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!!!