Access Modifiers in C# Interview Questions & Answers

Q: Can we use all access modifiers for all types?

No, not every access modifier can be applied to every kind of type or member. For example, top level classes and interfaces defined directly within a namespace can only be declared as either public or internal, not private or protected, since private and protected only make sense in the context of a containing type. Members declared inside a class, on the other hand, can use the full range of access modifiers, including private, protected, internal, public, and protected internal.

Q: Can derived classes have greater accessibility than their base types?

No, a derived class cannot have greater accessibility than its base class, since doing so would create a situation where code could access the derived class but not the base class it depends on, which would be inconsistent and is disallowed by the compiler. For example, a public class cannot inherit from an internal base class if that public class is meant to be used outside the current assembly, since external code would not be able to see the base type it depends on.

Q: Can the accessibility of a type member be greater than the accessibility of its containing type?

No, a member cannot be more accessible than the type that contains it, since that would create a contradiction where the member is technically reachable but the type needed to access it is not visible in the same context. For example, if a class is declared as internal, none of its members can be declared as public, because doing so would imply the member is accessible from outside the assembly while the class itself is not.

Q: Can destructors have access modifiers?

No, destructors in C# cannot have any access modifiers applied to them, since they are called automatically by the garbage collector rather than being called directly by other code, meaning concepts like public or private accessibility simply do not apply to them. Attempting to add an access modifier to a destructor results in a compiler error, since the syntax for a destructor does not support one.

Q: What do protected internal access modifiers mean?

The protected internal access modifier combines the rules of protected and internal using an or relationship, meaning a member marked this way is accessible from anywhere within the same assembly, just like internal, as well as from any derived class even if that derived class exists in a completely different assembly, just like protected. This gives broader access than either protected or internal alone would provide on its own.

Q: Can you specify an access modifier for an enumeration?

Yes, you can specify an access modifier for an enum declared at the top level, such as public or internal, following the same accessibility rules that apply to classes, and by default a top level enum without an explicit modifier is internal. However, the individual members inside an enum, such as its named constant values, cannot have their own separate access modifiers, since they always share the same accessibility as the enum itself.

What are Access Modifiers in C#?

Beginner

Access modifiers in C# are keywords that control the visibility and accessibility of classes, methods, properties, and other members from other parts of the code. The main access modifiers are public, which allows access from anywhere, private, which restricts access to only within the same class, protected, which allows access within the same class and any derived classes, internal, which allows access only within the same assembly, and protected internal, which combines protected and internal accessibility rules.

public class Customer {
    public string Name;       // accessible from anywhere
    private decimal _balance; // accessible only within Customer
    protected int LoyaltyPoints; // accessible in Customer and its subclasses
}

Real-world example A banking application marks its internal balance calculation logic as private, ensuring that no other class anywhere in the codebase can directly modify the balance except through the class's own carefully validated methods.

Common follow-ups: Which access modifier is used by default if none is specified?;How do access modifiers support the principle of encapsulation?

Can we use all access modifiers for all types?;What do protected internal access modifiers mean?

Can we use all access modifiers for all types?

Intermediate

No, not every access modifier can be applied to every kind of type or member. For example, top level classes and interfaces defined directly within a namespace can only be declared as either public or internal, not private or protected, since private and protected only make sense in the context of a containing type. Members declared inside a class, on the other hand, can use the full range of access modifiers, including private, protected, internal, public, and protected internal.

// Valid: top level class as public or internal
public class Order { }
internal class InternalHelper { }

// Not valid: top level class cannot be private
// private class Invalid { }

Real-world example A developer attempting to declare a top level helper class as private receives a compiler error, and instead marks it as internal, since the class is only meant to be used within the same project assembly.

Common follow-ups: Why can't top level classes be declared as private or protected?;What access modifiers are valid specifically for interface members?

What are Access Modifiers in C#?;Can derived classes have greater accessibility than their base types?

Can derived classes have greater accessibility than their base types?

Advanced

No, a derived class cannot have greater accessibility than its base class, since doing so would create a situation where code could access the derived class but not the base class it depends on, which would be inconsistent and is disallowed by the compiler. For example, a public class cannot inherit from an internal base class if that public class is meant to be used outside the current assembly, since external code would not be able to see the base type it depends on.

// This causes a compiler error if Base is internal and Derived is public
internal class Base { }
// public class Derived : Base { } // not allowed

Real-world example A library author keeps a helper base class internal while trying to expose a derived class as public, and encounters a compiler error explaining the accessibility mismatch, prompting them to make the base class public as well.

Common follow-ups: What compiler error is shown when this rule is violated?;Does this rule apply the same way to interfaces as it does to classes?

Can we use all access modifiers for all types?;Can the accessibility of a type member be greater than the accessibility of its containing type?

Can the accessibility of a type member be greater than the accessibility of its containing type?

Advanced

No, a member cannot be more accessible than the type that contains it, since that would create a contradiction where the member is technically reachable but the type needed to access it is not visible in the same context. For example, if a class is declared as internal, none of its members can be declared as public, because doing so would imply the member is accessible from outside the assembly while the class itself is not.

internal class Helper {
    // This causes a compiler error
    // public void DoWork() { }
    internal void DoWork() { } // valid, matches or is more restrictive than the class
}

Real-world example A developer marks a class as internal but initially declares one of its methods as public, triggering a compiler warning about inconsistent accessibility, which is resolved by changing the method to internal to match the class's own accessibility level.

Common follow-ups: What is the general rule for matching accessibility between a type and its members?;Does this restriction apply to nested classes as well?

Can derived classes have greater accessibility than their base types?;What are Access Modifiers in C#?

Can destructors have access modifiers?

Intermediate

No, destructors in C# cannot have any access modifiers applied to them, since they are called automatically by the garbage collector rather than being called directly by other code, meaning concepts like public or private accessibility simply do not apply to them. Attempting to add an access modifier to a destructor results in a compiler error, since the syntax for a destructor does not support one.

public class ResourceHolder {
    ~ResourceHolder() {  // no access modifier allowed here
        // cleanup logic
    }
}

Real-world example A developer new to C# tries to mark a class's destructor as protected out of habit from other object oriented languages, and learns that C# destructors simply do not accept access modifiers of any kind.

Common follow-ups: How does a destructor differ from a constructor in terms of syntax rules?;What is the recommended alternative to destructors for deterministic resource cleanup in C#?

What are Access Modifiers in C#?;Can you specify an access modifier for an enumeration?

What do protected internal access modifiers mean?

Intermediate

The protected internal access modifier combines the rules of protected and internal using an or relationship, meaning a member marked this way is accessible from anywhere within the same assembly, just like internal, as well as from any derived class even if that derived class exists in a completely different assembly, just like protected. This gives broader access than either protected or internal alone would provide on its own.

public class BaseClass {
    protected internal void Configure() {
        // accessible within the same assembly OR from derived classes anywhere
    }
}

Real-world example A shared library exposes a configuration method as protected internal, allowing other classes within the same library to call it freely while also allowing external applications that inherit from the base class to access it through their own derived classes.

Common follow-ups: How does protected internal differ from the newer private protected modifier?;When would you choose protected internal over plain protected or plain internal?

What are Access Modifiers in C#?;Can we use all access modifiers for all types?

Can you specify an access modifier for an enumeration?

Beginner

Yes, you can specify an access modifier for an enum declared at the top level, such as public or internal, following the same accessibility rules that apply to classes, and by default a top level enum without an explicit modifier is internal. However, the individual members inside an enum, such as its named constant values, cannot have their own separate access modifiers, since they always share the same accessibility as the enum itself.

public enum OrderStatus {
    Pending, Shipped, Delivered
}

internal enum InternalFlags {
    None, Active, Archived
}

Real-world example A shared library exposes its OrderStatus enum as public so that any application referencing the library can use these status values, while keeping an internal enum used only for internal flags hidden from external consumers.

Common follow-ups: What is the default accessibility of an enum if no modifier is specified?;Can an enum be nested inside a class and given a different accessibility than the outer class?

Can destructors have access modifiers?;What are Access Modifiers in C#?