C# Interview Questions

In C#, both const and readonly are used to define immutable values, but they serve different purposes and have different behaviors. Understanding these differences is important for using them effectively in your code.

const Keyword

1. Compile-time Constant:

   • The value of a const field is set at compile time and cannot be changed thereafter.
   • const fields are implicitly static and must be initialized with a constant expression.

2. Initialization:

   • Must be initialized at the time of declaration.
   • Cannot be assigned a value in a constructor or elsewhere in the code.

3. Scope:

   • const fields are implicitly static, so they belong to the type itself, not to instances of the type.

4. Usage:

   • Typically used for defining constant values that are known at compile time, such as mathematical constants, configuration values, etc.

5. Type:

   • Can be used with built-in types (int, float, double, char, string, etc.) and enum.

Example:

public class Constants
{
    public const int MaxValue = 100;
    public const string Greeting = "Hello, World!";
}

readonly Keyword

1. Run-time Constant:

   • The value of a readonly field can be set at runtime, but only in the constructor of the class.
   • Once assigned, the value cannot be changed.

2. Initialization:

   • Can be initialized at the time of declaration or in the constructor.
   • Allows for more flexible initialization, especially when the value is not known until runtime.

3. Scope:

   • readonly fields can be instance-specific or static. If declared as static, they belong to the type itself; otherwise, they belong to the instance.

4. Usage:

   • Used when the value is constant for the lifetime of the object but might be different for different instances, or when the value needs to be calculated or determined at runtime.

5. Type:

   • Can be used with any type, including complex types and objects.

Example:

public class Configuration
{
    public readonly int MaxItems;
    public readonly string AppName;

    public Configuration(int maxItems, string appName)
    {
        MaxItems = maxItems;
        AppName = appName;
    }
}

// Usage
var config = new Configuration(100, "MyApp");
Console.WriteLine(config.MaxItems); // Outputs: 100
Console.WriteLine(config.AppName);  // Outputs: MyApp

Summary of Differences

Feature	const	readonly
Value Set	At compile time	At runtime (constructor)
Mutability	Immutable	Immutable after construction
Initialization	At declaration	At declaration or in constructor
Scope	Implicitly static	Can be instance-specific or static
Types Allowed	Primitive types, enum, string	Any type
Flexibility	Less flexible, value must be known at compile time	More flexible, value can be calculated at runtime

When to Use Each

• Use const when:

  • You have a value that is known at compile time and will never change.
  • You need to define constants for configuration, mathematical values, or fixed data.

• Use readonly when:

  • You need to set the value at runtime, possibly based on constructor parameters.
  • The value is immutable once set, but might differ between instances or needs to be computed.

By understanding these differences, you can make informed decisions about when to use const and readonly in your C# applications to ensure immutability and proper initialization of your fields.