Practical Difference between Static class and Singleton Pattern

Static Class - 

1. static class can not implement interfaces or can not drive from class , it can drive from Object class only.

2. static class can not passed as a method parameter

3. static class is a sealed class

As is the case with all class types, the type information for a static class is loaded by the .NET Framework common language runtime (CLR) when the program that references the class is loaded. The program cannot specify exactly when the class is loaded. However, it is guaranteed to be loaded and to have its fields initialized and its static constructor called before the class is referenced for the first time in your program. A static constructor is only called one time, and a static class remains in memory for the lifetime of the application domain in which your program resides.

Singleton Pattern-

1.  Singleton  can  implement interfaces or can drive from class.

2.  Singleton  can  passed as a method parameter

3.. its a abstract class because of private constructor

Because the Singleton instance is referenced by a private static member variable, the instantiation does not occur until the class is first referenced by a call to the Instance property. This solution therefore implements a form of the lazy instantiation property, as in the Design Patterns form of Singleton.

using System;

public class Singleton
{
   private static Singleton instance;

   private Singleton() {}

   public static Singleton Instance
   {
      get 
      {
         if (instance == null)
         {
            instance = new Singleton();
         }
         return instance;
      }
   }
}

Static Initialization 

public sealed class Singleton
{
   private static readonly Singleton instance = new Singleton();
   
   private Singleton(){}

   public static Singleton Instance
   {
      get 
      {
         return instance; 
      }
   }
}

This implementation has two main advantages:

Because the instance is created inside the Instance property method, the class can exercise additional functionality (for example, instantiating a subclass), even though it may introduce unwelcome dependencies.

The instantiation is not performed until an object asks for an instance; this approach is referred to as lazy instantiation. Lazy instantiation avoids instantiating unnecessary singletons when the application starts.

For More - Singleton Pattern