MarzleyTech Learn

Home / Learn / C# / C# introduction: what C# and .NET are, where they're used, setup, your first program, types, strings, control flow and methods

C# introduction: what C# and .NET are, where they're used, setup, your first program, types, strings, control flow and methods

C# (pronounced "C sharp") is a modern, general-purpose language created by Microsoft (led by Anders Hejlsberg) and released in 2000. It runs on .NET, a free, open-source, cross-platform platform for building almost anything: web applications and APIs (ASP.NET Core), Windows desktop apps, mobile apps (.NET MAUI), cloud services on Azure, games (the Unity engine uses C#), and enterprise systems in banks, insurance companies, government and large organisations, including many in Kenya. C# is strongly typed, safe, productive, and well supported by excellent tools like Visual Studio.

Where C# is used

Area	Examples
Web apps and APIs	ASP.NET Core for business systems, portals, e-commerce, REST APIs
Enterprise	Banking, insurance, ERP integrations, internal tools
Cloud	Azure Functions, microservices
Desktop	Windows apps (WPF, WinForms, WinUI)
Mobile	.NET MAUI (Android, iOS, Windows, macOS from one codebase)
Games	Unity: a large share of mobile and indie games, AR/VR
Automation	Office add-ins, scripts, tools

Setup

Install the .NET SDK (an LTS version) from dotnet.microsoft.com.
Choose an editor:
Visual Studio Community (Windows, free): the full IDE.
VS Code with the C# Dev Kit extension (any OS).
JetBrains Rider (paid, free for some uses).
Create and run a console app:

Terminal

dotnet --version
dotnet new console -o HelloKenya
cd HelloKenya
dotnet run

In this hub, the Run button compiles and runs C# examples online.

Your first program

C# · runs live in the interactive lesson

using System;

class Program
{
    static void Main()
    {
        Console.WriteLine("Habari, Kenya!");
        Console.WriteLine("Learning C# on the Marzley learning hub.");
    }
}

using System; gives access to the System namespace (where Console lives).
Code lives inside classes; Main is the entry point.
Console.WriteLine prints a line.
Statements end with ;; blocks use { }; C# is case-sensitive.

Modern .NET projects created with dotnet new console use top-level statements: the file can simply contain Console.WriteLine("Hello"); and the compiler generates the class and Main for you. Both styles compile to the same thing.

Variables and types

C# · runs live in the interactive lesson

using System;

class Program
{
    static void Main()
    {
        int students = 45;
        long population = 53_000_000;
        double distanceKm = 160.5;
        decimal balance = 15000.75m;     // decimal: exact, best for money (m suffix)
        bool isPaid = true;
        char grade = 'A';
        string school = "Alliance High";
        var town = "Kikuyu";             // var: type inferred (string)
        const decimal Vat = 0.16m;

        Console.WriteLine(school + " in " + town + " has " + students + " students.");
        Console.WriteLine($"Population {population:N0}, distance {distanceKm} km");
        Console.WriteLine($"Balance KSh {balance:N2}, paid: {isPaid}, grade {grade}");
        Console.WriteLine($"VAT on KSh 1,000: KSh {1000 * Vat:N2}");
    }
}

Type	Use
`int`, `long`	Whole numbers
`double`	Decimals for measurements and science
`decimal`	Money and exact decimal arithmetic
`bool`	true/false
`char`	One character
`string`	Text
`var`	Let the compiler infer the type (still strongly typed)

Why decimal for money? double can't represent some decimals exactly (0.1 + 0.2 isn't exactly 0.3), which causes rounding errors in financial calculations.

Strings and formatting

C# · runs live in the interactive lesson

using System;

class Program
{
    static void Main()
    {
        string name = "Wanjiku Kamau";
        decimal amount = 2500000m;
        DateTime due = new DateTime(2026, 10, 31);

        Console.WriteLine($"Hello {name}!");                         // interpolation
        Console.WriteLine($"Amount: KSh {amount:N0}");                 // 2,500,000
        Console.WriteLine($"Due: {due:dd MMM yyyy}");                  // 31 Oct 2026
        Console.WriteLine(name.ToUpper());
        Console.WriteLine(name.Length + " characters");
        Console.WriteLine(name.Contains("Kamau"));
        Console.WriteLine(name.Split(' ')[0]);                         // first name
        Console.WriteLine("0712345678".Replace("0712", "254712"));
        string item = "Item", price = "Price", unga = "Unga";
        Console.WriteLine($"{item,-12}|{price,8}");                    // alignment: -12 left, 8 right
        Console.WriteLine($"{unga,-12}|{195,8}");
    }
}

Input and safe conversion

Reading input (run on your own computer):

C#

Console.Write("Enter amount: ");
string? input = Console.ReadLine();
if (decimal.TryParse(input, out decimal amount) && amount > 0)
{
    Console.WriteLine($"You entered KSh {amount:N2}");
}
else
{
    Console.WriteLine("Please enter a valid positive number.");
}

decimal.Parse("abc") throws an exception; TryParse returns false instead, which is safer for user input:

C# · runs live in the interactive lesson

using System;

class Program
{
    static void Main()
    {
        string[] inputs = { "1500", "2,500", "abc", "-20" };
        foreach (string s in inputs)
        {
            bool ok = decimal.TryParse(s.Replace(",", ""), out decimal value);
            Console.WriteLine(ok && value > 0 ? $"'{s}' -> {value}" : $"'{s}' is not a valid amount");
        }
    }
}

Conditions and switch

C# · runs live in the interactive lesson

using System;

class Program
{
    static string Grade(int mark)
    {
        if (mark >= 80) return "A";
        else if (mark >= 65) return "B";
        else if (mark >= 50) return "C";
        else if (mark >= 40) return "D";
        return "E";
    }

    static void Main()
    {
        Console.WriteLine($"72 -> {Grade(72)}");

        string status = "shipped";
        string label = status switch                // switch expression
        {
            "pending" => "Waiting for payment",
            "paid" or "processing" => "Preparing your order",
            "shipped" => "On the way",
            _ => "Unknown"
        };
        Console.WriteLine(label);

        int stock = 3;
        Console.WriteLine(stock < 5 ? "Reorder soon" : "Stock OK");   // ternary
    }
}

Loops

C# · runs live in the interactive lesson

using System;

class Program
{
    static void Main()
    {
        for (int i = 1; i <= 3; i++)
            Console.WriteLine($"7 x {i} = {7 * i}");

        int[] marks = { 67, 82, 45, 90 };
        int total = 0;
        foreach (int m in marks) total += m;
        Console.WriteLine($"Average: {(double)total / marks.Length:F1}");

        decimal balance = 0m;
        int months = 0;
        while (balance < 50000m)
        {
            balance = balance * 1.01m + 4500m;
            months++;
        }
        Console.WriteLine($"Months to save 50,000: {months}");
    }
}

Methods

C# · runs live in the interactive lesson

using System;

class Program
{
    static decimal Vat(decimal amount, decimal rate = 0.16m) => amount * rate;   // optional parameter, expression body

    static void PrintReceipt(string item, int qty, decimal price)
    {
        decimal total = qty * price;
        Console.WriteLine($"{item} x{qty} = KSh {total:N2}");
    }

    static (decimal min, decimal max) Range(decimal[] values)                    // returns a tuple
    {
        decimal lo = values[0], hi = values[0];
        foreach (var v in values) { if (v < lo) lo = v; if (v > hi) hi = v; }
        return (lo, hi);
    }

    static void Main()
    {
        Console.WriteLine(Vat(1000m));
        Console.WriteLine(Vat(1000m, rate: 0.08m));        // named argument
        PrintReceipt("Phone charger", 2, 800m);
        var (min, max) = Range(new decimal[] { 1200m, 450m, 3999m, 800m });
        Console.WriteLine($"Cheapest {min}, dearest {max}");
    }
}

Common errors

Error	Cause
`CS1002: ; expected`	Missing semicolon
`CS0103: The name 'x' does not exist in the current context`	Typo or variable out of scope
`CS0029: Cannot implicitly convert type 'double' to 'decimal'`	Mixing types: add the `m` suffix or convert
`FormatException`	`Parse` on invalid text: use `TryParse`
`NullReferenceException`	Using an object that's null: check for null (nullable reference types help)

Think about it: Why should a school fees system written in C# store amounts as decimal rather than double?Show answer

decimal represents base-10 numbers exactly (like 1500.10), so totals and balances don't drift with rounding errors. double is binary floating point and can't represent many decimal fractions exactly, so repeated calculations can be off by small amounts, unacceptable for money.

Summary

C# runs on .NET, a free cross-platform platform for web (ASP.NET Core), desktop, mobile (MAUI), cloud and games (Unity).
Install the .NET SDK; use Visual Studio, VS Code or Rider; dotnet new console and dotnet run.
Types: int, long, double, decimal (money), bool, char, string, var; strings support interpolation and formatting.
Use TryParse for safe input; if/else, switch expressions and the ternary operator for decisions; for, foreach and while loops.
Methods support optional/named parameters, expression bodies and tuple returns.

Check yourself

Which platform does C# run on?
Show answer
.NET
Which type should you use for money in C#?
Show answer
decimal
Which game engine uses C#?
Show answer
Unity
Which method converts text to a number without throwing an exception on bad input?
Show answer
TryParse
Which CLI command creates a new console project? (three words)
Show answer
dotnet new console
Which symbol starts an interpolated string in C#?
Show answer
$

Lesson 1 of 7 in C# · Printable course notes