C# Interview Questions & Answer 2

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.

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.

PrivateThe 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.

InternalThe 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.

What is Boxing and Unboxing?
Boxing - Converting a value type to reference type is called boxing. An example is shown below.
int i = 101;
object obj = (object)i; // Boxing

Unboxing - Converting a reference type to a value typpe is called unboxing. An example is shown below.
obj = 101;
i = (int)obj; // Unboxing

Is boxing an implicit conversion?Yes, boxing happens implicitly.

Is unboxing an implicit conversion?
No, unboxing is an explicit conversion.

What happens during the process of boxing?Boxing is used to store value types in the garbage-collected heap. Boxing is an implicit conversion of a value type to the type object or to any interface type implemented by this value type. Boxing a value type allocates an object instance on the heap and copies the value into the new object. Due to this boxing and unboxing can have performance impact.

What are constants in C#?
Constants in C# are immutable values which are known at compile time and do not change for the life of the program. Constants are declared using the const keyword. Constants must be initialized as they are declared. You cannot assign a value to a constant after it isdeclared. An example is shown below.

using System;
class Circle
{
public const double PI = 3.14;
public Circle()
{
//Error : You can only assign a value to a constant field at the time of declaration
//PI = 3.15;
}
}
class MainClass
{
public static void Main()
{
Console.WriteLine(Circle.PI);
}
}
Can you declare a class or a struct as constant?
No, User-defined types including classes, structs, and arrays, cannot be const. Only the C# built-in types excluding System.Object may be declared as const. Use the readonly modifier to create a class, struct, or array that is initialized one time at runtime (for example in a constructor) and thereafter cannot be changed.

Does C# support const methods, properties, or events?No, C# does not support const methods, properties, or events.

Can you change the value of a constant filed after its declaration?
No, you cannot change the value of a constant filed after its declaration. In the example below, the constant field PI is always 3.14, and it cannot be changed even by the class itself. In fact, when the compiler encounters a constant identifier in C# source code (for example, PI), it substitutes the literal value directly into the intermediate language (IL) code that it produces. Because there is no variable address associated with a constant at run time, const fields cannot be passed by reference.

using System;
class Circle
{
public const double PI = 3.14;
}
How do you access a constant field declared in a class?
Constants are accessed as if they were static fields because the value of the constant is the same for all instances of the type. You do not use the static keyword to declare them. Expressions that are not in the class that defines the constant must use the class name, a period, and the name of the constant to access the constant. In the example below constant field PI can be accessed in the Main method using the class name and not the instance of the class. Trying to access a constant field using a class instance will generate a compile time error.

using System;
class Circle
{
public const double PI = 3.14;
}
class MainClass
{
public static void Main()
{
Console.WriteLine(Circle.PI);
Circle C = new Circle();
// Error : PI cannot be accessed using an instance
// Console.WriteLine(C.PI);
}
}