Constant or ReadOnly?

ReadOnly properties andconstant fields may seem to achieve the same results. However, there are manydifferences which must be taken into consideration when choosing which one touse. 

Constant Field 

A constant field can be created in aclass for information that is to be hard coded into the application and willnot be changed. This often used for mathematical values such as Pi. Aconstant is considered a "Compile-Time Constant". This means thevalues is fully evaluated at compile time. It cannot beassigned to a value returned from a method or an object instance. The compilercompiles the constant value into every reference in the application. If youlater change the value of the constant, it will also change value in everylocation it is used in the code.  

public class Maths 

{ 

public const double Pi= 3.14  

A constant must be assigned ondeclaration. It can only be a number, string or Boolean as these can beevaluated at compile time. 

Must be assigned on declaration 

Can only be a number, Boolean orstring as these can be evaluated at compiled time 

They are always static  and canbe accessed using the class name as seen below: 

var area = Math.Pi * radius *radius; 

ReadOnly Property 

A Read-Only Property is a variable ina class that is initialised and then not changed again. It isconsidered a "Runtime Constant", which means that it can beassigned to any valid expression at runtime.  

The property can be assigned ondeclaration or in a constructor as seen below: 

In declaration: 

public class Maths { 

public readonly decimal myPrice = GetPrice(); 

In constructor: 

public readonly decimal myPrice; 

//constructor 

public Product(){ 
MyPrice = GetPrice();  } 

By default, a readonly field is notstatic so you must create an instance of the class in order to call it. 

var newProduct = new Product();

var newPrice = newProduct.myPrice;

Read-Only properties can have any datatype and can optionally be static.  

When to use each? 

It is better to use Constants withsimple data types and when you can be certain that the values will never change. 

You may use a Read-only property whenvalue is initialised from a code, a file or a table but then cannot be changedanywhere else in the application. If it is to be shared by all instances, thenit should be a static read-only field. If the field value could be differentfor each instance then leave the field as non-static.