Lesson 5 — Types

Data Types

C# has a rich type system including the critical decimal type for financial precision. Choosing the right type prevents silent bugs in production systems.

CSHARP — Numeric types 
// INTEGERS:
byte   b = 255;
short  s = 32_000;
int    i = 2_000_000;       // most common
long   l = 9_876_543_210L;

// FLOATING-POINT:
float  f = 3.14f;           // 7 decimal digits — use only when forced
double d = 3.14159265;      // 15-16 digits — general purpose

// EXACT DECIMAL — for financial data:
decimal price   = 1_299.99m; // 28-29 significant digits
decimal vatRate = 0.15m;
decimal total   = price * (1 + vatRate);
Console.WriteLine($"Total: R{total:F2}"); // R1 494,99

// WHY decimal matters:
double  wrong = 0.1 + 0.2;
Console.WriteLine(wrong); // 0.30000000000000004
decimal right = 0.1m + 0.2m;
Console.WriteLine(right); // 0.3

Reference Types

CSHARP — String operations 
// string is immutable — operations return new strings
string name  = "Thandi";
string upper = name.ToUpper(); // new string — name unchanged
Console.WriteLine($"{name} / {upper}");

Console.WriteLine(name.Length);
Console.WriteLine(name.Contains("han"));
Console.WriteLine(name.Replace("Th", "L")); // Landi
Console.WriteLine(string.IsNullOrWhiteSpace("  ")); // True

Type Conversion

CSHARP — Conversion 
// Implicit (widening — no data loss):
int    n = 42;
double d = n;   // int → double: safe

// Explicit cast (may lose data):
double avg   = 74.8;
int    floor = (int) avg; // 74 — truncated

// TryParse — safe, no exception on bad input:
if (int.TryParse("abc", out int result))
    Console.WriteLine(result);
else
    Console.WriteLine("Not a valid integer.");

// Parse — throws FormatException on bad input:
int parsed = int.Parse("100");

Practice Task

Your Turn

Store a product price as decimal, quantity as int, and discount percentage as decimal. Calculate total = price × quantity × (1 − discount/100). Print formatted to 2 decimal places. Use int.TryParse on the string "abc" and print whether it succeeded.

Common Mistakes

  • Using double for money — always use decimal.
  • Forgetting the m suffix on decimal literals — decimal d = 3.14; is a compile error.
  • Forgetting the f suffix on float literals.
  • Using Parse() without try-catch — use TryParse for user input.

Professional Tip

For any monetary value in a South African business application, use decimal. A rounding error of one cent across millions of transactions is a real financial problem.

Mini Quiz

Which C# type should you use for financial amounts?