Hi. In this video we're going to think about classes, how to define them, how to instantiate them, how to set attributes or properties of them. And, in this example, we're going to create some code from scratch. So, I've got a project already open, I'm going to right-click on the project's name in Solution Explorer, and I'm going to add a class, and I'm gonna call that class employee. Class turns up, and inside there I'm gonna want to add some properties. They could've been public fields, but properties are a better thing to use. What it's just-, we'll, we'll justify their use later on in the course. To start off with, I'm just gonna type, 'prop', and a code snippet pops up using-, from IntelliSense, and I can hit tab key twice to create the rest of the code for that. I need to define a string data type because my employees will have some kind of number, an employee number. And then there's a bunch of other properties that we'll need to set, which I've just pasted in to save a bit of time. The only difference between these things is the last property is a type integer rather than a type string, just to show you that, you know, any type will do at this point in time.

That's my class defined, and I'm now going over to the program file and the main function in order to start instantiating one of these things. I wanna bring an employee object into life, so I declare an instance of an employee class by declaring the class name, just like any other type integer string, it's an employee. Giving it a name, emp1, and bringing one into life. Something that's, kind of, new to us here is the fact that we are instantiating an instance of a class, and whenever you do that in C#, you need to use the new operator. Any time you need the new object, the new operator will do it for you. I'm instantiating an instance therefore of the employee class, and the parentheses are, kind of, important at that point. Next thing I want to do is initialise the properties. So, I'll go to the employee one object, emp1, put a dot, and IntelliSense will offer me a bunch of values. The spanner symbol means properties, and we can see that we've got the option of setting the employee number. I made that a string because, although employee numbers are very often numeric, you never want to do arithmetic with them, so you tend to set them as strings. I've got other things to set, like the first name and last name, and so I can do the same thing again. So, I've got Fred, I've got emp1 dot last name, and that could be Smith, and I've got emp1 dot department name, and that might be the IT department, and finally I've got emp1 dot age, and this guy can be 23.

To see the details of that, Console.WriteLine, and using my favourite notation, which is the formatted notation with the dollar symbol, I can say something like employee number colon, and then use the curly brackets to specify that I wanna get hold of emp1 dot employee ID-, employee number, rather. And then, maybe a comma, and then maybe name colon, and then curly brackets emp1 dot first name, and then a space, then a new set of curly brackets, and we can do emp1 dot last name. And we could go on printing out the other bits and pieces, should we so wish, but that should give use enough information to confirm that this stuff is behaving itself as we expect. So, I'll run the application and see the result of that, and lo and behold, 'Employee number 12345, name Fred Smith,' gets printed out. So, we've actually instantiated an employee object, set some of its properties, and then retrieved some of those properties in that print statement.

Now, I want to take this a little bit further, so I'm going to copy this code here. I'm going to paste it in beneath, I'm gonna create an employee number 2, so emp1 rather than emp1. These will, therefore, be a different person, so let's give them the different employee number. Let's call this Tina. Surname can be Jones. Jones would do, wouldn't it? And she can work in sales, and she can be 35. Now, the reason why I'm, I'm doing this is to highlight the fact that this is a, a-, an instance of an employee class, but it's a distinctly new instance. These are two completely different objects. They are employee objects, they will have these properties, but they won't interfere with each other. So, this Console.WriteLine will still print out the details of Fred Smith. If I was to copy this line, and paste this in, and change this to emp2, then, no great surprise, Tina's details will be printed out. So, they're not interfering with, with each other at all, they are, kind of, one and the same thing. So if I ran that-, in fact, they're not one and the same thing, I should say. And you can see that Fred's details have been printed out, as have Tina's details.

Now, I want to do one extra thing, which is, kind of, interesting. I'm gonna-, I'm gonna declare another employee variable, I'm gonna call it emp3, but I'm gonna make it equal to emp 2. Now, it looks like there are three different employees but, actually, there are only two, because I've declared a variable called emp3 of type employee, but I've made it equal to emp2. I haven't used the new operator, so I haven't instantiated a new employee. I've got two names for the same object. That's really, really important to get your head round. Two names for the same object. There is only one Tina, but she could be looked at through now employee 3, or employee 2, names. So, if I was to copy that, paste it in, and change this to emp3, emp3 and emp2 are the same person. They have alias names. So, we'll get Tina's details printed out twice when we now run the program. Here's the evidence, emp2, emp3, they are the same thing. They're both Tina Jones in this example.

So, in this video, we've seen how we can define a class, give it some properties, attributes, in your head think public fields, and instantiated them, seen how we can then interrogate them and-, well, set their properties and interrogate them. And also, recognise that if we don't use the new operator, we're not getting a new object, and if we make one equal to the other, then they are actually the same thing. They're both pointers pointing to the same object in memory.

Practical example of defining a data class and creating objects from it

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