C# Interview Questions & Answer 4

What are Properties in C#. Explain with an example?
Properties in C# are class members that provide a flexible mechanism to read, write, or compute the values of private fields. Properties can be used as if they are public data members, but they are actually special methods called accessors. This enables data to be accessed easily and still helps promote the safety and flexibility of methods.

In the example below _firstName and _lastName are private string variables which are accessible only inside the Customer class. _firstName and _lastName are exposed using FirstName and LastName public properties respectively. The get property accessor is used to return the property value, and a set accessor is used to assign a new value. These accessors can have different access levels. The value keyword is used to define the value being assigned by the set accessor. The FullName property computes the full name of the customer. Full Name property is readonly, because it has only the get accessor. Properties that do not implement a set accessor are read only.

The code block for the get accessor is executed when the property is read and the code block for the set accessor is executed when the property is assigned a new value.


using System;
class Customer
{
// Private fileds not accessible outside the class.
private string _firstName = string.Empty;
private string _lastName = string.Empty;
private string _coutry = string.Empty;

// public FirstName property exposes _firstName variable
public string FirstName
{
get
{
return _firstName;
}
set
{
_firstName = value;
}
}
// public LastName property exposes _lastName variable
public string LastName
{
get
{
return _lastName;
}
set
{
_lastName = value;
}
}
// FullName property is readonly and computes customer full name.
public string FullName
{
get
{
return _lastName + ", " + _firstName;
}
}
//Country Property is Write Only
public string Country
{
set
{
_coutry = value;
}
}

}
class MainClass
{
public static void Main()
{
Customer CustomerObject = new Customer();
//This line will call the set accessor of FirstName Property
CustomerObject.FirstName = "David";
//This line will call the set accessor of LastName Property
CustomerObject.LastName = "Boon";
//This line will call the get accessor of FullName Property
Console.WriteLine("Customer Full Name is : " + CustomerObject.FullName);
}
}
Explain the 3 types of properties in C# with an example?
1. Read Only Properties: Properties without a set accessor are considered read-only. In the above example FullName is read only property.
2. Write Only Properties: Properties without a get accessor are considered write-only. In the above example Country is write only property.
3. Read Write Properties: Properties with both a get and set accessor are considered read-write properties. In the above example FirstName and LastName are read write properties.

What are the advantages of properties in C#?
1. Properties can validate data before allowing a change.
2. Properties can transparently expose data on a class where that data is actually retrieved from some other source such as a database.
3. Properties can take an action when data is changed, such as raising an event or changing the value of other fields.

What is a static property. Give an example?
A property that is marked with a static keyword is considered as static property. This makes the property available to callers at any time, even if no instance of the class exists. In the example below PI is a static property.

using System;
class Circle
{
private static double _pi = 3.14;
public static double PI
{
get
{
return _pi;
}
}
}
class MainClass
{
public static void Main()
{
Console.WriteLine(Circle.PI);
}
}
What is a virtual property. Give an example?
A property that is marked with virtual keyword is considered virtual property. Virtual properties enable derived classes to override the property behavior by using the override keyword. In the example below FullName is virtual property in the Customer class. BankCustomer class inherits from Customer class and overrides the FullName virtual property. In the output you can see the over riden implementation. A property overriding a virtual property can also be sealed, specifying that for derived classes it is no longer virtual.


using System;
class Customer
{
private string _firstName = string.Empty;
private string _lastName = string.Empty;

public string FirstName
{
get
{
return _firstName;
}
set
{
_firstName = value;
}
}
public string LastName
{
get
{
return _lastName;
}
set
{
_lastName = value;
}
}
// FullName is virtual
public virtual string FullName
{
get
{
return _lastName + ", " + _firstName;
}
}
}
class BankCustomer : Customer
{
// Overiding the FullName virtual property derived from customer class
public override string FullName
{
get
{
return "Mr. " + FirstName + " " + LastName;
}
}
}
class MainClass
{
public static void Main()
{
BankCustomer BankCustomerObject = new BankCustomer();
BankCustomerObject.FirstName = "David";
BankCustomerObject.LastName = "Boon";
Console.WriteLine("Customer Full Name is : " + BankCustomerObject.FullName);
}
}
What is an abstract property. Give an example?
A property that is marked with abstract keyword is considered abstract property. An abstract property should not have any implementation in the class. The derived classes must write their own implementation. In the example below FullName property is abstract in the Customer class. BankCustomer class overrides the inherited abstract FullName property with its own implementation.


using System;
abstract class Customer
{
private string _firstName = string.Empty;
private string _lastName = string.Empty;

public string FirstName
{
get
{
return _firstName;
}
set
{
_firstName = value;
}
}
public string LastName
{
get
{
return _lastName;
}
set
{
_lastName = value;
}
}
// FullName is abstract
public abstract string FullName
{
get;
}
}
class BankCustomer : Customer
{
// Overiding the FullName abstract property derived from customer class
public override string FullName
{
get
{
return "Mr. " + FirstName + " " + LastName;
}
}
}
class MainClass
{
public static void Main()
{
BankCustomer BankCustomerObject = new BankCustomer();
BankCustomerObject.FirstName = "David";
BankCustomerObject.LastName = "Boon";
Console.WriteLine("Customer Full Name is : " + BankCustomerObject.FullName);
}
}
Can you use virtual, override or abstract keywords on an accessor of a static property?
No, it is a compile time error to use a virtual, abstract or override keywords on an accessor of a static property.

Will the following code compile?
using System;
public class Example
{
static void Main()
{
TestStruct T = new TestStruct();
Console.WriteLine(T.i);
}
}
public struct TestStruct
{
public int i=10;
//Error: cannot have instance field initializers in structs
}No, a compile time error will be generated stating "within a struct declaration, fields cannot be initialized unless they are declared as const or static"

Can a struct have a default constructor (a constructor without parameters) or a destructor in C#?
No

Can you instantiate a struct without using a new operator in C#?Yes, you can instantiate a struct without using a new operator

Can a struct inherit from another struct or class in C#?
No, a struct cannot inherit from another struct or class, and it cannot be the base of a class.

Can a struct inherit from an interface in C#?Yes

Are structs value types or reference types?
Structs are value types.

What is the base type from which all structs inherit directly?All structs inherit directly from System.ValueType, which inherits from System.Object.

What are the 2 types of data types available in C#?
1. Value Types
2. Reference Types

If you define a user defined data type by using the struct keyword, Is it a a value type or reference type?
Value Type

If you define a user defined data type by using the class keyword, Is it a a value type or reference type?
Reference type
Are Value types sealed?Yes, Value types are sealed.

What is the base class from which all value types are derived?
System.ValueType
Give examples for value types?Enum
Struct

Give examples for reference types?
Class
Delegate
Array
Interface
What are the differences between value types and reference types?
1. Value types are stored on the stack where as reference types are stored on the managed heap.
2. Value type variables directly contain their values where as reference variables holds only a reference to the location of the object that is created on the managed heap.
3. There is no heap allocation or garbage collection overhead for value-type variables. As reference types are stored on the managed heap, they have the over head of object allocation and garbage collection.
4. Value Types cannot inherit from another class or struct. Value types can only inherit from interfaces. Reference types can inherit from another class or interface.

Will the following code compile and run?
string str = null;
Console.WriteLine(str.Length);The above code will compile, but at runtime System.NullReferenceException will be thrown.

How do you create empty strings in C#?
Using string.empty as shown in the example below.
string EmptyString = string.empty;

What is the difference between System.Text.StringBuilder and System.String?
1. Objects of type StringBuilder are mutable where as objects of type System.String are immutable. 2. As StringBuilder objects are mutable, they offer better performance than string objects of type System.String.
3. StringBuilder class is present in System.Text namespace where String class is present in System namespace.

How do you determine whether a String represents a numeric value?To determine whether a String represents a numeric value use TryParse method as shown in the example below. If the string contains nonnumeric characters or the numeric value is too large or too small for the particular type you have specified, TryParse returns false and sets the out parameter to zero. Otherwise, it returns true and sets the out parameter to the numeric value of the string.

string str = "One";
int i = 0;
if(int.TryParse(str,out i))
{
Console.WriteLine("Yes string contains Integer and it is " + i);
}
else
{
Console.WriteLine("string does not contain Integer");
}

What is the difference between int.Parse and int.TryParse methods?
Parse method throws an exception if the string you are trying to parse is not a valid number where as TryParse returns false and does not throw an exception if parsing fails. Hence TryParse is more efficient than Parse.

Why should you override the ToString() method?
All types in .Net inherit from system.object directly or indirectly. Because of this inheritance, every type in .Net inherit the ToString() method from System.Object class. Consider the example below.

using System;
public class MainClass
{
public static void Main()
{
int Number = 10;
Console.WriteLine(Number.ToString());
}
}
In the above example Number.ToString() method will correctly give the string representaion of int 10, when you call the ToString() method.

If you have a Customer class as shown in the below example and when you call the ToString() method the output doesnot make any sense. Hence you have to override the ToString() method, that is inherited from the System.Object class.

using System;
public class Customer
{
public string FirstName;
public string LastName;
}
public class MainClass
{
public static void Main()
{
Customer C = new Customer();
C.FirstName = "David";
C.LastName = "Boon";
Console.WriteLine(C.ToString());
}
}
The code sample below shows how to override the ToString() method in a class, that would give the output you want.


using System;
public class Customer
{
public string FirstName;
public string LastName;

public override string ToString()
{
return LastName + ", " + FirstName;
}
}
public class MainClass
{
public static void Main()
{
Customer C = new Customer();
C.FirstName = "David";
C.LastName = "Boon";
Console.WriteLine(C.ToString());
}
}
Conclusion : If you have a class or a struct, make sure you override the inherited ToString() method.