Why do we use static classes in .NET?

Static classes in .NET are used for various reasons, and they serve specific purposes in the design and organization of code. Here are some common reasons why static classes are used in .NET:

1. **Utility Classes:**
   Static classes are often used to create utility classes that group related methods or functions together. These methods perform specific tasks that are not related to maintaining state, and they can be called without creating an instance of the class. This is useful for organizing and grouping functions logically.

    ```csharp
    public static class MathUtilities
    {
        public static double Square(double number)
        {
            return number * number;
        }

        public static int Add(int a, int b)
        {
            return a + b;
        }
    }
    ```

2. **Helper Classes:**
   Static classes are suitable for creating helper classes that provide shared functionality across an application. These classes can contain methods that perform common tasks, such as reading/writing files, formatting strings, or handling exceptions.

    ```csharp
    public static class FileHelper
    {
        public static string ReadFile(string filePath)
        {
            // Code to read and return content from a file
        }

        public static void WriteFile(string filePath, string content)
        {
            // Code to write content to a file
        }
    }
    ```

3. **Constants and Configuration:**
   Static classes are used to store constant values or configuration settings that are shared across the application. This provides a centralized location for managing such values.

    ```csharp
    public static class AppConfig
    {
        public const string ConnectionString = "your_connection_string_here";
        public const int MaxRetryAttempts = 3;
    }
    ```

4. **Stateless Operations:**
   When a class doesn't need to maintain any instance-specific state and its methods are stateless, making the class static can lead to more efficient memory usage and simpler code.

    ```csharp
    public static class Calculator
    {
        public static int Add(int a, int b)
        {
            return a + b;
        }

        // Other stateless math operations...
    }
    ```

5. **Extension Methods:**
   Extension methods are commonly defined as static methods within static classes. They allow you to add new methods to existing types without modifying those types, providing a form of syntactic sugar.

    ```csharp
    public static class StringExtensions
    {
        public static bool IsPalindrome(this string str)
        {
            // Check if the string is a palindrome
        }
    }
    ```

In summary, static classes are used in .NET to group related functionality, provide shared services, and handle stateless operations. They help improve code organization, maintainability, and performance in scenarios where instances are not needed, and state is not maintained.