Introduction to C# - interactive tutorial
This tutorial teaches you C# interactively, using your browser to write C# and see the results of compiling and running your code. It contains a series of lessons that begin with a "Hello World" program. These lessons teach you the fundamentals of the C# language.
Tip
When a code snippet block includes the "Run" button, that button opens the interactive window, or replaces the existing code in the interactive window. When the snippet doesn't include a "Run" button, you can copy the code and add it to the current interactive window.
Run your first program
Run the following code in the interactive window.
Console.WriteLine("Hello, World!");
Congratulations! You ran your first C# program. It's a simple program that prints the message "Hello World!" It used the Console.WriteLine method to print that message. Console is a type that represents the console window. WriteLine is a method of the Console type that prints a line of text to that text console.
Let's move on and explore more. The rest of this lesson explores working with the string type, which represents text in C#. Like the Console type, the string type has methods. The string methods work with text.
Declare and use variables
Your first program printed the string "Hello World!" on the screen.
Tip
As you explore C# (or any programming language), you make mistakes when you write code. The compiler finds those errors and report them to you. When the output contains error messages, look closely at the example code, and the code in the interactive window to see what to fix. That exercise helps you learn the structure of C# code.
Your first program is limited to printing one message. You can write more useful programs by using variables. A variable is a symbol you can use to run the same code with different values. Let's try it! Start with the following code:
string aFriend = "Bill";
Console.WriteLine(aFriend);
The first line declares a variable, aFriend, and assigns it a value, "Bill". The second line prints the name.
You can assign different values to any variable you declare. You can change the name to one of your friends. Add these two lines in the preceding interactive window following the code you already added. Make sure you keep the declaration of the aFriend variable and its initial assignment.
Important
Don't delete the declaration of aFriend. Add the following code at the end of the preceding interactive window:
aFriend = "Maira";
Console.WriteLine(aFriend);
Notice that the same line of code prints two different messages, based on the value stored in the aFriend variable.
You might notice that the word "Hello" was missing in the last two messages. Let's fix that now. Modify the lines that print the message to the following code:
Console.WriteLine("Hello " + aFriend);
Select Run again to see the results.
You've been using + to build strings from variables and constant strings. There's a better way. You can place a variable between { and } characters to tell C# to replace that text with the value of the variable.
This process is called String interpolation.
If you add a $ before the opening quote of the string, you can then include variables, like aFriend, inside the string between curly braces. Give it a try:
Select Run again to see the results. Instead of "Hello {aFriend}", the message should be "Hello Maira".
Console.WriteLine($"Hello {aFriend}");
Work with strings
Your last edit was our first look at what you can do with strings. Let's explore more.
You're not limited to a single variable between the curly braces. Try the following code:
string firstFriend = "Maria";
string secondFriend = "Sage";
Console.WriteLine($"My friends are {firstFriend} and {secondFriend}");
Strings are more than a collection of letters. You can find the length of a string using Length. Length is a property of a string and it returns the number of characters in that string. Add the following code at the bottom of the interactive window:
Console.WriteLine($"The name {firstFriend} has {firstFriend.Length} letters.");
Console.WriteLine($"The name {secondFriend} has {secondFriend.Length} letters.");
Tip
Now is a good time to explore on your own. You learned that Console.WriteLine() writes text to the screen. You learned how to declare variables and concatenate strings together. Experiment in the interactive window. The window has a feature called IntelliSense that makes suggestions for what you can do. Type a . after the d in firstFriend. You see a list of suggestions for properties and methods you can use.
You've been using a method, Console.WriteLine, to print messages. A method is a block of code that implements some action. It has a name, so you can access it.
Trim
Suppose your strings have leading or trailing spaces that you don't want to display. You want to trim the spaces from the strings. The Trim method and related methods TrimStart and TrimEnd do that work. You can just use those methods to remove leading and trailing spaces. Try the following code:
string greeting = " Hello World! ";
Console.WriteLine($"[{greeting}]");
string trimmedGreeting = greeting.TrimStart();
Console.WriteLine($"[{trimmedGreeting}]");
trimmedGreeting = greeting.TrimEnd();
Console.WriteLine($"[{trimmedGreeting}]");
trimmedGreeting = greeting.Trim();
Console.WriteLine($"[{trimmedGreeting}]");
The square brackets [ and ] help visualize what the Trim, TrimStart, and, TrimEnd methods do. The brackets show where whitespace starts and ends.
This sample reinforces a couple of important concepts for working with strings. The methods that manipulate strings return new string objects rather than making modifications in place. You can see that each call to any of the Trim methods returns a new string but doesn't change the original message.
Replace
There are other methods available to work with a string. For example, you probably used a search and replace command in an editor or word processor before. The Replace method does something similar in a string. It searches for a substring and replaces it with different text. The Replace method takes two parameters. These parameters are the strings between the parentheses. The first string is the text to search for. The second string is the text to replace it with. Try it for yourself. Add this code. Type it in to see the hints as you start typing .Re after the sayHello variable:
string sayHello = "Hello World!";
Console.WriteLine(sayHello);
sayHello = sayHello.Replace("Hello", "Greetings");
Console.WriteLine(sayHello);
Two other useful methods make a string ALL CAPS or all lower case. Try the following code. Type it in to see how IntelliSense provides hints as you start to type To:
Console.WriteLine(sayHello.ToUpper());
Console.WriteLine(sayHello.ToLower());
Search strings
The other part of a search and replace operation is to find text in a string. You can use the Contains method for searching. It tells you if a string contains a substring inside it. Try the following code to explore Contains:
string songLyrics = "You say goodbye, and I say hello";
Console.WriteLine(songLyrics.Contains("goodbye"));
Console.WriteLine(songLyrics.Contains("greetings"));
The Contains method returns a boolean value which tells you if the string you were searching for was found. A boolean stores either a true or a false value. When displayed as text output, they're capitalized: True and False, respectively. You learn more about boolean values in a later lesson.
Challenge
There are two similar methods, StartsWith and EndsWith that also search for substrings in a string. These methods find a substring at the beginning or the end of the string. Try to modify the previous sample to use StartsWith and EndsWith instead of Contains. Search for "You" or "goodbye" at the beginning of a string. Search for "hello" or "goodbye" at the end of a string.
Note
Watch your punctuation when you test for the text at the end of the string. If the string ends with a period, you must check for a string that ends with a period.
You should get true for starting with "You" and ending with "hello" and false for starting with or ending with "goodbye".
Did you come up with something like the following (expand to see the answer):
You completed the "Hello C#" introduction to C# tutorial. You can select the Numbers in C# tutorial to start the next interactive tutorial, or you can visit the .NET site to download the .NET SDK, create a project on your machine, and keep coding. The "Next steps" section brings you back to these tutorials.
For further reading on the string type:
