access modifiers and abstract, sealed class

What are Access Modifiers in C#?
In C# there are 5 different types of Access Modifiers.
Public
The public type or member can be accessed by any other code in the same assembly or another assembly that references it.

Private
The type or member can only be accessed by code in the same class or struct.

Protected
The type or member can only be accessed by code in the same class or struct, or in a derived class.

Internal
The type or member can be accessed by any code in the same assembly, but not from another assembly.

Protected Internal
The type or member can be accessed by any code in the same assembly, or by any derived class in another assembly.

What are Access Modifiers used for?
Access Modifiers are used to control the accessibilty of types and members with in the types.


Can you use all access modifiers for all types?
No, Not all access modifiers can be used by all types or members in all contexts, and in some cases the accessibility of a type member is constrained by the accessibility of its containing type.

Can derived classes have greater accessibility than their base types?
No, Derived classes cannot have greater accessibility than their base types. For example the following code is illegal.
using System;
internal class InternalBaseClass
{
   public void Print()
   {
      Console.WriteLine("I am a Base Class Method");
   }
}
public class PublicDerivedClass : InternalBaseClass
{
   public static void Main()
   {
      Console.WriteLine("I am a Public Derived Class Method");
   }
}

When you compile the above code an error will be generated stating "Inconsistent accessibility: base class InternalBaseClass is less accessible than class PublicDerivedClass".To make this simple, you cannot have a public class B that derives from an internal class A. If this were allowed, it would have the effect of making A public, because all protected or internal members of A are accessible from the derived class.


Is the following code legal?

using System;
private class Test
{
   public static void Main()
   {
   }
}

No, a compile time error will be generated stating "Namespace elements cannot be explicitly declared as private, protected, or protected internal"

Can you declare struct members as protected?
No, struct members cannot be declared protected. This is because structs do not support inheritance.

Can the accessibility of a type member be greater than the accessibility of its containing type?
No, the accessibility of a type member can never be greater than the accessibility of its containing type. For example, a public method declared in an internal class has only internal accessibility.

Can destructors have access modifiers?
No, destructors cannot have access modifiers.

What does protected internal access modifier mean?
The protected internal access means protected OR internal, not protected AND internal. In simple terms, a protected internal member is accessible from any class in the same assembly, including derived classes. To limit accessibility to only derived classes in the same assembly, declare the class itself internal, and declare its members as protected.

What is the default access modifier for a class,struct and an interface declared directly with a namespace?
internal

Will the following code compile?

using System;
interface IExampleInterface
{
   public void Save();
}

No, you cannot specify access modifer for an interface member. Interface members are always public.

Can you specify an access modifier for an enumeration?
Enumeration members are always public, and no access modifiers can be specified.

What is an abstract class?
An abstract class is an incomplete class and must be implemented in a derived class.

Can you create an instance of an abstract class?
No, abstract classes are incomplete and you cannot create an instance of an abstract class.

What is a sealed class?
A sealed class is a class that cannot be inherited from. This means, If you have a class called Customer that is marked as sealed. No other class can inherit from Customer class. For example, the below code generates a compile time error "MainClass cannot derive from sealed type Customer.
using System;
public sealed class Customer
{
}
public class MainClass : Customer
{
public static void Main()
{
}
}

What are abstract methods?
Abstract methods are methods that only the declaration of the method and no implementation.


Will the following code compile?
using System;
public abstract class Customer
{
public abstract void Test()
{
Console.WriteLine("I am customer");
}
}
public class MainClass
{
public static void Main()
{
}
}
No, abstract methods cannot have body. Hence, the above code will generate a compile time error stating "Customer.Test() cannot declare a body because it is marked abstract"

Is the following code legal?
using System;
public class Customer
{
public abstract void Test();
}
public class MainClass
{
public static void Main()
{
}
}

No, if a class has even a single abstract member, the class has to be marked abstract. Hence the above code will generate a compile time error stating "Customer.Test() is abstract but it is contained in nonabstract class Customer"

How can you force derived classes to provide new method implementations for virtual methods?
Abstract classes can be used to force derived classes to provide new method implementations for virtual methods. An example is shown below.
public class BaseClass
{
public virtual void Method()
{
// Original Implementation.
}
}

public abstract class AbstractClass : BaseClass
{
public abstract override void Method();
}

public class NonAbstractChildClass : AbstractClass
{
public override void Method()
{
// New implementation.
}
}

When an abstract class inherits a virtual method from a base class, the abstract class can override the virtual method with an abstract method. If a virtual method is declared abstract, it is still virtual to any class inheriting from the abstract class. A class inheriting an abstract method cannot access the original implementation of the method. In the above example, Method() on class NonAbstractChildClass cannot call Method() on class BaseClass. In this way, an abstract class can force derived classes to provide new method implementations for virtual methods.

Can a sealed class be used as a base class?
No, sealed class cannot be used as a base class. A compile time error will be generated.

Will the following code compile?
public abstract sealed class Test
{
public virtual void Method()
{
}
}
No, a class cannot be marked as sealed and abstract at the same time. This is because by definition, a sealed class cannot be a base class and an abstract class can only be a base class.