What is the use of static class in C# and .NET with examples?

In C# and .NET, a static class is a class that cannot be instantiated, and all its members (fields, properties, methods, etc.) must be static. This means that you can't create an instance of a static class, and you access its members using the class itself rather than an instance of the class. Static classes are often used to group related functionality together and provide a convenient way to organize code.

Here are some common use cases for static classes in C# and .NET:

1. **Utility Classes:**
   Static classes are commonly used to create utility classes that contain methods for common tasks, calculations, or operations. For example:

    ```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;
        }
    }
    ```

   Usage:

    ```csharp
    double squaredValue = MathUtilities.Square(5);
    int sum = MathUtilities.Add(3, 7);
    ```

2. **Helper Classes:**
   Static classes can be used to create helper classes that provide shared functionality across an application. For instance:

    ```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
        }
    }
    ```

   Usage:

    ```csharp
    string fileContent = FileHelper.ReadFile("example.txt");
    FileHelper.WriteFile("output.txt", "Hello, world!");
    ```

3. **Constants and Configuration:**
   You can use static classes to store constant values or configuration settings that are shared across the application:

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

   Usage:

    ```csharp
    string connectionString = AppConfig.ConnectionString;
    int maxRetries = AppConfig.MaxRetryAttempts;
    ```

Remember that static classes cannot be instantiated, so they don't have constructors, and you can't create instances of them using the `new` keyword. They are typically used when you have functionality that does not depend on maintaining state across multiple instances.