What is difference between constant and readonly?

In C#, both const and readonly are used to define values that cannot be modified. However, there are some key differences between const and readonly variables in terms of their declaration, initialization, and usage. Here are the main differences:

  1. Declaration:
    • const variables are declared and initialized at the same time, and their values must be known at compile-time.
    • readonly variables are declared separately from their initialization and can be assigned a value either at declaration or within the constructor of the class.
  2. Initialization:
    • const variables are implicitly static and must be initialized with a compile-time constant value. This means that their value cannot be changed once assigned.
    • readonly variables are not implicitly static and can have different values for each instance of a class. They can be initialized either at declaration or within the constructor, and their values can be changed only within the constructor.
  3. Usage:
    • const variables are typically used for values that are known at compile-time and will not change throughout the lifetime of the program. They are evaluated and substituted by their literal values during compilation.
    • 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.
  4. Memory Allocation:
    • const values are substituted with their literal values during compilation. Therefore, they do not consume any memory at runtime.
    • 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.
  5. Usage Scenarios:
    • Use const when you have values that are known and constant at compile-time, such as mathematical constants (e.g., π) or fixed configuration values.
    • Use readonly when you have values that need to be assigned once, possibly based on runtime conditions or initialization logic, and may vary between instances of a class.

Example usage of const and readonly:

class ExampleClass
{
    public const int MyConstant = 42;

    public readonly int MyReadonly;

    public ExampleClass(int value)
    {
        MyReadonly = value;
    }
}

In this example, MyConstant is a const variable that is assigned the value 42 at declaration. Its value cannot be changed after that. On the other hand, MyReadonly is a readonly variable that can be assigned a different value for each instance of the class, but once assigned, its value cannot be changed.

To summarize, const variables are compile-time constants with fixed values, while readonly variables are assigned at runtime and can vary between instances. const values do not consume memory at runtime, while readonly values have a memory footprint. 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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