static class, static constructor and private constructor

1. Static Class:
    Static classes are not to be instantiated. You can access the methods & properties without creating an object of it by making the class & their properties & methods are static.
    Use case: A class to have utility methods can be made as static

2. Static Constructor:
   Static Constructors are invoked only once to initialize the static fields & properties when any member of the class is referenced or instance is created.
   Use case: Reading required configuration data into static readonly fields, etc.

3. Private Constructor:
   Private Constructor cant be invoked outside.
   Use case: To create a singleton class.

Example:

Static class
static class Utils
{
    // can be accessed like Utils.SayHello();
    public static string SayHello()
   {
   }
}


Static Constructor
static class Utils
{
    static string helloString;
    // invoked when the SayHello is called.
    static Utils()
    {
       helloString = "something";
    }

    // can be accessed like Utils.SayHello();
    public static string SayHello()
   {
       return helloString;
   }
}

Private Constructor:

class Singleton
{
// Not accessible out side
private Singleton() { }
// But inside ti create singleton class
private static Singleton singletonInstance = new Singleton();
public static Singleton GetInstance() { return singletonInstance ; }
}