What is difference between constant and readonly?

In C#, both const and readonly are used to define values that cannot be modified, but they have several key differences in terms of declaration, initialization, usage, and memory allocation. Here are the main differences between const and readonly:

  1. Declaration and Initialization:
    • const: const variables are declared and initialized at the same time. Their values must be known at compile-time and cannot be changed afterward. They are implicitly static and must be assigned a compile-time constant value.
    • readonly: readonly variables are declared separately from their initialization. They can be assigned a value either at declaration or within the constructor of the class. They can have different values for each instance of a class, but once assigned, their values cannot be changed.
  2. Usage Scenarios:
    • const: const variables are typically used for values that are known and constant at compile-time, such as mathematical constants (e.g., π) or fixed configuration values. They are evaluated and substituted by their literal values during compilation.
    • readonly: readonly variables are used when a value needs to be assigned once and can vary between instances of a class. They can be useful for values that are not known until runtime or that depend on some initialization logic.
  3. Memory Allocation:
    • const: const values are substituted with their literal values during compilation. Therefore, they do not consume any memory at runtime.
    • readonly: readonly values are stored in memory and have a memory footprint. Each instance of the class will have its own copy of the readonly value.
  4. Accessibility:
    • const: const variables are implicitly static and are accessible using the class name. They can be used within the same class, in derived classes, and outside the class by using the class name.
    • readonly: readonly variables are instance-specific and can be accessed only through an instance of the class. They are accessible within the same class and in derived classes.
  5. Initialization Flexibility:
    • const: const variables must be initialized with a compile-time constant value. This restricts their initialization to values that can be determined at compile-time.
    • readonly: readonly variables can be assigned values at runtime, either at declaration or within the constructor of the class. This allows for more flexibility in assigning values that are determined at runtime or depend on initialization logic.

To summarize, const variables are compile-time constants with fixed values, while readonly variables can have different values for each instance of a class. const values do not consume memory at runtime, while readonly values have a memory footprint. const variables are implicitly static and accessible using the class name, while readonly variables are instance-specific. The choice between const and readonly depends on whether the value is known at compile-time or needs to be assigned at runtime, and whether it should be the same for all instances or can vary between instances.

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