Fields and Constants Interview Questions C#.NET

Fields and Constants Interview Questions and Answers C#.NET

In this article, I am going to discuss most frequently asked Fields and Constants Interview Questions and Answers on C#.NET.

What are the 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. The 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 is declared. An example is shown below.

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 read-only 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 the 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 runtime, const fields cannot be passed by reference.

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 the 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.

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);
    }
}

What is the difference between const and read-only?

The Read-only value can be changed at runtime however const value can never change.

What are the 2 broad classifications of fields in C#?

  1. Instance fields
  2. Static fields

What are instance fields in C#?

Instance fields are specific to an instance of a type. If you have a class T, with an instance field F, you can create two objects of type T, and modify the value of F in each object without affecting the value in the other object.

What is a static field? 

A static field belongs to the class itself and is shared among all instances of that class. Changes made from instance A will be visible immediately to instances B and C if they access the field.

Will the following code compile?
class Area
{
    public static double PI = 3.14;
}
class MainClass
{
    public static void Main()
    {
        Area A = new Area();
        Console.WriteLine(A.PI);
    }
}

No, a compile-time error will be generated stating “Static member ‘Area.PI’ cannot be accessed with an instance reference; qualify it with a type name instead”. This is because PI is a static field. Static fields can only be accessed using the name of the class and not the instance of the class. The above sample program is rewritten as shown below.

class Area
{
    public static double PI = 3.14;
}
class MainClass
{
    public static void Main()
    {
        Console.WriteLine(Area.PI);
    }
}
Can you declare a field readonly? 

Yes, a field can be declared readonly. A read-only field can only be assigned a value during initialization or in a constructor. An example is shown below.

class Area
{
    public readonly double PI = 3.14;
}
class MainClass
{
    public static void Main()
    {
        Area A = new Area();
        Console.WriteLine(A.PI);
    }
}
Will the following code compile? 
class Area
{
    public readonly double PI = 3.14;
}
class MainClass
{
    public static void Main()
    {
        Area A = new Area();
        A.PI = 3.15;
        Console.WriteLine(A.PI);
    }
}

No, PI is readonly. You can only read the value of PI in the Main() method. You cannot assign any value to PI.

What is wrong with the sample program below? 
class Area
{
    public const double PI = 3.14;
    static Area()
    {
        Area.PI = 3.15;
    }
}
class MainClass
{
    public static void Main()
    {
        Console.WriteLine(Area.PI);
    }
}

You cannot assign a value to the constant PI field.

What is the difference between a constant and a static readonly field? 

A static readonly field is very similar to a constant, except that the C# compiler does not have access to the value of a static read-only field at compile time, only at runtime.

Static members cannot access with an object can anybody give the reason why?

Because static member belongs to the class not to a specific instance of that class, so we access them using the class name itself. Static members are instantiated before the main method of the class calls.

SUMMARY

In this article, I try to explain most frequently asked Fields and Constants Interview Questions and Answers on C#.NET. I hope this article will help you with your need. I would like to have your feedback. Please post your feedback, question, or comments about this article.

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