Discuss the difference between constants and read-only variables

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Constant fields are created using const keyword and their value remains the same throughout the program. The Read-only fields are created using a read-only keyword and their value can be changed. Const is a compile-time constant while Read-only is a runtime constant.

In .NET interviews, when discussing the difference between constants and read-only variables, it’s important to highlight their distinctions in terms of declaration, initialization, and usage. Here’s a breakdown:

  1. Constants:
    • Constants are declared using the const keyword.
    • They must be initialized at the time of declaration, and their value cannot be changed throughout the program’s execution.
    • Constants are resolved at compile-time.
    • They are implicitly static, meaning they belong to the type, not to the instance of the type.
    • Constants are typically used for values that are known at compile-time and are not expected to change, such as mathematical constants or configuration values.

Example:

csharp
public class ExampleClass
{
public const int MyConstant = 10;
}
  1. Read-only variables:
    • Read-only variables are declared using the readonly keyword.
    • They can be initialized either at the time of declaration or within the constructor of the class.
    • Unlike constants, their value can be assigned at runtime, but once assigned, they cannot be modified.
    • Read-only variables are resolved at runtime.
    • They can vary from one instance of the class to another.

Example:

csharp
public class ExampleClass
{
public readonly int MyReadOnlyVariable;

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

Key Differences:

  • Constants are initialized at compile-time, whereas read-only variables can be initialized at runtime.
  • Constants have to be initialized at the time of declaration, whereas read-only variables can be initialized in the constructor.
  • Constants are implicitly static, while read-only variables are not.
  • Constants are typically used for values that are known at compile-time and do not change, while read-only variables are used for values that may be determined at runtime but should not change thereafter.

In summary, constants are immutable at compile-time, while read-only variables are initialized at runtime and immutable thereafter.