Lesson 5: Variables

At the end of this lesson, you should be able to answer the following:

• What is a variable?

• How do I declare a variable in C#?

• How do I assign a value to a variable in C#?

When C# evaluates an expression and returns a new value, it would be nice if the value could be stored somewhere. That's so we could use it later in the program. Otherwise, we'd need to keep typing the same expression to get the value.

In programming, we have a concept called a variable. A variable is a named representation of some value. Variables can hold values of the type it was declared with. For example, a string variable can contain values of the string type.

Declaring Variables

Declaring a variable means letting C# know to set some memory aside to store a value.

To declare a variable in C#, we need the type of the variable and a name to call it. We use the C# keyword in Lesson 3 to specify the type of the variable.

int number;

The statement above is called a declaration statement. We have declared a variable called number that has the type of int. This variable can hold integer numbers.

What's in a Name?

User-defined names in C# are called identifiers. Identifiers must follow certain rules - for example, they must start with a letter or an underscore (_). See this page for more information about identifiers.

When we declare a variable, we can also assign a value to it.

int number = 50;

We can use this variable in a Console.WriteLine() statement.

int number = 50;
Console.WriteLine(number);

Can you guess what the output will be? That's right, the console will display 50.

If we don't assign a value to a variable, like the first statement in this lesson, the variable will have a default value. For numeric types like int, that value is 0.

Remove the = 50 from the program above and run it. The value in the output will be 0.

int number;
Console.WriteLine(number);

Assigning to Variables

Once you've declared a variable, we can assign a different value to it. The syntax will be similar to the declaration, except that we don't need to specify the type again. C# already knows its type from the declaration.

We can also assign expressions to a variable, like below:

number = 5 + 20;

This statement is an assignment statement. We have assigned a value to the number variable.

Remember that C# will evaluate expressions like 5 + 20 into its resulting value.

Type the program below into your code box. Can you guess what the output will be?

int number = 50;
number = 5 + 20;
Console.WriteLine(number);

If you answered 25, you're correct! The program won't display 50 because we assigned a new value to number in Line 2.

An important thing to remember is that once a variable has been declared with a certain type, that type can't be changed. The value can be changed, but only values of its declared type are acceptable.

For example, we can't assign a string or bool value to an int variable.

int number = "Will it blend?"; // This statement will cause an error!

int anotherNumber;
anotherNumber = false;    // This statement will ALSO cause an error!

Using the var keyword

There's another way to declare a variable in C#. Instead of specifying the type ourselves, we can let C# guess the type for us.

var title = "Full Metal Alchemist";

The var keyword is used to declare variables like title above. But what is the data type of title?

Because we have assigned the value "Full Metal Alchemist" to it, which is a string value, the type of title is string. Recall that strings are text values surrounded by double quotes (Lesson 3).

Since C# needs an initial value in order to guess the type of the variable, using the var keyword means we can't declare without an assigned value.

var myVariable; // This will cause an error - C# doesn't know what type it is!

Why use the var keyword at all?

Some types in C# can get really long, like Dictionary<string, string> and using var instead of the type name will make our declarations more concise. Read more about the var keyword here.

Question

What is the output of this program?

var colour = "blue";
colour = "yellow";
Console.WriteLine(colour);

Questions

True or False:

1. You can assign a value of any type to a variable after it's been declared.

2. When you declare a variable, you cannot assign a value to it in the same statement.

3. You can assign an expression to a variable.

Challenge

Declare three variables with different types: string, int, and bool. Assign either a value or an expression to each variable. Then print each variable using Console.WriteLine(). Did you get the output you were expecting?