Difference Between Const, ReadOnly and Static ReadOnly in C#

The cost, readonly, and static readonly in C# are keywords used to define a constant, a readonly, and a static readonly types of variables. These variables are used in a class so that the caller class cannot update the values of these variables once the values are assigned. In this post, learn the difference between const vs. readonly vs. static readonly and how to use them in a C# application. 

Const in C#

The cost keyword declares a constant type variable. That means a variable of which the value is constant but at compile time. And it's mandatory to assign a value to it. By default, a const is static, and we cannot change the value of a const variable throughout the entire program. So constant variables are useful when you already know a variable's value and know it will never change in the entire program. 


Here I have created a class named Variables and defined all three variables, so first, let's play with a const type variable.


Here I tried to reassign the value of the const variable, but it gave me an error, "A const field requires a value to be provided". So, OK, now I initialize a value for this variable and try to change it further in the class. 


Here I have created a static constructor, a default constructor, parameterized constructor, and a Simple Method. I tried to change the value of the const variable everywhere, but once I assign the value, I cannot change it again since when I do, it gives me a compile time error, as you can see in the snapshot above.

Now let's move on to the readonly keyword.

Readonly in C#

Readonly is the keyword whose value we can change during runtime or assign it at run time but only through the non-static constructor. Not even a method. Let's see:


Here first, I try to initialize the value in the static constructor. It gives me an error. Which you can see above. Now I try to change the value in a method, see what happens,

Here, it also gives an error that you can only assign a value through a variable or a constructor. Now try to change the value in the default constructor.


The snapshot above shows that it's built successfully without an error, warning, or message. Let's check if there is a runtime error. OK.


Here we can see no runtime error, and the value was assigned successfully to the Readonly variable. Now one gotcha is direct that you have assigned the value. Can you change this value again ??? Let's try to change the value again.


Here I created a parameterized constructor and created a new object, passing a value as "Hello Frend'z," As I built it, it gave me the result "Build Succeeded." Now let's move ahead and check for a runtime error:


See guys. There is no runtime error !! And the value can be changed again and again through a constructor.

Here is an article on when to use readonly vs. cost in C#: Readonly and Constant variables in C#.

Now move ahead to Static Readonly variables.

Static ReadOnly in C#

A Static Readonly type variable's value can be assigned at runtime or assigned at compile time and changed at runtime. But this variable's value can only be changed in the static constructor. And cannot be changed further. So it can change only once at runtime. Let's understand it practically.


In the preceding, you can see that I used two variables, one is not assigned, another is assigned, and the static constructor. Now in the static constructor, you can see that the unassigned variable is being assigned, and the assigned value is being changed. And there is no compile time error. Further, I try to change this variable's value again. See what happened:


As you can see above, I created a Default, Parameterized Constructor and Method and tried to change the value again here. But I am getting a compile-time error for all.


This article taught you the difference between cost vs. readonly vs. static readonly in C#. You also learned when to use these variables in your program. 

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