Just4Fun - Comparaison between const and readonly in C#

/* By Dylan SUN */

Today let us talk about const and readonly.

• const is considered as compile-time constant
• readonly is considered as runtime constant.

I will demonstrate some example code to clarify their usages and differences.

I. Const usages

Firstly, you can see that const variable can be declared and assigned at class level.

public class ConstExample
{
    public const string CstValue1 = "hello"; //Correct
}

You can also declare a const variable in constructor

public ConstExample()
{
    const string cstValue6 = "hehe"; //Correct
}

You can see that const can be declared at method level too

public void Display()
{
    const string cstValue2 = "cst 2"; //Correct : Constant can be declared in method
}

If you want to assign another value to CstValue1, you will get an compile-time exception. Because const can only be assigned when declaration.

CstValue1 = "hello2"; //Compile time exception : Can not resolve symbol CstValue1

You can not also assign another value to CstValue1 in the constructor. You will get an compile-time exception.

public ConstExample()
{
    CstValue1 = "hello2"; //Compile time exception : Constant can not be used as an assignment target
}

May be you want to assign a normal string variable to a const like this. But you will see it doesn’t work too.

public string v1;
const string CstValue2 = v1; //Compile time exception : Constant initializer must be compile-time constant

But you can assign the operation result of several consts to another const like this.

public const string CstValue3 = "world"; //Correct

public const string CstValue4 = CstValue1 + CstValue3; //Correct : Constants can only be assigned with values or with consts

You can also declare a const, and assign it with the addition of two consts in the method.

const string cstValue5 = CstValue1 + CstValue3;

When you display them you can see

Console.WriteLine(CstValue1); //=> hello

Console.WriteLine(cstValue2); //=> cst 2

Console.WriteLine(CstValue4); //=> helloworld

Console.WriteLine(cstValue5); //=> helloworld

II. readonly usages

Firstly, you can declare an readonly variable at class level.

public class ReadOnlyExample
{
    public readonly string rdValue1 = "good";
}

You can also just declare a readonly variable without assigning any value to it.

public readonly string rdValue2;

You can not declare a readonly variable in a method.

readonly string rdValue6 = "hohoho"; //compile time exception : statement expected

You cannot assign a value to variable rdValue1 after the declaration, except for, you assign a value in the class constructor.

rdValue1 = "good 2"; //Compile time exception : Can not resolve symbol rdValue1

You can not assign a readonly variable to another readonly variable at class level.

readonly string rdValue3 = rdValue1; //Compile time exception : cannot access non-static field 'rdValue1' in static context

You can not assign a normal variable to a readonly variable neither at class level.

public string value1 = "hey";
readonly string rdValue6 = value1; //Compile time exception : cannot access non-static field 'value1' in static context

You can not assign value to a readonly variable in a method.

public void Display()
{
    rdValue1 = "good one"; //Compile time exception: Readonly field can not be used as an assignment target
    rdValue2 = "good too"; //Compile time exception: Readonly field can not be used as an assignment target
}

But you can assign a normal variable to a readonly variable in class constructor.

You can even assign the operation result of several readonly variables to another readonly variable.

public readonly string rdValue4;

public readonly string rdValue5;

public ReadOnlyExample(string value)
{
    rdValue2 = value; //Correct

    rdValue4 = rdValue1 + rdValue2; //Correct : Assign another readonly variable to another readonly variable can only be done in constructor

    rdValue5 = rdValue1 + value1; //Correct : Assign readonly variable with normal variable to another readonly variable
}

when you display them, you will see

Console.WriteLine(rdValue1); //=> good

Console.WriteLine(rdValue2); //=> day

Console.WriteLine(rdValue4); //=> goodday

Console.WriteLine(rdValue5); //=> goodhey

If you want to access to a const, you must access it via the class

Console.WriteLine(ConstExample.CstValue1); //Const variable can only be accessed by class

If you want to access to a readonly variable, you must access it via an instance of class.

var re = new ReadOnlyExample("day");
Console.WriteLine(re.rdValue1); //Readonly value can only be accessed by instance of class

So to conclude,

Const :

• Can only be assigned at declaration

• Can be declared at class level, in constructor and in method.

• Can be assigned with operation result of several consts at class
  
  level or in constructor or in method. (like addition, multiplication
  
  etc)

• Once the declaration is done, you can never modify a const’s value, neither in constructor, nor just after declaration.

• You can access const variable only by class.

Readonly:

• Can only be assigned at declaration or in constructor.

• Can be declared only at class level. You can declare a readonly
  
  variable neither in constructor nor in method.

• Can be assigned with operation result of several readonly variables only in constructor (like addition, multiplication etc). You can not assign them to a readonly variable when declaration or in a method.

• You can access Readonlyvariable only by the instance of class.

I hope you find this article helpful!

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