Access Modifiers

Access Modifiers

What it means

Access modifiers control which code can see and use a class, method, or field. The main ones: public (accessible from anywhere), private (only within the same class — the default if you write nothing), protected (the class and its subclasses), internal (anywhere in the same assembly/project), and protected internal / private protected (combinations of the above).

Examples

public class BankAccount
{
    private decimal _balance;                  // only this class can touch it directly
    protected string AccountType;               // this class and subclasses
    internal string InternalNotes;               // anywhere in this project

    public decimal GetBalance() => _balance;     // anyone can call this

    public void Deposit(decimal amount)
    {
        if (amount <= 0)
            throw new ArgumentException("Deposit must be positive");
        _balance += amount;                      // internal state, only modified through this method
    }
}

public class SavingsAccount : BankAccount
{
    public void PrintType() => Console.WriteLine(AccountType);   // OK -- protected, visible to subclass
    // public void PrintBalance() => Console.WriteLine(_balance); // Error: _balance is private to BankAccount
}

Common mistake

Making fields public for convenience instead of exposing controlled access through a property or method — this lets any external code set the field to an invalid value, bypassing whatever validation logic you intended.

public class BankAccount
{
    public decimal Balance;   // anyone can do account.Balance = -1000; — no validation possible
}

// Better: keep the field private, expose validated access
public class BankAccount
{
    private decimal _balance;
    public decimal Balance => _balance;   // read-only from outside

    public void Deposit(decimal amount)
    {
        if (amount <= 0) throw new ArgumentException("Must be positive");
        _balance += amount;
    }
}

Quick practice

1. What’s the default access level for a class member if you write no modifier at all?

   Answer

   private.

2. What’s the difference between private and protected?

   Answer

   private members are visible only within the declaring class; protected members are also visible to derived (subclass) types.

3. Why is exposing a field as public generally worse than exposing it through a public property with a private setter?

   Answer

   A public field lets any external code assign any value with no validation; a property (even a simple one) gives you a single controlled point where you can add validation, logging, or change the implementation later without breaking callers.