Difference between Readonly and Const in C# Basic Knowledge

In C#, both the readonly and const keywords are used to define constants, but there are some differences between them.

1. The const keyword can only be used inside a class or namespace, while the readonly keyword can be used inside a class, namespace, or constructor.
2. Assignment method: Constants defined with the const keyword must be assigned a value at the time of declaration and cannot be modified; whereas constants defined with the readonly keyword can be assigned a value at the time of declaration or in the constructor, but cannot be modified once assigned.
3. Compile-time calculation: Constants defined with the const keyword are computed at compile-time and will be replaced with specific values everywhere they are used. Constants defined with the readonly keyword, on the other hand, are computed at runtime and will be recalculated each time they are used.
4. The data types for constants defined by the “const” keyword can be any built-in data type, such as integers, floating-point numbers, boolean values, and so on; whereas constants defined by the “readonly” keyword can also be classes, structures, arrays, and so on, in addition to built-in data types.

In general, variables defined with the “const” keyword have their values determined at compile time and cannot be changed, whereas variables defined with the “readonly” keyword have their values determined at runtime and can be assigned in the constructor.