Variables in C#

Programmes work by manipulating data stored in memory. These storage areas come under the general heading of Variables. In this section, you'll see how to set up and use variables. You'll see how to set up both text and number variables. By the end of this section, you'll have written a simple calculator programme. We'll start with something called a String variable.

String Variables in C#.NET

The first type of variable we'll take a look at is called a String. String variables are always text. We'll write a little programme that takes text from a text box, store the text in a variable, and then display the text in a message box.
But bear in mind that a variable is just a storage area for holding things that you'll need later. Think of them like boxes in a room. The boxes are empty until you put something in them. You can also place a sticker on the box, so that you'll know what's in it. Let's look at a programming example.
If you've got your project open from the previous section, click File from the menu bar at the top of Visual C#. From the File menu, click Close Solution. Start a new project by clicking File again, then New Project. From the New Project dialogue box, click on Windows Application. For the Name, type String Variables.
Click OK, and you'll see a new form appear. Add a button to the form, just like you did in the previous section. Click on the button to select it (it will have the white squares around it), and then look for the Properties Window in the bottom right of Visual Studio. Set the following Properties for your new button:

Name: btnStrings
Location: 90, 175
Size: 120, 30
Text: Get Text Box Data

Your form should then look like this:

We can add two more controls to the form, a Label and a Text Box. When the button is clicked, we'll get the text from the text box and display whatever was entered in a message box.
A Label is just that: a means of letting your users know what something is, or what it is for. To add a Label to the form, move your mouse over to the Toolbox on the left. Click the Label item under Common Controls:

Now click once on your form. A new label will be added:

The Label has the default text of label1. When your label is selected, it will have just the one white square in the top left. When it is selected, the Properties Window will have changed. Notice that the properties for a label are very similar to the properties for a button - most of them are the same!
Change the following properties of your label, just like you did for the button:

Location: 10, 50
Text: Name

You don't really need to set a size, because Visual C# will automatically resize your label to fit your text. But your Form should look like this:

Move your mouse back over to the Toolbox. Click on the TextBox entry. Then click on your form. A new Text Box will be added, as in the following image:

Instead of setting a location for your text box, simply click it with your left mouse button. Hold your left mouse button down, and the drag it just to the right of the Label.
Notice that when you drag your text box around, lines appear on the form. These are so that you can align your text box with other controls on the form. In the image below, we've aligned the text box with the left edge of the button and the top of the Label.

OK, time to add some code. Before you do, click File > Save All from the menu bar at the top of Visual C#. You can also run your programme to see what it looks like. Type some text in your text box, just to see if it works. Nothing will happen when you click your button, because we haven't written any code yet. Let's do that now. Click the red X on your form to halt the programme, and you'll be returned to Visual C#.

Assigning Text to a String Variable

Double click your button to open up the coding window. Your cursor will be flashing inside of the curly brackets for the button code:

Notice all the minus symbols on the left hand side. You can click these, and it will hide code for you. Click the minus symbol next to public Form1( ). It will turn into a plus symbol, and the code for just this Method will be hidden:

Hiding code like this makes the rest of the coding window easier to read. Back to the button code, though. We're going to set up a string variable. To do this, you need two things: the Type of variable you want, and a name for your variable.
Click inside the two curly brackets of the button code, and add the following:

string firstName;

After the semi-colon, press the enter key on your keyboard to start a new line. Your coding window will then look like this:

What you have done is to set up a variable called firstName. The Type of variable is a string. Note that the coding editor will turn the word "string" blue. Blue denotes the variable type - a string, in this case. (Other variable types are int, float, and double. These are all number variables that you'll meet shortly.)
After you have told C# which type of variable you want, you then need to come up with a name for your variable. This is like the sticker on an empty box. The empty box is the variable type. Think of these empty boxes as being of different sizes and different materials. A big, cardboard box is totally different from a small wooden one! But what you are really doing here is telling C# to set aside some memory, and that this storage area will hold strings of text. You give it a unique name so as to tell it apart from other items in memory. After all, would you be able to find the correct box, if they were all the same size, the same shape, the same colour, and had no stickers on them?
The name you pick for your variables, firstName in our case, can be almost anything you want - it's entirely up to you what you call them. But you should pick something that is descriptive, and gives you a clue as to what might be in your variable.
We say you can call your variables almost anything. But there are some rules, and some words that C# bags for itself. The words that C# reserves for itself are called Keywords. There are about 80 of these words, things like using, for, new, and public. If the name you have chosen for your variable turns blue in the coding window, then it's a reserved word, and you should pick something else.

Characters you can use for your Variables

The only characters that you can use in your variable names are letters, numbers, and the underscore character ( _ ). And you must start the variable name with a letter, or underscore. You'll get an error message if you start your variable names with a number. So these are OK:

firstName
first_Name
firstName2

But these are not:

1firstName (Starts with a number)
first_Name& (Ends with an illegal character)
first Name (Two words, with a space in between)

Notice that all the variable names above start with a lowercase letter. Because we're using two words joined together, the second word starts with an uppercase letter. It's recommended that you use this format for your variables (called camelCase notation.) So firstName, and not Firstname.
After setting up your variable (telling C# to set aside some memory for you), and giving it a name, the next thing to do is to store something in the variable. Add the following line to your code (don't forget the semi-colon on the end):

firstName = textbox1.Text;

Your coding window will then look like this:

To store something in a variable, the name of your variable goes on the left hand side of an equals sign. After an equals sign, you type what it is you want to store in the variable. For us, this is the Text from textbox1.
Except, there's a slight problem. Try to run your code. You should see an error message like this one:

Click No, and have a look at your code:

There is a blue wiggly line under textbox1. Hold your mouse over this and Visual Studio will tell you that:

The name 'textbox1' does not exist in the current context.

If you see an error like this, which is quite common, it means that Visual C# cannot find anything with the name you've just typed. So it thinks we don't have a textbox called textbox1. And we don't! It's called textBox1. We've typed a lowercase "b" when it should be an uppercase "B". So it's important to remember that C# is case sensitive. This variable name:

firstName

Is different to this variable name:

FirstName

The first one starts with a lowercase "f" and the second one starts with an uppercase "F".
Delete the lowercase "b" from your code and type an uppercase "B" instead. Run your programme again and you won't see the error message. Now stop your programme and return to the coding window. The blue wiggly line will have disappeared.
What have so far, then, is the following:

string firstName;
firstName = textBox1.Text;

The first line sets up the variable, and tells C# to set aside some memory that will hold a string of text. The name of this storage area will be firstName.
The second line is the one that actually stores something in the variable - the Text from a text box called textBox1.
Now that we have stored the text from the text box, we can do something with it. In our case, this will be to display it in a message box. Add this line to your code:

MessageBox.Show(firstName);

The MessageBox.Show( ) Method is one you've just used. In between the round brackets, you can either type text surrounded by double quotes, or you can type the name of a string variable. If you're typing the name of a variable, you leave the double quotes off. You can do this because C# knows what is in your variable (you have just told it on the second line of your code.)
Run your programme again. Type something in your text box, and then click the button. You should see the text you typed:

Halt your programme and return to the coding window.

Assigning text to a String Variable

As well as assigning text from a text box to your variable, you can assign text like this:

firstName = "Home and Learn";

On the right hand side of the equals sign, we now have some direct text surrounded by double quotes. This then gets stored into the variable on the left hand side of the equals sign. To try this out, add the following two lines just below your MesageBox line:

firstName = "Home and Learn";
MessageBox.Show(firstName);

Your coding window will then look like this:

Run your programme again. Type something in the text box, your own first name. Then click the button. You should see two message boxes, one after the other. The first one will display your first name. But the second will display "Home and Learn".
We're using the same variable name, here: firstName. The first time we used it, we got the text directly from the text box. We then displayed it in the Message Box. With the two new lines, we're typing some text directly in the code, "Home and Learn", and then assigning that text to the firstName variable. We've then added a second MessageBox.Show( ) method to display whatever is in the variable.

 

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