What is “Static" class? When to use static class in real application? with example

A static class is a class which object can not be created, and which can not be inherited.

Use of static class:Static classes are used as containers for static members like methods, constructors and others. 

In C#, a static class is a class that cannot be instantiated and is used to hold static members such as static methods, static properties, and static fields. Here's an example of a static class in C#:

public static class MathUtils
{
    public static double SquareRoot(double value)
    {
        // Square root calculation implementation
    }
    
    // Other utility methods...
}

Now, let's discuss when you might use a static class in a real application:

Utility functions: If you have a set of related utility functions that don't require any state or instance-specific data, you can group them together in a static class. For example, a MathUtils static class might contain various mathematical calculations like square roots, logarithms, or trigonometric functions.

public static class MathUtils
{
    public static double SquareRoot(double value)
    {
        // Square root calculation implementation
    }
    
    // Other utility methods...
}

Helper classes: Static classes can be used to define helper methods or classes that provide common functionality across your application. For example, a StringUtils static class might contain methods for string manipulation such as formatting, parsing, or searching.

public static class StringUtils
{
    public static bool IsNullOrEmpty(string value)
    {
        return string.IsNullOrEmpty(value);
    }
    
    // Other helper methods...
}

Constants or configuration: You can use a static class to hold constants or configuration values that are shared across your application. By making the class static, you can access these values without needing an instance of the class.

public static class AppConfig
{
    public const string ApplicationName = "MyApp";
    public const int MaxItemCount = 100;
    
    // Other configuration values...
}

Remember, when you declare a class as static in C#, it cannot be instantiated, and you cannot create instances of it. It is designed to provide a container for static members and offer a convenient way to group related functionality or data without the need for instantiation.