C# Variables and Data Types

A program could repeat the number 4.50 everywhere, but the number alone does not say whether it is a price, a tax rate, or a distance. It also becomes difficult to update consistently. A variable connects a meaningful name to a value so later instructions can use that value without rediscovering it.

In int quantity = 3;, int is the type, quantity is the name, and 3 is a literal value written directly in the code. The equals sign assigns the value on its right to the variable on its left. It does not ask whether the sides are equal; comparison comes later.

using System;

class Program
{
  static void Main()
  {
      int unreadMessages = 2;
      Console.WriteLine(unreadMessages);

      unreadMessages = unreadMessages + 1;
      Console.WriteLine(unreadMessages);
  }
}

2
3

Types are not labels added only for the compiler. They document what a value represents and rule out operations that do not fit. A message can be joined with more text; a count can be added; a Boolean can decide whether code runs. C# checks those expectations before execution whenever it can.

Use int for ordinary whole-number counts and long when the range may be much larger. double is the normal choice for measurements and scientific calculations. decimal represents base-10 fractions more predictably and is commonly chosen for money; decimal literals use an m suffix. char holds one UTF-16 code unit in single quotes, while string holds text in double quotes.

A variable can be assigned another compatible value. That change is useful for a score, balance, or current position. If a named value must not change, declare it with const. The compiler will then reject later assignments. Constants are useful for fixed rules inside the program, not for settings that users or administrators may need to change without rebuilding it.

The var keyword asks the compiler to infer a variable's type from its initial value. var total = 12.5m is still statically typed as decimal; it does not become a container for anything. Use var when the right side makes the type obvious, and spell out the type when doing so communicates important meaning.

const decimal TaxRate = 0.15m;
decimal subtotal = 80m;
var tax = subtotal * TaxRate; // inferred as decimal
decimal total = subtotal + tax;

The following program stores facts about an order, derives a new value, and presents the result. Notice that total is not an independent fact: it is calculated from quantity and unitPrice. Keeping that relationship in code avoids manually synchronizing three values.

String interpolation begins with a dollar sign and places expressions inside braces. The :C format displays a number as currency using the environment's current culture. Formatting changes the displayed text, not the numeric value stored in total.

using System;

class Program
{
  static void Main()
  {
      string item = "Water bottle";
      int quantity = 3;
      decimal unitPrice = 7.25m;
      decimal total = quantity * unitPrice;

      Console.WriteLine($"Item: {item}");
      Console.WriteLine($"Quantity: {quantity}");
      Console.WriteLine($"Total: {total:0.00}");
  }
}

Item: Water bottle
Quantity: 3
Total: 21.75