Session Goals
By the end of this session, you will be able to:
- Declare variables using the correct data type for a given situation
- Convert between types safely using built-in methods
- Control program flow using conditions and loops
- Declare, initialize, and traverse arrays
Value Types
Stored directly on the stack. Each variable holds its own copy of the data.
| Type | Description | Example |
|---|
int | 32-bit integer | int age = 25; |
long | 64-bit integer | long pop = 8_000_000_000L; |
double | 64-bit floating point | double price = 9.99; |
decimal | 128-bit high-precision | decimal tax = 0.15m; |
bool | true or false | bool isActive = true; |
char | Single Unicode character | char grade = 'A'; |
Tip
Use
decimal for money and financial calculations —
double has floating-point rounding errors.
Reference Types & Type Inference
Reference types — stored on the heap; the variable holds a reference, not the value itself.
| Type | Description |
|---|
string | Sequence of characters |
object | Base of every .NET type |
var — the compiler infers the type. The type is still fixed at compile time.
Remember
var is not dynamic. You cannot change the type after assignment.
// Reference types
string name = "Alice";
object anything = 42;
// Type inference with var
var count = 10; // int
var price = 9.99m; // decimal
var label = "Hello"; // string
// This would be a compile error:
// var x = 1;
// x = "hello"; // cannot change type
Strings: Interpolation & Verbatim
// String interpolation — embed expressions with $""
string firstName = "Alice";
int year = 2025;
string message = $"Hello, {firstName}! Born in {year - 25}.";
// → "Hello, Alice! Born in 2000."
// Verbatim string — backslashes are treated literally, no escape needed
string path = @"C:\Users\Alice\Documents\notes.txt";
string multiLine = @"Line one
Line two
Line three";
Note
You can combine both:
$@"C:\Users\{userName}\files" gives interpolation inside a verbatim string.
Implicit vs Explicit Conversion
Implicit — safe, no data loss. The compiler does it automatically when the destination type can hold all values of the source type.
int x = 100;
long y = x; // int → long
double d = x; // int → double
Explicit cast — possible data loss. You must declare it with (type).
double pi = 3.14159;
int truncated = (int)pi;
// truncated = 3 — decimal part lost!
Caution
Casting a
long to
int will silently overflow if the value is too large. The compiler will not warn you.
Convert class — converts between unrelated types. Throws on failure.
int n = Convert.ToInt32("42");
bool b = Convert.ToBoolean("true");
double d = Convert.ToDouble("3.14");
Parse & TryParse
// Parse — throws FormatException if the string is not a valid number
int a = int.Parse("100"); // fine
int b = int.Parse("abc"); // throws!
// TryParse — returns false instead of throwing; always prefer for user input
string userInput = Console.ReadLine();
if (int.TryParse(userInput, out int result))
{
Console.WriteLine($"You entered: {result}");
}
else
{
Console.WriteLine("That is not a valid integer.");
}
Best Practice
Always use
TryParse when handling any input you did not create yourself — user input, file reads, API responses.
if / else & Ternary Operator
Multi-branch with if
int score = 75;
if (score >= 90)
{
Console.WriteLine("A");
}
else if (score >= 75)
{
Console.WriteLine("B");
}
else
{
Console.WriteLine("C or below");
}
Ternary — single-expression two-branch
// condition ? valueIfTrue : valueIfFalse
string label = score >= 50 ? "Pass" : "Fail";
// Useful for assignments and return values
bool isAdult = age >= 18;
string message = isAdult
? "Access granted"
: "Access denied";
Tip
Keep ternary expressions simple. If the branches are complex, use a full
if statement instead.
switch Statement & switch Expression
Classic switch statement
string day = "Saturday";
switch (day)
{
case "Saturday":
case "Sunday":
Console.WriteLine("Weekend");
break;
case "Monday":
Console.WriteLine("Back to work");
break;
default:
Console.WriteLine("Weekday");
break;
}
switch expression (C# 8+) — returns a value
string type = day switch
{
"Saturday" or "Sunday" => "Weekend",
"Monday" => "Back to work",
_ => "Weekday"
};
// Works great for mapping enums too:
string label = status switch
{
TaskStatus.Pending => "Not started",
TaskStatus.Completed => "Done",
_ => "In progress"
};
Common Mistake
Forgetting
break in a
switch statement causes fall-through to the next case.
Multi-dimensional Arrays
// 2D array — rows × columns
int[,] grid = new int[3, 3];
grid[0, 0] = 1;
grid[1, 2] = 7;
// Initialize inline
int[,] matrix = {
{ 1, 2, 3 },
{ 4, 5, 6 },
{ 7, 8, 9 }
};
Console.WriteLine(matrix[1, 1]); // 5 (row 1, column 1)
// Iterate with nested loops
for (int row = 0; row < matrix.GetLength(0); row++)
for (int col = 0; col < matrix.GetLength(1); col++)
Console.Write(matrix[row, col] + " ");
Common Mistakes to Avoid
Mistake 1 — Float Comparison
Never use
== to compare
double or
float. Use
Math.Abs(a - b) < 0.0001 instead — floating-point arithmetic is imprecise.
Mistake 2 — Missing break in switch
Forgetting
break causes fall-through: execution continues into the next
case block, producing unexpected results.
Mistake 3 — Off-by-One in for loops
Decide once: upper bound is
< Length or
<= Length - 1. Mixed usage causes skipped or out-of-bounds elements.
Mistake 4 — Array Index Out of Range
Accessing
arr[arr.Length] throws at runtime. Arrays are zero-based: valid indices are
0 to
arr.Length - 1.
What's Next
Session 04 — C# Methods, Strings & Exception Handling
- Defining and calling methods with various parameter types
- Method overloading, optional and named parameters
- String manipulation and
StringBuilder
- Handling
null safely — nullable types, ??, ?.
- Exception handling with
try / catch / finally
Before next session
Complete the exercises on the next slide before Session 04.
Assignment
Build a small C# console program that demonstrates your understanding of types, control flow, and arrays.
Acceptance criteria:
- Accept an integer from the user and print whether it is even or odd — use
TryParse to handle invalid input gracefully
- Print the multiplication table (1 to 10) for the entered number using a
for loop
- Declare an array of 5 integers, fill it with user input, then print the minimum and maximum values without using
Array.Sort
- Use a
switch expression to map the maximum value to a label: <10 = "Low", 10–99 = "Medium", ≥100 = "High"
- All output must be clearly labeled so the user knows what each line means
Bonus
Extend the program to accept a second array of integers and print only the values that appear in both arrays (without using any collection classes — arrays only).
