyButton6 = new Button();
            MoneyButton6.Text = "€2,00";
            MoneyButton6.Location = new Point(28,325);
            MoneyButton6.Click += new System.EventHandler(this.MoneyButton6_Click);
            this.Controls.Add(MoneyButton6);

            // Drinks & Drink Buttons

            Label Drinks;
            Drinks = new Label();
            Drinks.Text = "Choose Your Drink:";
            Drinks.Location = new Point(315 , 100);
            Drinks.AutoSize = true;
            this.Controls.Add(Drinks);

            Button DrinkButton1;
            DrinkButton1 = new Button();
            DrinkButton1.Text = "Coca-Cola";
            DrinkButton1.Location = new Point(328,125);
            this.Controls.Add(DrinkButton1);

                        Button DrinkButton2;
            DrinkButton2 = new Button();
            DrinkButton2.Text = "Coca-Cola Light";
            DrinkButton2.Location = new Point(328,165);
            this.Controls.Add(DrinkButton2);

                        Button DrinkButton3;
            DrinkButton3 = new Button();
            DrinkButton3.Text = "Fanta";
            DrinkButton3.Location = new Point(328,205);
            this.Controls.Add(DrinkButton3);

                        Button DrinkButton4;
            DrinkButton4 = new Button();
            DrinkButton4.Text = "Sprite";
            DrinkButton4.Location = new Point(328,245);
            this.Controls.Add(DrinkButton4);

                        Button DrinkButton5;
            DrinkButton5 = new Button();
            DrinkButton5.Text = "Spa Blauw";
            DrinkButton5.Location = new Point(328,285);
            this.Controls.Add(DrinkButton5);

                        Button DrinkButton6;
            DrinkButton6 = new Button();
            DrinkButton6.Text = "Red Bull";
            DrinkButton6.Location = new Point(328,325);
            this.Controls.Add(DrinkButton6);

            //Header & Machine Display

            Label Header;
            Header = new Label();
            Header.Text = "Coca-Cola Machine";
            Header.Font = new Font("Arial" , Header.Font.Size +5);
            Header.ForeColor = Color.DarkRed;
            Header.Location = new Point(132, 20);
            Header.AutoSize = true;
            this.Controls.Add(Header);



            TextBox TextBox1 ;
            TextBox1 = new TextBox();

            if(InsertedCoins == 0.00)
                TextBox1.Text = "Buy Your Ice Cold Drinks Here!";
            else
                TextBox1.Text = "Inserted Coins: €" + InsertedCoins;

            TextBox1.BackColor = Color.Black;
            TextBox1.ForeColor = Color.Red;
            TextBox1.Font = new Font("Arial" , TextBox1.Font.Size +3);
            TextBox1.ReadOnly = true;
            TextBox1.Size = new Size(210,300);
            TextBox1.Location = new Point(112,50);

            // I tried to get the text scrolling here... :)
            TextBox1.SelectionStart = TextBox1.Text.Length;
            TextBox1.ScrollToCaret();
            TextBox1.Refresh();

            this.Controls.Add(TextBox1);
        }


        public double InsertedCoins;

        // Money Button Click Events

        private void MoneyButton1_Click(object sender, EventArgs e)
        {
            InsertedCoins = InsertedCoins + 0.05;
        }

        private void MoneyButton2_Click(object sender, EventArgs e)
        {
            InsertedCoins = InsertedCoins + 0.10;
        }

        private void MoneyButton3_Click(object sender, EventArgs e)
        {
            InsertedCoins = InsertedCoins + 0.20;
        }

        private void MoneyButton4_Click(object sender, EventArgs e)
        {
            InsertedCoins = InsertedCoins + 0.50;
        }

        private void MoneyButton5_Click(object sender, EventArgs e)
        {
            InsertedCoins = InsertedCoins + 1.00;
        }

        private void MoneyButton6_Click(object sender, EventArgs e)
        {
            InsertedCoins = InsertedCoins + 2.00;
        }

        private static void Main()
        {

            ColaMachine Scherm;
            Scherm = new ColaMachine();
            Application.Run(Scherm);
        }
    }
}

Also, if you have any tips on my general programming (eg to make things easier-to-follow for others trying to read my code), please tell me!

Answer 1

When I think about a Coke machine I see a button for each type of drink in the machine, but not buttons for different amounts of money. Maybe you mean a coke costs 50 cents so pressing the coke button I need to charge 50 cents.

Buttons and Event Handlers

When you press a button on the screen it generates a click event . You need to write a method to respond to that click. Any method that we use to respond to an event (generally speaking) is called an event handler . You must tell your program what buttons go with what event handlers. We call this registering the event handler

By convention, if your button is named 'CokeButton' then the event handler associated with that specific button would be named 'CokeButton_ClickHandler'. Or something like that.

General Advice

Think about the thing you are modeling and define things in code to reflect the real world. The things in your model typically will end up as classes, class properties, and class fields. What these things do typically end up as methods w/in the appropriate class. Then you think about how these things interact.

You do not need to figure out everything about a coke machine before you begin writing code. And you should write little bits at a time, test those and then build on what you've tested. Do not write oodles of complex-ish interacting code and then test. You'll end up spinning in circles chasing your tail. Write a little, test a little, repeat. Hear me now and believe me later; write a little, test a little, repeat. Heed this advice now and forever.

So here's how I might think about a Coke Machine. First there is a coke machine itself.

public class CokeMachine {}

A coke machine has a money slot, a return slot, and drink buttons. I can't really put money in a slot, so off hand, I'd say I'll type into a text box. Then I'll click a button and the coke will dispense. I feel like I've defined enough of the model to get started. There's lots of other things about a Coke Machine but I'm not going to worry about them right now.

But I need to know how much each drink costs.

Well, OK. then there must be "CokeCost", "7UpCost", etc. fields. So define them! We'll figure out how and where to use them as we go along.

   public class CokeMachine {
     Button Coke;
     Button 7Up;
     Button RootBeer;
     TextBox MoneySlot;

     double CokeCost = .75;
     double 7UpCost = .65;
}

I said the buttons need handlers, so we can write some code shells at least. I expect they'll all work the same way so I'll focus on one for now. Note that as I write code I realize other things that must be dealt with. I'll put in comments, calls to methods that don't exist yet, etc.

   public class CokeMachine {
     Button Coke;
     Button 7Up;
     Button RootBeer;
     TextBox MoneySlot;

     double CokeCost = .75;
     double 7UpCost = .65;

     // "wiring up" the coke button click event to it's handler.
     // We do this in C# by declaring an new EventHandler object (a .NET framework supplied class)
     // and we pass in the name of our method as a parameter.
     // This new EventHandler is *added* to the button's click event.
     // An event can have multiple handlers, that's why we do "+="
     // instead of just "=". Otherwise we would have accidentally "unhooked" any
     // previously registered handlers.
     Coke.Click += new EventHandler(Coke_ClickHandler);

     // this is the .NET event handler method signature.
     Public void Coke_ClickHandler (object sender, EventArgs args){
          if (MoneySlot.Value >= CokeCost) {
             DispenseDrink();
             // How do I handle returning change? Maybe DispenseDrink() can do that.
          }else {
             // tell customer to put in more money
          }
     }

     private void DispenseDrink() {
       // An empty method is enough to get it to compile so for now that's fine.
       // I need to test the Coke_EventHandler logic that I've written so far.
     }

  }

Now I need to test what I've written so far. After that I need to decide what to focus on next. But realize that when you're writing new code that depends on already written code, if that existing code has not been tested - and now you see errors, you've just made it much harder on yourself. You could have tested when the code is simpler. Now there's more, it's more complex, and will be harder to debug and fix.

Suggestions, Part II

At the risk of messing things up, I offer this folow-up to my original answer:

You can see that every drink button does the same thing, and given the above code we would write the same logic over and over for every button. If anything needs to change we have to change it everywhere.

More General Advice

One Object Oriented Progamming heuristic is encapsulate that which stays the same . You should always be on the lookout for places where things may be candidates for common code.

I want to emphasize that this common button behavior was not immediately obvious to me. Only after I wrote the code above did I get to thinking that all my drink button handlers would start to look the same and I realized that on a real drink machine they actually do behave the same way. My coding spidey-sense told it definitely is a good thing when the code reflects the identifiable behaviors of your real thing (pun intended!).

Refactoring

Actually is a technical term that means reworking existing code to make it more flexible, re-useable, readable, etc. In a word maintainable.

Refactoring should be in your thought processes all the time. But be sure you have a legitimate reason for making any change. Reshaping code is a normal, integral part of software development.

Let's refactor by extracting a method

     Public void Coke_ClickHandler (object sender, EventArgs args){
          PurchaseDrink("Coke", CokeCost);
     }

     // now we have a method that stands out and says THIS is how it works
     // and a single point of change, rather than ump-teen button handlers.
      private PurchaseDrink (string whatKind, double cost) {

         // all I did so far is move the code and change "Cokecost" to "cost"
         // Now I'm beginning to think I may need to pass "whatKind" to
         // DispenseDrink() - but first I need to test the changes I've
         // made at this level.
         // ***** and since I already tested the code when I 1st wrote it,
         // this refactoring will be easier & quicker to test.. GET IT??!! ******

         if (MoneySlot.Value >= cost) {
             DispenseDrink();
             // How do I handle returning change? Maybe DispenseDrink() can do that.
          }else {
             // tell customer to put in more money
          }
     }

     private void DispenseDrink() {
       // An empty method is enough to get it to compile so for now that's fine.
       // I need to test the Coke_EventHandler logic that I've written so far.
     }

Enumerations

I hate using strings the way I used "Coke" above. Typo's and casing (upper/lower, that is) can cause problems that Visual Studio won't catch. When I have a limited list of things - kinds of drinks - I really like using enumerations. They show up in intellesense and I can use them in switch statements (and research the idea of "type safe"). And what I REALLY like is that they absolutely define in one place all the drink types our program knows about. It's like documentation!

Answer 2

You can store the amount for each button i buttons tag property, and use the following code in your eventhandler to read the amount:

void ValueButton_Click(object sender, EventArgs e)
{
  Button button = sender as Button;
  if (button == null) return;
  if (button.Tag == null) return;
  double amount = (double)button.Tag;

  // Process the amount here....
  InsertedCoins += amount; 
}

Answer 3

First think : You should divide the problem into two classes (class Test and class ColaMachine).

It looks like :

public class ColaMachine : Form
{
  public ColaMachine()
  {  
     ...
  }
}

public class Test 
{
   private static void Main()
   {
      ColaMachine Scherm;
      Scherm = new ColaMachine();
      Application.Run(Scherm);
   }
}

Next one : If you want to return a variable that is private , use properties . IC will be a public method (properties). InsertedCoins will be a private variable.

public double IC
{
   get
   {
      return InsertedCoins;
    }
   set
   {
      InsertedCoins = value;
   }
}

Don't forget, that the machine has a lot of states . You should use the design Pattern, exactly State pattern .