what is private constructor?why we use private constructor?

Just an addition to the excellent answers above, in earlier C#, private constructor was used to hide a class containing static fields and static methods, that is in language we can think that we used private constructor to "seal" the class. The question is why? Well, in our real programming environment we have so many methods which are used across the solution. So, for reusing purpose, we create "Utility" classes containing all this static methods. As this class is very reusable so we protect it and do not want to be inherited further. That time we used private constructor for this class.


Later C#, we actually don't use private constructors often. Rather, we chose "static" class which eventually serve the same purpose.With a Static Class you no longer had to write a Private Constructor, didn't need to mark them 'Sealed, etc.


Another tendency is to use Singleton pattern. Read it more about this design pattern from Google. But I really never used this pattern in my coding life time, so can't really tell about this.


Hope this helps. Mark the answer as accepted if you are satisfied :)

Private Constructor are those which has static fields. You create a static constructor to initialized static fields. Static constructors are not called explicitly with the new statement. They are called when the class is first referenced.  There are some limitation of the static constructor as following;

• ·         Static constructors are parameterless.

• ·         Static constructors can't be overloaded.

• ·         There is none of accessibility of Static 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:
public class Counter
{
private Counter() { }

public static int currentCount;

public static int IncrementCount()
{

return ++currentCount;
}
}

class TestCounter
{

static void Main()
{

// If you uncomment the following statement, it will generate 

// an error because the constructor is inaccessible:
  // Counter aCounter = new Counter();
// Error
Counter.currentCount = 100;
Counter.IncrementCount();
Console.WriteLine("New count: {0}", Counter.currentCount); // Keep the console window open in debug mode.

Console.WriteLine("Press any key to exit.");
Console.ReadKey(); } } // Output: New count: 101