Const vs Readonly in C#
In this article, we will talk about the two most commonly used keywords “const” and “readonly” and the difference between them. Let`s get started.
const:
We can declare a variable as a constant using the const keyword in C#.
We need to assign a value to a constant variable at the time of declaration by using a hard-coded value or an expression that can be fully evaluated at compile time.
Only built-in data types like int, string, boolean etc can be marked as constant variables.
Once the value is assigned to a constant variable at the time of declaration, then we can`t change its value. If we try to change the value, then we will get the compilation error “The left-hand side of an assignment must be a variable, property or indexer”. Because of this constant variables are most commonly called Compile time constant.
The compiler compiles the constant value to every location that references it. For example, initially, we declare a constant variable with value X and we are using that variable in a method named foo(). After that we change the value to Y then we need to compile the code again so that the change will get reflected in method foo().
Constant variables are static. So we can access constant variables using the class name.
The best practice for naming the constant variable is to use Pascal casing.
readonly:
We can declare a variable as a readonly variable using the readonly keyword in C#.
We can assign a value to a readonly variable at the time of declaration or in the constructor. Once the value is assigned, then we can`t change it.
As we can assign a value to readonly variable using a constructor, it can have a different value for a different constructor. Because of this readonly variables are most commonly called Run time constants.
We can use any data type as a readonly variable.
We can use static keyword with readonly variables.
The best practice for naming the read-only variable is to use Pascal casing.
When to use constant and when to use readonly variable:
So the main key points are:
If we know that the value of the variable is not going to change in any part of our application, then we can declare that variable as a constant variable. For example, PI=3.14.
If we are unsure about the value of a variable, but we don`t want other classes to change the value after initialization then, we can declare that variable as readonly.
Conclusion:
In this article, we talked about the difference between constant and readonly variables in C#. I hope you enjoyed reading the article.
Happy Coding!!!