What is Private Constructors ?
A private constructor is a special instance constructor. It is generally used in classes that contain static members only. If a class has one or more private constructors and no public constructors, other classes (except nested classes) cannot create instances of this class. For example:
class NLog { // Private Constructor: private NLog() { } public static double e = Math.E; //2.71828... }
The declaration of the empty constructor prevents the automatic generation of a default constructor.
Note : if you do not use an access modifier with the constructor it will still be private by default.
However, the private modifier is usually used explicitly to make it clear that the class cannot be instantiated.
Private constructors are used to prevent creating instances of a class when there are no instance fields or methods, such as the Math class, or when a method is called to obtain an instance of a class. If all the methods in the class are static, consider making the complete class static.
What is Static Class ?
A static class is basically the same as a non-static class, but there is one difference: a static class cannot be instantiated. In other words, you cannot use the new keyword to create a variable of the class type. Because there is no instance variable, you access the members of a static class by using the class name itself.
E.g: Math 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.
Features of Static class :
- Contains only static members.
- Cannot be instantiated.
- Is sealed.
- Cannot contain Instance Constructors.
What is Static Constructor ?
A static constructor is used to initialize any static data, or to perform a particular action that needs to be performed once only. It is called automatically before the first instance is created or any static members are referenced.
E.g:
class SimpleClass { // Static variable that must be initialized at run time. static readonly long baseline; // Static constructor is called at most one time, before any // instance constructor is invoked or member is accessed. static SimpleClass() { baseline = DateTime.Now.Ticks; } }
Properties :
static constructors have the following properties:
- A static constructor does not take access modifiers or have parameters.
- A static constructor is called automatically to initialize the class before the first instance is created or any static members are referenced.
- A static constructor cannot be called directly.
- The user has no control on when the static constructor is executed in the program.
- A typical use of static constructors is when the class is using a log file and the constructor is used to write entries to this file.
- Static constructors are also useful when creating wrapper classes for unmanaged code, when the constructor can call the LoadLibrary method.
- If a static constructor throws an exception, the runtime will not invoke it a second time, and the type will remain uninitialized for the lifetime of the application domain in which your program is running.
