how to use enum and what is the usage of enum

Enums are strongly typed constants, meaning that an enum of one type may not be implicitly assigned to an enum of another type even though the underlying value of their members are the same. They are essentially unique types that allow you to assign symbolic names to integral values.

Well,the best usage would be, constants can be referred to in a consistent, expressive and type safe way via Enums. See a live example why we need Enums from here:
What is main use of Enumeration?[^]

Check this CP article over Enums:
Enums in C#[^]

And complete also these 2 articles which will clear you most:
C# Station Tutorial (Lesson 17): Enums[^]
Working with enums in C# [^]

If you have a number of constants that are logically related to each other, then you can group together these constants in an enumeration.

Enum declaration

enum Tempurature
{
    Low,
    Medium,
    High,
};

Enums are strongly typed constants which makes the code more readable and less prone to errors. It is useful when you have a set of values that are functionally significant and unchanged. The main advantage of Enum is make it easy to change values in the future, also you can reduces errors caused by transposing or mistyping numbers.

More about ...C# Enum

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