A programmatic close look at constant and read-only variables in C#

Posted by SATYA SAHOO in C# category on 12/30/2010 for Intermediate level | Points: 250 | Views : 13197

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Many time we get question in interview that what is the difference between constant and readonly.I have listed out most of the difference and to make it more clear I have also given sample code/steps to test every point in this comparison. I hope this will be useful.

Introduction

This article explains the difference between constant and read-only variables using simple programs.

Constant

Read-only

This is like static variables and can not be accessed through object.

Example:

class Program

{

static void Main()

{

X o = new X();

int k = o.i;//Error

int k = X.i; //No Error

}

class X

{

public const int i=10;

}

Can be accessed through objects like a normal instance variables.

Example:

class Program

{

static void Main(string[] args)

{

X o = new X();

int k = o.i;//No Error

int j = X.i;//Error

}

class X

{

public readonly int i=10;

}

Can't be declared static (when declared in a class by default it is a class level variable )

Example:

class X

{

public static const int i=10;//Error

}

Can be declared static.

Example:

class X

{

public static readonly int i=10; //No Error

}

Value is evaluated at compile time.

Example :

static int k = 10;//assigned at runtime

const int i=k;//Error since k value is assigned at runtime.

Value is evaluated at runtime like a normal variable

Example :

static int k = 10;//assigned at runtime

readonly int i=k; //No error

Initialized at declaration only.

Example:

(1)public const int i = 25; //No Error

(2)public const int i ;//Error

i=25;//Error

(3)

class X

{

const int i;//Error

public X()

{

i=10;

}

(4)

class X

{

const int i;//Error

public void f()

{

i=10;

}

Can be initialized at declaration or by code in the constructor.

Example:

(1)public readonly int i = 25; //No Error

(2)public readonly int i ;//Error

i=25;//Error

(3)

class X

{

readonly int i;//No Error

public X()

{

i=10;

}

(4)

class X

{

readonly int i;//Error

public void f()

{

i=10;

}

Can be declared and initialized in a method.

Example:

class X

{

public void f()

{

const int i = 10;//No Error

}

Can not be declared and initialized in a method.

Example:

class X

{

public void f()

{

readonly int i = 10;//Error

}

Referring assembly preserves the value at compile time.

Example:

Follow the below steps to understand this :

1. Create an assembly MYTEST.dll using below code

namespace MYTEST

{

public class X

{

public const int i = 10;

}

2. Create a Console application(say Constant) and add reference to MYTEST.dll and write below code to print the value of the constant i and compile the program to create Constant.exe assembly.

namespace Constant

{

class Program

{

static void Main()

{

Console.WriteLine(MYTEST.X.i);

Console.Read();

}

3. Now open the bin/Debug directory of constant application and run/doubleclick on Constant.Exe. Output will be 10.

4.Change the value of the constant in MYTEST program as below and recompile it to MYTEST.dll

namespace MYTEST

{

public class X

{

public const int i = 20;

}

5. Open the bin/Debug directory of constant application and replace MYTEST.dll with the new dll.

6. Run/doubleclick on Constant.Exe. Output will be still 10.

Value changes in referring assembly if modified in the source assembly.

Example:

Follow the below steps to understand this :

1. Create an assembly MYTEST.dll using below code

namespace MYTEST

{

public class X

{

public readonly int i = 10;

}

2. Create a Console application(say Readonly) and add reference to MYTEST.dll and write below code to print the value of the readonly variable i and compile the program to create Readonly.exe assembly.

namespace Readonly

{

class Program

{

static void Main()

{

Console.WriteLine(MYTEST.X.i);

Console.Read();

}

3. Now open the bin/Debug directory of Readonly application and run/doubleclick on Readonly.Exe. Output will be 10.

4.Change the value of the Readonly variable in MYTEST program as below and recompile it to MYTEST.dll

namespace MYTEST

{

public class X

{

public readonly int i = 20;

}

5. Open the bin/Debug directory of Readonly application and replace MYTEST.dll with the new dll.

6. Run/doubleclick on Readonly.Exe. Output will be still 20.

Conclusion

I hope this will help to get a clear understanding on const and read only keyword.

About the Author

Full Name: SATYA SAHOO
Member Level: Starter
Member Status: Member
Member Since: 12/28/2010 8:47:18 AM
Country: India

Working in Misys Software solution having interest in learning and writing on MS.Net technologies.

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Comments or Responses

Posted by: Rtpharry on: 1/5/2011 | Points: 25

Is this article technically correct? I don't know all the technical details about the differences but it seems that the readonly examples are wrong in several places.

"Can be declared static."

class X

{

   public static int i=10; //No Error

}

this is just a normal static int, shouldnt it be:

class X

{

   public static readonly int i=10; //No Error

}

The next box uses

const int i=k; //No error

which I would expect to be

readonly int i=k; //No error

And then the "Can be initialized in declaration or by code in the constructor." box uses both readonly and const in a single declaration.

Posted by: Esensahoo on: 1/5/2011 | Points: 25

Thanks harry for pointing it ....while copying pate from my visual studio I have done some mistake.I will correct it and post it again.

Posted by: Esensahoo on: 1/5/2011 | Points: 25

Harry, I have updated the post.There were some places where I had missed readonly keyword and some places I had used constant key word in read-only column.It was a mistake during copy paste .I apologizes all the readers for this mistake. Please feel free to comment if you find any mistake in the post.Many thanks to Harry once again.

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