Instance vs Static Data

Part of the Object Oriented Programming section of Coddy's C# journey — lesson 15 of 70.

Now that you understand static members, let's clarify when to use instance data versus static data. This decision shapes how your class stores and shares information.

Instance data belongs to each object individually. Every object gets its own copy, and changes to one object don't affect others:

public class BankAccount
{
    public string Owner { get; set; }      // Instance - unique per account
    public decimal Balance { get; set; }   // Instance - unique per account
}

Each BankAccount has its own owner and balance. That makes sense because different accounts hold different amounts for different people.

Static data belongs to the class itself and is shared across all instances:

public class BankAccount
{
    public static decimal InterestRate = 0.05m;  // Static - same for all
    public static int TotalAccounts = 0;         // Static - tracks all accounts
    
    public string Owner { get; set; }
    public decimal Balance { get; set; }
}

The interest rate applies equally to every account, so it makes sense as static. The total account counter tracks class-wide information, not individual account data.

A simple rule: if the data describes a specific object, make it instance. If it describes the class as a whole or should be shared, make it static. Mixing them incorrectly leads to bugs where data unexpectedly changes across objects or isn't accessible when needed.

Challenge

Easy

Let's build a game session tracker that demonstrates the difference between data that belongs to individual players versus data shared across the entire game.

You'll create two files to organize your code:

Player.cs: Define a Player class in the GameSession namespace. Each player has their own Username (string) and Score (int) - these are unique to each player. The class should also track information that applies to all players: a static field GameName (string) that stores the name of the game everyone is playing, and a static field TotalPlayers (int) that counts how many players have joined. Include a constructor that takes the username and starting score, and automatically increments the total player count. Add a static method SetGameName(string name) to configure the game name.
Program.cs: In your main file, set up the game and create players based on input. First set the game name, then create two players and display their individual information along with the shared game data.

You will receive five inputs:

The game name
First player's username
First player's starting score
Second player's username
Second player's starting score

Print the output in this format:

Game: {GameName}
Player: {Username}, Score: {Score}
Player: {Username}, Score: {Score}
Total Players: {TotalPlayers}

For example, if the inputs are BattleArena, DragonSlayer, 1500, ShadowNinja, and 2200, the output should be:

Game: BattleArena
Player: DragonSlayer, Score: 1500
Player: ShadowNinja, Score: 2200
Total Players: 2

Think carefully about which data should be instance-based (different for each player) and which should be static (shared across all players). The game name and total player count describe the session as a whole, while username and score are unique to each individual.

Cheat sheet

Use instance data for properties that are unique to each object:

public class BankAccount
{
    public string Owner { get; set; }      // Each account has its own owner
    public decimal Balance { get; set; }   // Each account has its own balance
}

Use static data for properties shared across all instances of a class:

public class BankAccount
{
    public static decimal InterestRate = 0.05m;  // Same for all accounts
    public static int TotalAccounts = 0;         // Tracks all accounts
    
    public string Owner { get; set; }
    public decimal Balance { get; set; }
}

Decision rule: If the data describes a specific object, use instance data. If it describes the class as a whole or should be shared, use static data.

Try it yourself

using System;

namespace GameSession
{
    class Program
    {
        public static void Main(string[] args)
        {
            // Read inputs
            string gameName = Console.ReadLine();
            string username1 = Console.ReadLine();
            int score1 = Convert.ToInt32(Console.ReadLine());
            string username2 = Console.ReadLine();
            int score2 = Convert.ToInt32(Console.ReadLine());

            // TODO: Set the game name using the static method

            // TODO: Create two Player objects with the input data

            // TODO: Print the output in the required format
            // Game: {GameName}
            // Player: {Username}, Score: {Score}
            // Player: {Username}, Score: {Score}
            // Total Players: {TotalPlayers}
        }
    }
}