What is the difference between constants and readonly?

Asked 06-Dec-2019
Updated 07-Apr-2023
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Constants and readonly variables are two important concepts in C# that are used to define variables with a fixed value. While both constants and readonly variables are used to define variables that cannot be changed after they are initialized, there are some key differences between the two.

Constants: A constant is a variable that has a fixed value that cannot be changed during the execution of a program. Constants are typically used to define values that will not change throughout the lifetime of a program, such as mathematical constants or configuration settings.

In C#, constants are declared using the 'const' keyword, and must be initialized at the time of declaration. Constants are implicitly static and cannot be modified once they are declared.

Readonly Variables: Readonly variables are similar to constants in that they also have a fixed value that cannot be changed after they are initialized. However, unlike constants, readonly variables can be initialized at runtime, which means that they can be initialized with a value that is determined at runtime.

In C#, readonly variables are declared using the 'readonly' keyword, and can be initialized either at the time of declaration or in a constructor. Once a readonly variable is initialized, it cannot be modified.

Differences Between Constants and Readonly Variables: There are several key differences between constants and readonly variables in C#. One of the main differences is that constants must be initialized at the time of declaration, while readonly variables can be initialized either at the time of declaration or in a constructor.

What is the difference between constants and readonly

Another key difference is that constants are implicitly static, which means that they are shared across all instances of a class. Readonly variables, on the other hand, can be either static or non-static, depending on how they are declared.

A third difference between constants and readonly variables is that constants are resolved at compile-time, while readonly variables are resolved at runtime. This means that constants can be faster to access than readonly variables, but they are less flexible.

In summary, both constants and readonly variables are used to define variables with a fixed value that cannot be changed after they are initialized. Constants are typically used to define values that will not change throughout the lifetime of a program, while readonly variables are used to define values that are determined at runtime. Understanding the differences between constants and readonly variables is important for writing efficient and maintainable C# code.